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Site & Board Matters => Luminous Landscape Video => Topic started by: Gregory on August 08, 2007, 12:48:34 am

Title: Camera-to-Print -- exposing to the right question
Post by: Gregory on August 08, 2007, 12:48:34 am
I was a little confused by one statement in the tutorial regarding exposing to the right (which I'm familiar with).

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...you'll get a better result by increasing the exposure a little bit to the right, which is the same thing as lowering the ISO...
you shouldn't follow the 'expose to the right' religion if you're shooting at high ISO. there's no point in shooting at ISO 400 and exposing to the right. you may as well drop to ISO 200.
I don't understand this. the purpose of exposing to the right is to get more levels in the middle of the histogram because the first stop provides 4096 levels, the second stop 2048, etc. if you shoot at ISO 800 and expose to the right, you're getting more levels for the middle and low regions. how does this equate to shooting at ISO 400 without exposing to the right in terms of levels and detail? i.e., ISO 800 ev +1.0 == ISO 400 ev 0.0

please explain. details and levels in the lower region of the histogram are important to me because many of the birds I photograph have dark regions which don't contain as much detail as I would prefer.

and... is this level structure carried into the RAW processor and beyond? i.e., after processing, is the lowest stop still restricted to only 128 levels?

and... the Canon Mark III shoots in 14-bit (from Canon's page: "The extra power of dual DIGIC III processors has also allowed analog-to-digital conversion to improve from 12 to 14 bits per channel, meaning better tonal gradation for RAW images."). would this increase the levels of the lower stop in the histogram by 2^2 from 2^7 to 2^9 == 512 levels?

sincerely,
Gregory
Title: Camera-to-Print -- exposing to the right question
Post by: digitaldog on August 08, 2007, 08:57:51 am
Lets say you find that exposing plus one stop over the meter does indeed give you plenty of highlight detail to pull back in a Raw converter (I've actually found 1 ½ stops on my 5D). OK so you're starting out at ISO 400 at F8 based on the meter but using ETTR which tells you you'd get better data at F5.6. Well isn't that the same as shooting at ISO 200? If you have a scene that requires you to shoot at F8, not 5.6, well ETTR isn't going to fly for you.

Where I see ETTR really making a big difference is when I set the ISO higher and expose to the right which is the equivalent of using a lower ISO setting in practicality.

Extra bit depth in the new Canon doesn't necessarily mean that you have less noise or more dynamic range. You just have more levels in whatever you shot. That could be more levels of noise in theory.
Title: Camera-to-Print -- exposing to the right question
Post by: Gregory on August 08, 2007, 09:39:44 am
"Lets say you find that exposing plus one stop over the meter does indeed give you plenty of highlight detail to pull back in a Raw converter."

I'm not looking for more detail in the highlights. I want more detail in the lowlights, hence ETTR.


"I've actually found 1 ½ stops on my 5D."

how did you find that extra 1 ½ stops? I'd be interested to know. in my images, any highlights above 255 are blown out and irretrievable.


"OK so you're starting out at ISO 400 at F8 based on the meter but using ETTR which tells you you'd get better data at F5.6. Well isn't that the same as shooting at ISO 200?"

I don't think it is the same. in terms of overall exposure, it's the same. in terms of levels per stop, it's different, at least from my understanding.

let's see if I can try to explain what I'm referring to. assuming 6 stops range in my image:

       0                                      255
stop   |  6  |  5  |  4   |   3  |   2  |   1  |
levels | 128 | 256 | 512  | 1024 | 2046 | 4096 | ISO 400 ev 0.0
levels | 128 | 256 | 512  | 1024 | 2046 | 4096 | ISO 800 ev +1.0
levels | 256 | 512 | 1024 | 2046 | 4096 |   ?  | ISO 800 ev +1.0 adjusted back to ev 0.0 in the RAW processor
so by using ETTR, I get twice as many levels in the lowest 3 stops of data and therefore more detail in the dark areas of my images which for me is relatively important.

I could be completely wrong. one thing I'm very unsure of is whether the extra levels are preserved in the RAW Processor's output. is the converted file made up of logarithmic data which will always only preserve 128 levels in stop 6 or is the converted file made up of linear data or... ?

if I'm missing the mark, please enlighten me. I need to understand this.

sincerely,
Gregory
Title: Camera-to-Print -- exposing to the right question
Post by: digitaldog on August 08, 2007, 09:53:16 am
Quote
"Lets say you find that exposing plus one stop over the meter does indeed give you plenty of highlight detail to pull back in a Raw converter."
Quote
I'm not looking for more detail in the highlights. I want more detail in the lowlights, hence ETTR.

In order to use ETTR, you have to find the point of full sensor saturation and back off a tad. So you need to use the adjustments in the Raw processor to see if you can get just shy of 255 in all three rendered channels. I'm not referring to highlight detail, I'm referring to shooting so that specular highlights might be 255 but anything you wish to reproduce is just below that. ETTR isn't about over exposure, its about proper exposure and that means deciding where you want to clip and not clip. The results are more data in the shadows.


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how did you find that extra 1 ½ stops? I'd be interested to know. in my images, any highlights above 255 are blown out and irretrievable.

Use a very white, spectrally neutral target, setup lighting and exposure tests (bracket). Bring images into Raw converter and find image where you can neutralize exposure to be at the ideal ETTR values (not clipping highlight data you want).

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I don't think it is the same. in terms of overall exposure, it's the same. in terms of levels per stop, it's different, at least from my understanding.

