How do you really know when you are ETTR?

[bwana]

As I understand it, most histograms displayed on the back of a camera LCD are based on the jpg derivation of the raw image. Since the jpg is a compressed version of the raw, then to really go to the right edge of the raw histogram, you have to clip a little in the jpeg?

In addition, cameras are coming with more and more controls to change the processing in camera. The latest olympus micro 4/3 cameras even allow application of tone curves. Would these manipulations also not skew the histogram and therefore distort where ETTR should be?

Then again, this 'inability to know where the right edge of the raw histogram really is might actually be a bonus. It is the 'insurance' that allows us to recover highlights. So although we do not expose right up to the edge, we get out of the dark noise areas. So how far to the right do we REALLY need to go?

Maybe we dont need to go as far to the right any more. I guess it really depends when the S/N ratio comes out of the darkness. Really you just have to get the histogram out of the the low S/N region on the left side. I might still have a lot of headroom on the right side (as sensors come out with greater dynamic range).

Is there a program that will calculate S/N of a camera sensor from an exposure of a ramp of dark tones? But then you might say, what good is a number like S/N anyway. What does it really mean?

For S/N to have any meaning, we have to know what the human eye can perceive. Keeping S/N above that threshold in the raw capture would be the goal.

[K.C.]

What does it really mean?

You shouldn't rely on the camera histogram for anything more than a ruff guide.

And bracketing still works.

[Tony Jay]

There is no absolute answer to this question, no equation that can just simply be applied and hey presto the perfect ETTR exposure.

How to expose to the principles of ETTR are somewhat dependent on the performance of the sensor as already suggested, nonetheless individual scenes will have a large impact on what determines the actual exposure.

Specular highlights are a case in point. Does one want to include the specular highlights in determining how to expose or is it OK to let them blow.
How important are other highlights in the composition or is OK to let them blow.
No equation can determine this only the goal and vision of the photographer can determine how to expose for the specific scene in question.

No doubt that when one uses a particular camera for a time one does get a feel for how far to push the exposure but I am absolutely with K.C. when he suggests bracketing exposures around an apparent ideal exposure. Only round-tripping between the field and ones computer can one start to get a real idea of ETTR. There is also a learning curve associated with post-processing ETTR images. One may get the impression that ETTR does not work if the post-processing is not good.

Additionally, even if using the same hardware, my judgement of an ideal ETTR exposure may not square with anyone elses. Although the principles of ETTR are based on sound principles of physics ones own artistic vision and tolerances will actually determine how one exposes for ETTR.

Regards

Tony Jay

[marcmccalmont]

After some practice you can judge how many "blinkies" on your rear screen jpeg correlates to RAW clipping.
Just shoot a lot of ETTR's and compare your raw histograms that are ETTR'd  just right to how severe the "blinkies" are prior to clipping. About the best you can do without a RAW histogram displayed. As mentioned several exposures increases your odds. Just be careful on those blue skies the blue channel might clip before the others and your "blinkies" might not be an accurate representation.
Marc

[aduke]

In additional to knowing how far you can or should go with your exposure, you should also set the JPG processing parameters to obtain the most neutral processing possible. I think this means, at least, minimum contrast and sharpening.

Search the forum for ETTR. There have have been many discussions of how best the set those parameters.

Alan

[MarkL]

Take a shot with the histogram right up to the edge then load it into your raw converter with usual starting settings and see how exposure you need to increase before it clips. Personally, I just use this as insurance

[Tony Jay]

The histogram generated by the camera is a representation of a processed JPEG not the RAW file.
In order for the camera histogram to more closely represent the RAW file change settings such as contrast to zero (in most cameras changing the picture style to neutral should do the trick).
Nonetheless the camera histogram does not exactly match what the RAW file has captured.

Camera manufacturers have been lobbied (not least by Michael Reichman himself) to change to, or at least allow the option of viewing, a histogram representing a RAW file. To date, no dice.
So only once a RAW image has been loaded into a RAW converter can the dynamic range of the image really be appreciated.

Hope I am not selling snow to eskimos

Tony Jay

[Guillermo Luijk]

'How do you really know when you are ETTR?'

Unfortunately you can only know this once at home, and analysing your RAW files with an appropiate piece of software (Rawnalyze, RAW Digger, DCRAW,...).

At shooting time in the camera or at home using conventional RAW developers, today it's impossible to make sure if you ETTR'ed or not.

Regards

[ErikKaffehr]

Hi,

I agree with Guillermo, but in my view it's more about a mindset than achieving the absolutely correct exposure. The mindset is that we really want to maximize exposure. My experience with my equipment (Sony) is that blinking highlights and histogram are quite OK. Each time I went over blinking highlights and clipped the histogram I got burned out highlights.

Best regards
Erik

'How do you really know when you are ETTR?'

