Brightness vs Exposure - What's the Difference?

[Peter_DL]

In step 1, which is blown out in all channels, it is not possible to decrease the pixel value below 255. In step 2 there is valid date in the red channel and recovery is possible, but red=green=blue--i.e. the recovery is neutral as you and Guillermo mentioned. This behavior is also present in steps 3 and 4 which have valid data in the red and blue channels. Finally, in step 5 were all channels have intact data, color information appears. The white balance is slight off, and the RGB values are different.

When color information is lacking in one channel (in this the green channel), ACR has to guess, and using a neutral value is reasonable since extreme highlights often have little color. The Raw Developer program guesses that the color in areas with incomplete color information is similar to that in adjacent areas with intact channels.

This behavior can be useful in some instances, but it is better to expose so that channels are not blown. If the dynamic range exceeds that of the camera, two or more exposures can be made if the subject permits.

Bill,

Agreed, however, my post was referring to a different aspect:
What happens to non-clipped highlight colors when we apply a.) Brightness, or b.) Exposure + Recovery, or c.) Brightness + Recovery via the respective sliders and positive settings thereof ?
Result: in all cases saturation is reduced and highlight colors may appear washed out.

In Photoshop for comparison this can be avoided by starting (likewise) with a 'linear conversion/rendition' from any suitable converter, and then by furnishing the tone curve with an inverted luminosity mask. A kind of prevention of 'damage' rather than belated recovery.

Peter

--

[bjanes]

Should be one on my iDisk in a folder called "For Drew" that produced the screen grabs above. There's two brackets, the one needing recovery is _MG_1247.CR2

Out of interest I looked at _MG_1246.CR2 and _MG_1247.CR2 with Rawnalize. _MG_1246.CR2 is underexposed (green channels) by about 1 1/3 EV and _MG_1247.CR2 is overexposed by perhaps 0.5 EV, according to ETTR goals. The underexposure in the the first image when evaluated with ACR is partially masked by the baseline exposure of +0.4 EV that is used for the 5D Mark II. This is why I dislike the use of that offset. Unless you know about that offset and its value, it is difficult to evaluate exposure with ACR histograms.

[attachment=15371:Rodney1.gif]

The blue and red channel have unclipped data for recovery. Since the red multiplier for this camera is about 2, recovery should work for about to 1 stop overexposure for recovery.

[attachment=15372:Rodney2.gif]

[Guillermo Luijk]

Bill, the baseline exposure is a value that is added to the RAW developer exposure adjustment value, or the RAW developer adjustment is set to the baseline exposure as a starting default value than can then be changed?

I.e., if you set 0.0EV in ACR you are applying the baseline exposure, or you are just cancelling it?

Regards

[bjanes]

Bill, the baseline exposure is a value that is added to the RAW developer exposure adjustment value, or the RAW developer adjustment is set to the baseline exposure as a starting default value than can then be changed?

I.e., if you set 0.0EV in ACR you are applying the baseline exposure, or you are just cancelling it?

Regards

This is what the DNG specification says: "Because of these differences, a raw converter needs to vary the zero point of its exposure compensation control from model to model. BaselineExposure specifies by how much (in EV units) to move the zero point. Positive values result in brighter default results, while negative values result in darker default results.". However, this seems backwards to me. The BaselineExposure for the D3 is -0.5. However, ACR renders the image too light with no exposure compensation, and I have to use a negative exposure adjustment of -0.5 EV to get correct results. By correct, I mean a sensor saturation of 18% when exposure is made according to the light metered exposure, resulting in a pixel value of 118 in a gamma 2.2 space. With your 350D, the BaselineExposure is +0.25, so I would recommend trying an exposure compensation of +0.25 EV for correct tone placement.

The BaselineExposure for the 5D MII is +0.4. According to an article on Iliah Berg's web site, this camera places mid gray at 6.5% saturation. He has indicated that this is because of certain limitations of the sensor (see here. For the 5D MII one could try using +0.4 EV exposure compensation in ACR. However, this might not be enough if Iliah's obsrvations are correct.