OK so my meter tells me that a normal exposure is F8 at 125th. I know I can shoot at F5.6 at 125th and not blow out highlights and the net result is more data in the shadows due to the ETTR. IF I set my external meter to a stop less ISO, it would have indicated I should shoot at F 5.6 at 125th right?
Title: Camera-to-Print -- exposing to the right question
Post by: Gregory on August 08, 2007, 10:06:42 am
thank you digitaldog for trying to explain this to me.


I'm referring to shooting so that specular highlights might be 255 but anything you wish to reproduce is just below that. ETTR isn't about over exposure, its about proper exposure and that means deciding where you want to clip and not clip.

yep. that's how I understood it and that's how I try to shoot when possible.


OK so my meter tells me that a normal exposure is F8 at 125th. I know I can shoot at F5.6 at 125th and not blow out highlights and the net result is more data in the shadows due to the ETTR. IF I set my external meter to a stop less ISO, it would have indicated I should shoot at F 5.6 at 125th right?

right, but the histogram would be in the middle instead of to the right and you'd have less detail in the shadows. right?
Title: Camera-to-Print -- exposing to the right question
Post by: digitaldog on August 08, 2007, 10:13:41 am
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right, but the histogram would be in the middle instead of to the right and you'd have less detail in the shadows. right?


The on camera histogram doesn't show you the Raw data but instead what the JPEG would look like if you shot this way and let the camera process the data as a JPEG. So, not real useful!

The idea is to put as many real levels of data in the last stop, that means moving every level over to the right without clipping highlights you wish to reproduce.
Title: Camera-to-Print -- exposing to the right question
Post by: Gregory on August 08, 2007, 10:27:12 am
The idea is to put as many real levels of data in the last stop, that means moving every level over to the right without clipping highlights you wish to reproduce.

that's my point. with an ISO setting one stop lower and using F5.6, this wouldn't happen.

ISO 800 ev +1.0 --> F5.6 1/125
--> levels moved to the right to get more levels in the shadows.

ISO 400 ev +0.0 --> F5.6 1/125
--> same exposure but the sensor is 'less sensitive' so the histogram is in its normal 'middle' position rather than to the right and there are less details in the shadows.

I feel like I'm missing something, possible something really obvious to everyone but me ;-)
Title: Camera-to-Print -- exposing to the right question
Post by: digitaldog on August 08, 2007, 12:11:51 pm
In both scenario’s the ISO of the camera is the same. You set it for ISO 100, you get a fixed amount of noise at normal exposure (normal being what the meter tells you, not based on ETTR). You keep the unit at ISO 100 but open one stop (the effective equivalent of shooting at at ISO 50). You don't blow out highlights and move more levels from last stop of shadow to first stop highlight, the result being less noise in that last stop.

I think the point made on the DVD is if you need higher ISO, its because you don't have enough light and using ETTR reduces the effectiveness here so get the shot and live with a bit more noise (more due to the exposure and the ISO).
Title: Camera-to-Print -- exposing to the right question
Post by: Gregory on August 08, 2007, 12:51:34 pm
and then it's a matter of balancing detail and levels in the shadows with ETTR against the extra noise that might come with the higher ISO.

thank you again digitaldog. you've been patient and helpful.

regards,
Gregory
Title: Camera-to-Print -- exposing to the right question
Post by: picnic on August 08, 2007, 12:57:48 pm
Quote
and then it's a matter of balancing detail and levels in the shadows with ETTR against the extra noise that might come with the higher ISO.

thank you again digitaldog. you've been patient and helpful.

regards,
Gregory
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I'll say thank you too because I was a bit confused by what was said.   This clarifies it for me.  

Diane
Title: Camera-to-Print -- exposing to the right question
Post by: KenL on January 03, 2008, 10:58:22 pm
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In both scenario’s the ISO of the camera is the same. You set it for ISO 100, you get a fixed amount of noise at normal exposure (normal being what the meter tells you, not based on ETTR). You keep the unit at ISO 100 but open one stop (the effective equivalent of shooting at at ISO 50). You don't blow out highlights and move more levels from last stop of shadow to first stop highlight, the result being less noise in that last stop.

I think the point made on the DVD is if you need higher ISO, its because you don't have enough light and using ETTR reduces the effectiveness here so get the shot and live with a bit more noise (more due to the exposure and the ISO).
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Thanks for this discussion.

I just tried a little experiment:

I set the camera on a tripod in front of an evenly lite wall and took shots at ISO 100  @+1ev and ISO 400 @+1ev. Looking at the histogram for each they are very similar.

From the discussion above I was expecting ISO 100 @+1ev to have a similar histogram to ISO 400 @+2ev or +3ev.

Am I missing something?

Ken
Title: Camera-to-Print -- exposing to the right question
Post by: kaelaria on January 03, 2008, 11:29:42 pm
1 stop is 2x or 1/2 the light - same as one ISO step.
Title: Camera-to-Print -- exposing to the right question
Post by: larsrc on January 04, 2008, 09:02:09 am
Quote
I was a little confused by one statement in the tutorial regarding exposing to the right (which I'm familiar with).