Unfortunately you can only know this once at home, and analysing your RAW files with an appropiate piece of software (Rawnalyze, RAW Digger, DCRAW,...).

At shooting time in the camera or at home using conventional RAW developers, today it's impossible to make sure if you ETTR'ed or not.

Regards

[bwana]

thank you all for taking the time to answer the first part of this question. but the flip side of the coin still nags me. to put it simply, how far to the right do i have to go to get out of the noise regime of my sensor?

i know this varies depending on the sensor obviously- but i dont want to buy DXOanalyzer to check my camera.and even if i did, i wouldnt know how to translate those numbers into a useful guide. here for example is an evaluation of the canon 1D mark2
http://www.clarkvision.com/articles/evaluation-1d2/
S/N at iso 50 is 283, at iso 400 it's 115
how much do i have to adjust the EV to get clean data out of an image of a dirty,dark closet? If on my histogram on the back of the camera, 255 is white and 0 is black, what should be my lower limit where my darkest black should be captured?The goal is so that when i adjust the raw in PP and make that black 0, there is no noise.

now i know some of you will say that i might be clipping highlights, but i am not focussed on that at the moment. assume the image is low key or that multiple exposures with hdr technique will be used to capture the upper end.

and i did google extensively before asking this.
http://lmgtfy.com/?q=measure+camera+sensor+noise
but i am searching for meaning, not just information
(i think i should add that to my signature)

[Guillermo Luijk]

Your camera sensor is a tri-band photon counter. This means it averages the number of photons coming from 3 areas of the spectrum, one around the red wavelengths, one around the green wavelengths and one around the blue wavelengts.

Perfect ETTR would mean no information gets clipped in any of the three channels, and this depends on the scene. Most scenes will have a dominant G channel, but some others will have R or even B dominant channels. So unless your camera provides you with RAW histograms (which unfortunately is a very silly thing to achieve, but camera makes are not interested in), you cannot figure out a simple rule of thumb appicable to all cases. This thread shows why.

The good part is that cameras get more and more DR with improved sensors, so perfect ETTR is less and less necessary with time. If you make sure your JPEG has no clipped areas, or minimal clipped areas, you can be 99% sure your RAW file is intact.

 

[Michael H. Cothran]

If the camera histogram is generated by a processed jpeg, how does that work on a digital back that only produces RAW files? Is there a difference? It is discouraging to find right clippings in post processing software when one sees none in the camera in the field.

[Tony Jay]

I have to confess that I do not know the mode of generation of MFDB 'in camera' histograms.
As far as I am aware with regard to 35mm equivalent DSLR's my statement was accurate.

I do feel that camera manufacturers need to give photographers at least the option of viewing a histogram representative of the RAW capture.

Regards

Tony Jay

[Tony Jay]

http://www.brisk.org.uk/photog/histo5.html

This link explains some of the limitations of in camera histograms and their analysis.
It is a bit dated so newer information may be available.

Regards

Tony Jay

[bwana]

I dont feel I am getting the answer I am looking for , probably because I am not communicating well. Consider driving down a road to avoid an obstacle. What I am hearing from most of you is to watch out for the curb on the right side of the road and go as close to it as possible. (Apologies to those of you in the UK, just pretend you are driving in Europe) I am more interested in focussing on the obstacle to avoid it. What this translates to is a vertical guideline on the histogram chart that marks where the left side of the histogram *should* end so there is not perceptible noise. I know that noise is constant across the entire exposure range but it is just noticeable more in dark scenes. Only when the brightness of the signal exceeds the brightness of the noise does the noise become visually insignificant. This is a specific point on the horizontal axis of the histogram and is related to the sensor. Can this point be measured? Tnx.

[digitaldog]

What I am hearing from most of you is to watch out for the curb on the right side of the road and go as close to it as possible.

Yes but this isn’t really much different from exposing transparency film. There may be highlights in a scene you wish to record, you don’t want to blow them out. You want to get as close to that as possible with ETTR. The problem is, we don’t have a good on-screen histogram of the raw data. But then we didn’t have histograms for film! If you know how your film+processing responds along with your metering techniques, you can do the job as you desire and that is much the same with digital. ETTR is what it is because the feedback on the camera isn’t accurate for the raw capture (so the recommendations are to expose to the right of this incorrect indicator).

My suggestion is to get to better know your camera system and ignore the stuff on the back of the camera in terms of exposure for raw. If in doubt, it is better to have a tad more noise than blow out highlights you wish to record. If you do that, nothing will bring that back. If possible, bracket. Find the limitations of what the correct exposure settings based on a meter provides onto the raw data along with the raw converter you’ll use to ‘normalize’ the rendering.

[sandymc]

Only when the brightness of the signal exceeds the brightness of the noise does the noise become visually insignificant. This is a specific point on the horizontal axis of the histogram and is related to the sensor. Can this point be measured? Tnx.