Quote
...you'll get a better result by increasing the exposure a little bit to the right, which is the same thing as lowering the ISO...
you shouldn't follow the 'expose to the right' religion if you're shooting at high ISO. there's no point in shooting at ISO 400 and exposing to the right. you may as well drop to ISO 200.
I don't understand this. the purpose of exposing to the right is to get more levels in the middle of the histogram because the first stop provides 4096 levels, the second stop 2048, etc. if you shoot at ISO 800 and expose to the right, you're getting more levels for the middle and low regions. how does this equate to shooting at ISO 400 without exposing to the right in terms of levels and detail? i.e., ISO 800 ev +1.0 == ISO 400 ev 0.0

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My understanding of that bit was that if you ETTR into high enough ISO that the camera itself pushes the exposure, you won't get any more detail than if you kept within the range where the ISO is "real".  Having the camera multiply the readings after the A/D conversion doesn't give you any more shadow detail. However, I don't know what modern camera it would be where ISO 400 is pushed, so it puzzled me a bit, too.

-Lars
Title: Camera-to-Print -- exposing to the right question
Post by: bjanes on January 04, 2008, 11:12:40 am
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In order to use ETTR, you have to find the point of full sensor saturation and back off a tad. So you need to use the adjustments in the Raw processor to see if you can get just shy of 255 in all three rendered channels. I'm not referring to highlight detail, I'm referring to shooting so that specular highlights might be 255 but anything you wish to reproduce is just below that. ETTR isn't about over exposure, its about proper exposure and that means deciding where you want to clip and not clip. The results are more data in the shadows.
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If ETTR requires exposing to just short of sensor saturation, then ETTR applies only at base ISO. When you increase ISO 1 EV over base, then the sensor would only be half saturated and clipping would occur in the ADC rather from saturation. The same considerations apply to even higher ISOs.

If you are shooting at ISO 400 rather than ISO 200, then presumably you need a higher shutter speed, smaller aperture, or both. If ETTR requires a positive exposure adjustment of +1 EV then you are back to where you started with ISO 200 exposure settings. However, if you decide to use ISO 200, then the histogram would indicate that an exposure adjustment of +1 EV is still needed, so the same argument would indicate that you should drop back to ISO 100.

One does not need to invoke religion into ETTR theory. The physical principles are well understood. Under normal photographic conditions, noise in a digital image is composed almost entirely of shot noise (photon noise, related to exposure, and read noise). In the case of the ISO 200 exposure without ETTR and ISO 400 exposure with +1 EV, the exposure is the same and the shot noise will be the same. Read noise varies with the camera and ISO. According to Roger Clark's data for the Canon 1D MII, read noise at ISO 200 is 8.95 electrons and at ISO 400, it is 5.56 electrons. These differences could result in small differences in the deep shadows. However, the ISO 200 exposure without ETTR compensation will have more headroom in the highlights. Overall, an objective analysis confirms Jeff's statement.
Title: Camera-to-Print -- exposing to the right question
Post by: Gregory on January 04, 2008, 12:09:06 pm
Quote
If you are shooting at ISO 400 rather than ISO 200, then presumably you need a higher shutter speed, smaller aperture, or both. If ETTR requires a positive exposure adjustment of +1 EV then you are back to where you started with ISO 200 exposure settings. However, if you decide to use ISO 200, then the histogram would indicate that an exposure adjustment of +1 EV is still needed, so the same argument would indicate that you should drop back to ISO 100.
confused!!! let me see if I can understand this.

"exposure settings" means aperture and shutter speed without regard for the ISO speed? so:

with ISO 400, the camera calculates that correct exposure would require f8 @ 1/125.
you add 1 EV by increasing the aperture to f5.6.

with ISO 200, the camera calculates the correct exposure would require f5.6 @ 1/125; i.e., the same aperture and shutter speed as that required at ISO 400 +1 EV.

SO... to understand this, I need to know how ISO speed affects the light received by the sensor and how it affects the number of levels per stop. to my uneducated mind, your explanation seems to infer that the sensors pick up the same amount of light and levels per stop regardless of the ISO speed, and that the ISO speed affects the ADC's 'multiplier' effect rather than the sensors; i.e., how it multiplies the original analog signal to produce the final digital data.

further, if it is correct that the sensors pick up the same amount of light regardless of the ISO speed, then it follows that (as bjanes has stated) ETTR can only be used with benefit at the base (true) ISO speed, because only then can more light be directed to the lower stops of the image. higher ISO speeds increase the apparent exposure by multiplying the original signal, and exposure is calculated incorporating that multiplication factor. consequently, using a higher ISO speed may in fact reduce the amount of light and detail in the lower stops of the image rather than increase it. i.e., if you need to shoot with a higher ISO speed because of a minimum aperture or shutter speed requirement, then forget ETTR, "decide[ing] where you want to clip and not clip" and shoot accordingly.

whew!

(corrections definitely welcome)
Title: Camera-to-Print -- exposing to the right question
Post by: bjanes on January 04, 2008, 01:41:28 pm
Quote
confused!!! let me see if I can understand this.

"exposure settings" means aperture and shutter speed without regard for the ISO speed? so:

with ISO 400, the camera calculates that correct exposure would require f8 @ 1/125.
you add 1 EV by increasing the aperture to f5.6.

with ISO 200, the camera calculates the correct exposure would require f5.6 @ 1/125; i.e., the same aperture and shutter speed as that required at ISO 400 +1 EV.

SO... to understand this, I need to know how ISO speed affects the light received by the sensor and how it affects the number of levels per stop. to my uneducated mind, your explanation seems to infer that the sensors pick up the same amount of light and levels per stop regardless of the ISO speed, and that the ISO speed affects the ADC's 'multiplier' effect rather than the sensors; i.e., how it multiplies the original analog signal to produce the final digital data.
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I think your assumptions are true here. If the shutter speed and f/stop are the same (and other factors are the same), then the sensor will receive the same amount of light regardless of the ISO setting. Increasing the ISO does not change the amount of light received by the sensor, but merely increased the analog amplification so that the ADC is operating at full scale.

In general, you lose DR when you increase ISO. This can be seen from the  Excel plots derived from Roger Clark's data for the 1D MII. The base ISO for that camera is not 100, but is used here because of peculiarities of that camera. The DR is determined by the noise floor. If you require a Signal to Noise (SN) of 5 in the deepest shadows, then the DR at ISO 100 would be slightly better than 9 f/stops and that at ISO 400 would be slightly less than 8 stops.

The number of levels is not taken into account. DR is usually limited by noise and not posterization in the shadows.

ShotN = shot noise, ReadN = read noise, TN = total noise, S/N = signal / noise. The amount of light received by the sensor is directly proportional to the electron count.

(http://bjanes.smugmug.com/photos/239600421-O.png)

(http://bjanes.smugmug.com/photos/239600439-L.png)
Title: Camera-to-Print -- exposing to the right question
Post by: Schewe on January 04, 2008, 02:54:25 pm
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I need to know how ISO speed affects the light received by the sensor and how it affects the number of levels per stop.
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Just to be clear...changing the ISO setting will _NOT_ alter the amount of light falling on the sensor...if you think about, that would be SciFi because the camera can't alter the reality of a scene. So, setting F stop & shutter speed is the ONLY way the camera has to alter or control the amount of light hitting the sensor. If you change the ISO and keep the F stop & shutter the same, the same amount of light will be falling...only the ADC will alter the resulting capture.

The other thing you gotta understand is that ETTR is a technique to maximize the quality potential of a capture. ETTR isn't really something that is useful for an ISO other than the camera's optimum ISO. So, there would be no reason to increase the ISO and then also ETTR. You would be better off (I'm pretty sure) simply using a lower ISO.

Also, you must comprehend with ETTR is that it's really only applicable if the scene dynamic range fits within the dynamic range of the sensor. If you are shooting a really contrasty scene, ETTR won't really help the issue of noise in the shadow because you run the risk of blowing highlights.

For a lot of shooting, "correct exposure" for the scene is better than ETTR–it doesn't ALWAYS work. But if you are in a low contrast situation either outside or in a studio (easy to control contrast range by lighting and fill light), then ETTR can produce technically better images as it relates to noise. Note however that with the increased exposure you lose the potential for depth of field and faster shutter speed. If the shot demands either or both, ETTR will NOT result in a technically better "overall" shot.
Title: Camera-to-Print -- exposing to the right question
Post by: digitaldog on January 04, 2008, 03:06:42 pm
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The other thing you gotta understand is that ETTR is a technique to maximize the quality potential of a capture. ETTR isn't really something that is useful for an ISO other than the camera's optimum ISO. So, there would be no reason to increase the ISO and then also ETTR. You would be better off (I'm pretty sure) simply using a lower ISO.

That's what my tests showed. IF you need higher ISO (to capture the scene), you need a higher ISO.

But before we can set minimal or optimal ISO, we need to know the exposure sensitivity of the chip.

Doing that, then exposing for ETTR may not work however, the point about ETTR is about getting the best data possible using the ISO/Exposure/Aperture (its not about over exposing). We are lucky to be able to use various ISO's and exposure's within reason and get a Raw that we can massage into a good looking image. ETTR would simply place the most data within the image, doing so requires the proper setting of ISO. So I don't see ETTR and differing ISO as mutual exclusive, what I do see is this: If you need a higher ISO to capture the image, and you're basing this exposure on highlights (expose for highlights), you're setting said ISO based on the initial premise of ETTR. The proper exposure based on ISO requires you know the actual sensitivity of your chip based upon exposing for highlights such you produce as much useable data in the shadows as possible.
Title: Camera-to-Print -- exposing to the right question
Post by: bjanes on January 04, 2008, 03:41:13 pm
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The other thing you gotta understand is that ETTR is a technique to maximize the quality potential of a capture. ETTR isn't really something that is useful for an ISO other than the camera's optimum ISO. So, there would be no reason to increase the ISO and then also ETTR. You would be better off (I'm pretty sure) simply using a lower ISO.

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And what is the optimum ISO of the camera--base ISO? In most cases base ISO will give the best results if one can deal with shutter speed/aperture constraints. However, astronomers will get better results at higher ISOs where read noise is lower.
Title: Camera-to-Print -- exposing to the right question
Post by: bjanes on January 04, 2008, 05:22:34 pm
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That's what my tests showed. IF you need higher ISO (to capture the scene), you need a higher ISO.

The proper exposure based on ISO requires you know the actual sensitivity of your chip based upon exposing for highlights such you produce as much useable data in the shadows as possible.
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That is where ISO 12232:2006, the ISO standard for digital camera ISO, comes in. According to this specification, a standard light meter reading from an 18% target should give 12.7% sensor saturation.  With a 12 bit ADC the corresponding DN would be would be 522, corresponding to 101 in an 8 bit gamma 2.2 space. This standard leaves a headroom of 0.5 EV for specular highlights. If you want to take a highlight reading and place the highlights at 95% saturation (249 in a gamma 2.2 space), you would increase the exposure by 2.9 stops. Since in camera JPEGs and raw converters apply a saturation and contrast boost prior to encoding into a 2.2 gamma space, one should set the contrast and saturation to low to get a better idea of the data in the raw file.