Sorry, but no, it's not a specific point. It's hugely dependent on your perception (different people have entirely different opinions of what "visually insignificant" is), and it's also hugely dependent on post processing. E.g., with default settings the same image post processed with LR3 will look less noisy than if it were processed with LR2, just because LR3's noise processing is much better. LR4's extensive "content aware" processing adds a whole additional layer of complexity.

As suggested by Andrew, you'll do better working on understanding how your camera and workflow behaves end-to-end under various circumstances than trying to make decisions based on a theoretical ETTR threshold.

Sandy

[Tony Jay]

Sorry, but no, it's not a specific point...

Exactly what we all have been trying to communicate the last few days!

Regards

Tony Jay

[bjanes]

I dont feel I am getting the answer I am looking for , probably because I am not communicating well. Consider driving down a road to avoid an obstacle. What I am hearing from most of you is to watch out for the curb on the right side of the road and go as close to it as possible. (Apologies to those of you in the UK, just pretend you are driving in Europe) I am more interested in focussing on the obstacle to avoid it. What this translates to is a vertical guideline on the histogram chart that marks where the left side of the histogram *should* end so there is not perceptible noise. I know that noise is constant across the entire exposure range but it is just noticeable more in dark scenes. Only when the brightness of the signal exceeds the brightness of the noise does the noise become visually insignificant. This is a specific point on the horizontal axis of the histogram and is related to the sensor. Can this point be measured? Tnx.

I agree with the Digitaldog and Sandy that you are approaching the problem in the wrong way, but it is easy to use the DXO data to see how many stops a given noise floor is from saturation (100% ETTR). DXO has a full SNR plot showing the SNR in dB for a given percent of saturation. The example here is for the new Nikon D800. The Y values of 18, 24 and 30 dB correspond to SNRs of 8:1, 16:1 and 32:1 respectively. For a SNR of 18 dB, the sensor saturation is 0.16%, which is 9.27 stops from saturation (stops = log base 2 (100%/0.16%). For 24 and 30 dB, the corresponding number of stops are 7.52 and 5.52 stops respectively.

What constitutes an acceptable SNR is a personal and subjective matter. See Fig 13 of Emil Martinec's post for an illustration.

Unfortunately, the camera histogram does not have a log base 2 x-axis (f/stop increments) and the right end of the histogram is usually short of 100% saturation. In practice, it is best to place the highlights just short of clipping and the shadow SNR will be optimized. Because of problems with the camera histogram, one could use a spot meter to place the highlights and then measure the shadows downward from this point.

Regards,

Bill

[Nick Walker]

Sports photography rarely allows time for precise spot metering. I still use an incident meter at times, as I did with slide film. I am not suggesting my simplistic metering/RAW exposure method is bullet proof but it gives me a good starting point and has served me well since moving from transparency to digital, some 11 years ago.

With any new camera I record a Colorchecker under daylight conditions for Lightroom camera calibration purposes, taking an incident reading as my starting point. I then include a white cotton T shirt, bracketing the exposures again by 1/3rd of a stop. Many sports involve the use of whites.

I make a note of the shutter speed and aperture that caused the first highlight warning to show on the cameras LCD. In Lightroom I flick through the images to locate the exposure which is as close to 255 as possible for my 'normal' exposure - on average this results in 1/10th of a stop shy of 255.

I note the shutter speed and aperture for this 'normal' exposure, checking it against the original Incident reading - I leave the incident meter on the original exposure reading and recalibrate (internally) to meet the same shutter speed and aperture for the 'normal' Lightroom result.

I also experiment to see if I can recover over exposed highlights (daylight conditions) I normally don't go above 2/3rds for safety - there is often one, or two channels, intact at only +2/3rds.

Using a light meter has trained me to remember the exposure off by heart in full sunlight for subjects that contain whites - film and digital. With Fuji Velvia rated at 40 ISO for my E6 processing lab I could not better 1/640 at f/4 - (600mm f/4 lens related exposure recall). With the Nikon D3S and D4 In full light, subjects with whites, if I want to stay just shy of clipping, F/4 at 1/2000 sec (100 ISO) does the trick.

I can turn down various jpeg camera settings but tend not to bother - Uni White balance is not for me. If competitors are not playing ball and wearing all black clothing (some golfers occasionally do) I have to change tactics to ETTR satisfactorily.

Important texture in whites, also caucasian skin tones from good ETTR (not clipped) stay too thin and require pulling down. Lightromm 4 is so powerful, it's the first RAW converter I have used (tried many over the years, C1 Pro, RPP, Raw Therapee, etc) which has enabled me to reduce lighter tones using a combination of the powerful whites and highlights sliders to mimic the rolled off highlights which film provides.