Of course, some actual testing of your light meter and camera is indicated, but the above calculations could be a starting point. I would take a meter reading from a uniform reflecting surface (it can be gray or white), increase the indicated exposure by 3 stops, bracket up and down by 0.3 EV and use the compensation factor that gives a reading of 249 in a gamma 2.2 space.

You can leave the data on the CF card and then compare the histogram and blinking highlight data to know where you stand when using these tools on your camera. Taking of highlight readings required a spot reading, preferably at 1 degree.

I would venture to say that >90% of the members of this forum who use ETTR do so with ISOs above base and with the in camera light meter.
Title: Camera-to-Print -- exposing to the right question
Post by: Panopeeper on January 04, 2008, 11:52:23 pm
Two notes to the above.

1. Higher ISO does not mean automatically more noise.  saw a demonstration on DPReview (one of the authors was John Sheehy) proving, that not the higher ISO but low light causes noise.

The consequence of this is, that in such situations, when the lighting is enough but the exposure can not be increased to achieve ETTR because of other considerations, then increasing the ISO helps gaining more detail in the shadows.

Higher ISO is certainly no substitute for higher exposure - when the latter is possible.

2. The optimal ISO is not always the base. Example: the Canon D40. The DR with ISO 200 is about 0.2 EV larger than that of ISO 100. This is due to the fact, that the range of pixel values is larger with ISO 200 than with ISO 100. 0.2 EV is not much wort (though it could be important), but perhaps other cameras will follow suit and offer a much higher value range with higher ISOs. 14 bits should be plenty enough for that.
Title: Camera-to-Print -- exposing to the right question
Post by: bjanes on January 05, 2008, 12:00:47 am
Quote
Two notes to the above.

1. Higher ISO does not mean automatically more noise.  saw a demonstration on DPReview (one of the authors was John Sheehy) proving, that not the higher ISO but low light causes noise.

The consequence of this is, that in such situations, when the lighting is enough but the exposure can not be increased to achieve ETTR because of other considerations, then increasing the ISO helps gaining more detail in the shadows.
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I don't know what the above is in reply to, but my analysis clearly showed that noise is related almost entirely to exposure, not ISO. However, you can not expose at full well much above base ISO because you will get clipping in the ADC. Some cameras place the highlights some distance below ADC clipping, and you might be able to increase ISO somewhat and still expose at the full well of the sensor if you have one of those. In addition, there is often less read noise at high ISO and that could improve the S:N in the deep shadows. However, highlight S:N would be worse owing to the effect of shot noise.
Title: Camera-to-Print -- exposing to the right question
Post by: larsrc on January 05, 2008, 05:13:13 am
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2. The optimal ISO is not always the base. Example: the Canon D40. The DR with ISO 200 is about 0.2 EV larger than that of ISO 100. This is due to the fact, that the range of pixel values is larger with ISO 200 than with ISO 100. 0.2 EV is not much wort (though it could be important), but perhaps other cameras will follow suit and offer a much higher value range with higher ISOs. 14 bits should be plenty enough for that.
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I get the impression that optimal ISO does not vary from one instance of a particular model to another.  Is there a database of optimal ISOs for various (SLR) cameras somewhere? Could we make one?  I could certainly do the database setup.

-Lars
Title: Camera-to-Print -- exposing to the right question
Post by: John Sheehy on January 14, 2008, 06:21:38 pm
Quote
Quote
...you'll get a better result by increasing the exposure a little bit to the right, which is the same thing as lowering the ISO...
you shouldn't follow the 'expose to the right' religion if you're shooting at high ISO. there's no point in shooting at ISO 400 and exposing to the right. you may as well drop to ISO 200.
I don't understand this. the purpose of exposing to the right is to get more levels in the middle of the histogram because the first stop provides 4096 levels, the second stop 2048, etc. if you shoot at ISO 800 and expose to the right, you're getting more levels for the middle and low regions. how does this equate to shooting at ISO 400 without exposing to the right in terms of levels and detail? i.e., ISO 800 ev +1.0 == ISO 400 ev 0.0

The number of levels in a stop is not necessarily the real issue.  It works as a model because it parallels the real issue; signal-to-noise ratios.  Also, in theory, if there were no noise or very little noise, then more levels would avoid RAW quantization, but that doesn't seem to be an issue with the RAW data in current cameras; all have far more than enough levels for the highlights and midtones, and 12 bits is just good enough for the best ISO 100s out there, in the deepest shadows (the first place RAW quantization would show).  When you start comparing different relative (but similar absolute) exposures at different ISOs, things get a little bit more complex.  If you expose manually, and set the Av and Tv values, and vary the ISO, the optimal ISO may be a low one, or the highest one that doesn't clip desired highlights, depending on the camera.  Some cameras have horrible amplifiers in them for high ISO, and ruin the high ISOs.  They could give better results by shooting at a lower ISO and under-exposing.  This is certainly not true of your Canon DSLR, though.  Canon DSLRs, except for the original 1D, add less noise to the signal (relative to it) at the highest ISOs.

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please explain. details and levels in the lower region of the histogram are important to me because many of the birds I photograph have dark regions which don't contain as much detail as I would prefer.

and... is this level structure carried into the RAW processor and beyond? i.e., after processing, is the lowest stop still restricted to only 128 levels?

"The lowest stop" most likely refers to the lowest stop under consideration or considered to be useful, and everything below it, as a unit.  There really is no "lowest stop", per se, except for a few cameras that clip slightly above the true black level in their RAW data.  The only issue as you go down through all these stops is that the noise gets stronger and stronger, relative to the weak signal There is no great difference in kind between a signal being slightly below or slightly above the noise floor; the SNR is just different by degree.  There is signal below the noise floor.

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and... the Canon Mark III shoots in 14-bit (from Canon's page: "The extra power of dual DIGIC III processors has also allowed analog-to-digital conversion to improve from 12 to 14 bits per channel, meaning better tonal gradation for RAW images."). would this increase the levels of the lower stop in the histogram by 2^2 from 2^7 to 2^9 == 512 levels?
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The amount of IQ benefit that comes from 14-bit data is maximum at the lowest ISOs, and is nearly infinitesimal.  There is too much analog noise before and during digitization for the extra bits to carry any significant extra signal.  However, being in a format with a higher precision, the extra bits may force converters to use more precision in the conversion process; something that could be done, also with 12-bit data.
Title: Camera-to-Print -- exposing to the right question
Post by: Mark D Segal on January 15, 2008, 11:06:56 pm
Quote
The amount of IQ benefit that comes from 14-bit data is maximum at the lowest ISOs, and is nearly infinitesimal.  There is too much analog noise before and during digitization for the extra bits to carry any significant extra signal.  However, being in a format with a higher precision, the extra bits may force converters to use more precision in the conversion process; something that could be done, also with 12-bit data.
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You may find this research of interest: [a href=\"http://tinyurl.com/2e4nss]12 vs 14 bit[/url]

Now, let us set aside the question of IQ differences between 12 and 14 bit (though Canon claims more levels = somoother tonal gradations - in principle should be true, but the in practice the issue is whether or not it is or one day will be noticeable).

I would like to gain clarity on clarity    if I may. Let us accept that ETTR (without clipping desired highlight detail) is "a good thing" because the extra exposure improves S/N visibly especially in the three-quarter tones and below and should provide smoother tonal gradations because it puts those tones further to the right in the histogram where the number of levels is higher (i.e. more exposure). Then we rebalance the image to taste in the raw converter - Lightroom, ACR or whatever.

The question on which I would appreciate more clarity is how best to do ETTR when the camera's automatic metering is not doing so. As I see it, I have two choices (switching to manual exposure); (i) increase the ISO leaving aperture and shutter speed unchanged, or (ii) increase exposure by leaving the ISO unchanged and either opening the aperture or slowing the shutter speed. Let us say compensation worth 1 to 1.5 stops is generally needed to achieve this (though it could vary by scene). Which of these approaches (i.e. (i) or (ii) ) will give me an image with less visible noise and why?
Title: Camera-to-Print -- exposing to the right question
Post by: Ray on January 16, 2008, 08:56:23 am
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The question on which I would appreciate more clarity is how best to do ETTR when the camera's automatic metering is not doing so. As I see it, I have two choices (switching to manual exposure); (i) increase the ISO leaving aperture and shutter speed unchanged, or (ii) increase exposure by leaving the ISO unchanged and either opening the aperture or slowing the shutter speed. Let us say compensation worth 1 to 1.5 stops is generally needed to achieve this (though it could vary by scene). Which of these approaches (i.e. (i) or (ii) ) will give me an image with less visible noise and why?
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Mark,
The way I see it is, there's a primary requirement for a specific aperture for DoF purposes and a primary requirement for a shutter speed sufficient to freeze subject movement and/or camera shake.

In order to obtain those two requirements, an appropriate ISO should be selected.

Whatever ISO is selected, it is usually advantageous to expose to the right, especially with Canon DSLRs, but not apparently with some models of cameras.

I recall Edmund writing in another thread that he thought he was getting better results underexposing a P45+ DB by 3 stops at ISO 100, rather than using the same exposure at ISO 800 which would result in an ETTR in relation to that ISO setting.

My own experiments with the 20D and 5D have demonstrated very clearly that an ETTR at a higher ISO is always better than the same exposure at a lower ISO setting. However, if you can be flexible about the choice of either aperture or shutter speed, the lower ISO will generally produce better results, provided it's an ETTR.

In order to achieve accurate ETTR it may be necessary to bracket exposure or to adopt a more time-consuming method in manual mode. One such method, if your camera has a spot meter mode, is to take a reading of the brightest part of the scene, then increase exposure by 3 stops.

Another method is to take the shot, examine the histogram and if the assessment is that it is not far enough to the right, take the shot again at a higher ISO such that the histogram is pushed more to the right.

In order to get the appearance of the histogram and the highlight warning on the preview image as accurate as possible, it's probably advisable to reduce contrast, saturation and sharpening to a minimum in the camera's menu. However, this is a matter for experimentation. My own cameras are set up so that a very small amount of highlight flashing is just right for a full ETTR.
Title: Camera-to-Print -- exposing to the right question
Post by: John Sheehy on January 16, 2008, 08:58:32 am
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You may find this research of interest: 12 vs 14 bit (http://tinyurl.com/2e4nss)

The 12-bit and 14-bit readouts of the D300 might as well be on different cameras.

6fps vs 2.5 tells you that there is a lot more going on than bit depth here.  12-bit is most likely a quick'n'dirty readout, quite a different than than if the 2.5 fps readout only recorded 12 bits in the RAW.

Quote
The question on which I would appreciate more clarity is how best to do ETTR when the camera's automatic metering is not doing so. As I see it, I have two choices (switching to manual exposure); (i) increase the ISO leaving aperture and shutter speed unchanged, or (ii) increase exposure by leaving the ISO unchanged and either opening the aperture or slowing the shutter speed. Let us say compensation worth 1 to 1.5 stops is generally needed to achieve this (though it could vary by scene). Which of these approaches (i.e. (i) or (ii) ) will give me an image with less visible noise and why?
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My paradigm (even if not directly supported by the camera) is to choose real world exposure based on a compromise of photon collection, shutter speed, and aperture, and choose the highest ISO that won't clip desired highlights, because the highest ISO will give the least "added" read noise to the absolute exposure (on most cameras, to a small or great degree).  The real tradeoff isn't between ISO and anything; the real tradeoff is absolute exposure.  IQ only parallels ISO when you are exposing the same relative way in each ISO.
Title: Camera-to-Print -- exposing to the right question
Post by: Mark D Segal on January 16, 2008, 10:39:17 am
Interesting, both you and Ray are saying that higher ISOs are less noisy - specifically in John's post - less read noise. But do they not also produce more "shot" noise? Is it not normally the case that we see more noise in images shot at high ISO?

Ray - some qualifications on aspects of your suggestions, I think. Firstly, if the camera is in an auto exposure mode, changing ISO inversely changes the exposure so one shouldn't get closer or further to/from the desired ETTR setting unless one bumps into a constraint. If the approach is to change ISO, I do believe one must move into manual mode to preserve the exposure settings and move the histogram rightward, or again in manual mode over-ride the auto setting at a given ISO and expose more.  (John, I believe, perhaps incorrectly?, that there are trade-offs between these options.)

Secondly, when shooting raw of course, contrast, saturation and sharpening is not relevant - it should all be turned off/neutral so one imports an image as raw as it gets from the camera to the raw converter. Working from camera-baked jpegs I can see may be another story.
Title: Camera-to-Print -- exposing to the right question
Post by: bernie west on January 20, 2008, 08:02:11 pm
Just when I thought I had ettr worked out, I came across this thread, and it seems there is more to it.  It seems as if the original concept (which I have been following) as proposed by Reichmann has progressed some.  Can the argument be summarized like this:

Shooting above base iso and ettr may not give you are better SNR than shooting the same exposure at base iso (even though the histogram may only reach half way along the scale)?

If that's the case then clearly we are better shooting at base iso, than employing analogue gain via iso boost.  But obviously at some point (say iso 800 vs iso 100) the iso 100 shot is going to be underexposed and will require boosting via the raw converter.  At this point then it must become a case of diminishing returns, as boosting continuous analogue signal should always be better than boosting discrete digital data.  Am I right in this observation?

I did a test at iso 400 vs 100.  Iso 400 was ettr, and pulled back -1ev in DPP, and iso 100 was pushed 1ev.  Both were shot with the same absolute exposure (ie. same Av and Tv).  The following images show a 100% crop from both (iso 400 on the left).  You can clearly see more noise in the iso 100 +1ev vs the iso 400 -1ev.
Title: Camera-to-Print -- exposing to the right question
Post by: Ray on January 20, 2008, 09:00:59 pm
Quote
Ray - some qualifications on aspects of your suggestions, I think. Firstly, if the camera is in an auto exposure mode, changing ISO inversely changes the exposure so one shouldn't get closer or further to/from the desired ETTR setting unless one bumps into a constraint. If the approach is to change ISO, I do believe one must move into manual mode to preserve the exposure settings and move the histogram rightward, or again in manual mode over-ride the auto setting at a given ISO and expose more.

Mark,
Yes, of course you're right. I forgot to mention that. It's necessary to go into manual mode and here's a situation which can be a bit of a hassle. Getting into manual mode is easy with a turn of a dial. Changing ISO is easy with a press of a button and a turn of a dial. But one also has to change the f stop and shutter speed in manual mode. One could easily miss the shot with all that stuffing around.

Michael has made a big deal of the lack of a dedicated MLU button on Canon cameras. How about a button that gets you into manual mode whilst preserving the aperture and shutter speed that was last used in the previous mode. Perhaps there's a way of programming some cameras to do this, perhaps even my own camera. I sometimes miss the obvious.

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Secondly, when shooting raw of course, contrast, saturation and sharpening is not relevant - it should all be turned off/neutral so one imports an image as raw as it gets from the camera to the raw converter. Working from camera-baked jpegs I can see may be another story.

It's relevant to the appearance of the LCD preview and the histogram since that is based on an in-camera jpeg conversion (so I'm told). My experience is, the amount of highlight warning flashing that takes place is significantly affected primarily by the contrast setting. All you have to do is try shooting an ETTR of the same scene with contrast at a minimum and contrast at a maximum. When contrast is at a maximum, you could have an entire sky flashing in the preview. But the same exposure, with in-camera contrast set at a minimum, will produce a preview with just a small area of the brightest part of the sky flashing.
Title: Camera-to-Print -- exposing to the right question
Post by: Mark D Segal on January 20, 2008, 10:13:40 pm
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My experience is, the amount of highlight warning flashing that takes place is significantly affected primarily by the contrast setting. All you have to do is try shooting an ETTR of the same scene with contrast at a minimum and contrast at a maximum. When contrast is at a maximum, you could have an entire sky flashing in the preview. But the same exposure, with in-camera contrast set at a minimum, will produce a preview with just a small area of the brightest part of the sky flashing.
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Ray, this is interesting - do you find in general the minimum contrast setting produces an impression of clipping that is closer to what you would eventually observe in the raw converter?
Title: Camera-to-Print -- exposing to the right question
Post by: John Sheehy on January 20, 2008, 10:34:54 pm
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Interesting, both you and Ray are saying that higher ISOs are less noisy - specifically in John's post - less read noise. But do they not also produce more "shot" noise? Is it not normally the case that we see more noise in images shot at high ISO?

What I meant is that they are less noisy in an absolute sense.

IOW, for a given real world exposure, higher ISOs add less noise to the RAW data.  If you are using a fixed set of Tv and Av values, and just varying the ISO, shot noise does not vary.  Shot noise has nothing directly to do with the ISO setting; it is only related in the sense that a high ISO clips away the sensor ranges of exposure where the lowest shot noise occurs.

In simple theory, shot noise should be the only noise of significance in the midtones and highlights, but read noise is usually kind of nasty, in the sense that it isn't totally random, and my have some patterns which make it much more visible than its statistical strength might suggest.  Case in point, the other guy's test this evening.  He is looking at midtones, but with the same absolute exposure, with the same shot noise, he got significantly more visible noise in the low ISO shot.

Quote
Ray - some qualifications on aspects of your suggestions, I think. Firstly, if the camera is in an auto exposure mode, changing ISO inversely changes the exposure so one shouldn't get closer or further to/from the desired ETTR setting unless one bumps into a constraint. If the approach is to change ISO, I do believe one must move into manual mode to preserve the exposure settings and move the histogram rightward, or again in manual mode over-ride the auto setting at a given ISO and expose more.

Well, you can do that just as quickly sometimes with the EC control.

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(John, I believe, perhaps incorrectly?, that there are trade-offs between these options.)

I'm not sure I'm parsing your grammar there, but the general idea is that whatever amount of light you decide to capture, the best ISO is the one that comes just short of clipping desired highlights.  This is very valuable with Canon DSLRs, the more recent Nikons, and possibly others I'm not aware of, and it applies to a smaller degree to older Nikons and most other non-Canon DSLRs and P&S cameras.  There are indeed some cameras out there that do worse by using high ISOs than by under-exposing at low ISOs.  My Panasonic FZ50 is like that.  For any ISO greater than 200, it is better to shoot RAW and under-expose ISO 200.  It seems to use a really nasty amplifier to achieve high ISOs.

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Secondly, when shooting raw of course, contrast, saturation and sharpening is not relevant - it should all be turned off/neutral so one imports an image as raw as it gets from the camera to the raw converter. Working from camera-baked jpegs I can see may be another story.
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Well, you can apply the same principles to JPEG, but JPEG will just have more limitations on how far you can take things.  I often use the 2.5 MB embedded JPEGs to make small images, even though I shot in RAW, and exposed to the right, but I have the JPEG contrast set to minimum, and the saturation at -1, and I can pull down ETTR JPEGs by lowering the gamma to 0.8 in post-processing.
Title: Camera-to-Print -- exposing to the right question
Post by: Panopeeper on January 20, 2008, 10:51:22 pm
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I often use the 2.5 MB embedded JPEGs to make small images

You have a 30D, don't you? The embedded image is 1728x1152, which is ok for that purpose, but all sizes I have seen were in the range 260KB-500KB, i.e. the quality can't be that great.
Title: Camera-to-Print -- exposing to the right question
Post by: Ray on January 21, 2008, 12:56:49 am
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Ray, this is interesting - do you find in general the minimum contrast setting produces an impression of clipping that is closer to what you would eventually observe in the raw converter?
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Mark,
Yes. I find the Landscape Picture Style on my 5D set to minimum contrast results in a more reliable preview and histogram for assessing ETTR. Small areas of highlight flashing, sometimes barely discernible, are usually about right for ETTR. That's my guide. No flashing at all usually means underexposure. Significant flashing means overexposure.
Title: Camera-to-Print -- exposing to the right question
Post by: francois on January 21, 2008, 04:44:25 am
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Ray, this is interesting - do you find in general the minimum contrast setting produces an impression of clipping that is closer to what you would eventually observe in the raw converter?
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Andrew Rodney, in his [a href=\"http://www.digitalphotopro.com/index2.php?option=com_content&task=view&id=316&pop=1&page=0&Itemid=41]Exposing for RAW[/url] article, has someting interesting to say about contrast & settings:

…
Unfortunately, at least with my Canon EOS 5D, the “best” exposure often produced
 the worst-appearing LCD previews.
…
When I set my 5D Picture Style from the default to -4 contrast, the clipping of highlights
 on the LCD didn’t appear until I had “overexposed” 2⁄3 of a stop. This is better than
the original default settings, yet in actuality, I was able to overexpose 1! stops beyond
what my meter suggested while fully retaining  highlight data in my RAW file. The clipping
indicators are still far from correctly describing what’s  happening to the RAW data.
…
Title: Camera-to-Print -- exposing to the right question
Post by: Mark D Segal on January 21, 2008, 08:58:18 am
Ray and Francois,

Thanks - this means one thing and one thing only: experiment with a range of settings, make notes at the time the image is processing, then put them up in Camera Raw and see what works best. It could be that the settings which work well enough for most situations with one camera model are not satisfactory for another.