Hopefully, Eric Chan is still following this thread and can comment.

Bill

[Andrew Fee]

This is what the DNG specification says: "Because of these differences, a raw converter needs to vary the zero point of its exposure compensation control from model to model. BaselineExposure specifies by how much (in EV units) to move the zero point. Positive values result in brighter default results, while negative values result in darker default results.". However, this seems backwards to me. The BaselineExposure for the D3 is -0.5. However, ACR renders the image too light with no exposure compensation, and I have to use a negative exposure adjustment of -0.5 EV to get correct results. By correct, I mean a sensor saturation of 18% when exposure is made according to the light metered exposure, resulting in a pixel value of 118 in a gamma 2.2 space. With your 350D, the BaselineExposure is +0.25, so I would recommend trying an exposure compensation of +0.25 EV for correct tone placement.

The BaselineExposure for the 5D MII is +0.4. According to an article on Iliah Berg's web site, this camera places mid gray at 6.5% saturation. He has indicated that this is because of certain limitations of the sensor (see here. For the 5D MII one could try using +0.4 EV exposure compensation in ACR. However, this might not be enough if Iliah's obsrvations are correct.

Hopefully, Eric Chan is still following this thread and can comment.

Bill

Perhaps I'm misunderstanding this, but if the baseline exposure for a camera is +0.25, doesn't that mean ACR is automatically adding 0.25 exposure compensation "behind the scenes" when set to 0 and that you should set it to -0.25 to cancel it out and see the actual RAW exposure?

If you were to add +0.25, wouldn't that be 0.5 over what the RAW exposure actually is?

[Panopeeper]

Perhaps I'm misunderstanding this, but if the baseline exposure for a camera is +0.25, doesn't that mean ACR is automatically adding 0.25 exposure compensation "behind the scenes" when set to 0 and that you should set it to -0.25 to cancel it out and see the actual RAW exposure?

If you were to add +0.25, wouldn't that be 0.5 over what the RAW exposure actually is?

You do understand it right; the baseline exposure correction is applied automatically. What worse is the fact, that it is not only automatic, but clandestine. The slider does not show it, and it is not described anywhere (the amount depends on the camera model and often on the ISO as well).

For example Canon's Highlight Tone Protection option reduces the effective ISO from the displayed one by a full stop, i.e. the shot is underexposed, with the aim of increasing the intensity in the raw conversion, except for the very highlights. ACR increases the intensity by applying +1 EV (but that's it, ACR does not mimic the in-camera and DPP adjustment of the curve). The user does not see this adjustment at all; thus (s)he believes the exposure was correct, which is misleading pure.

MFDB users are regularly misled by ACR into believing, that they understand how their camera works and are exposing correctly.

[bjanes]

Perhaps I'm misunderstanding this, but if the baseline exposure for a camera is +0.25, doesn't that mean ACR is automatically adding 0.25 exposure compensation "behind the scenes" when set to 0 and that you should set it to -0.25 to cancel it out and see the actual RAW exposure?

If you were to add +0.25, wouldn't that be 0.5 over what the RAW exposure actually is?

To answer your question with an example for which I have data, let's use the D3, which has a BaselineExposure value of -0.5 EV as shown in a previous post. If that exposure adjustment were automatically used behind our back so to speak, it would darken the image. I photographed a gray card according to the camera exposure meter reading. The resulting pixel value in the raw file is approximately 1820 or 94 in terms of 8 bit sRGB as shown in this Rawnalize capture. (The Nikon metering is the the equivalent of 12.5% reflection, not 18%. If you want 18% saturation for mid gray you need to add 0.5 EV).

[attachment=15429:Rawnalize.png]

Setting the ACR tone curve to linear and leaving the exposure slider at zero, the sRGB pixel value is 110, which is too light. The BaselineExposure of -0.5 lightens the image rather than darkening it.

[attachment=15430:ACR_LinearExp0.png]

Setting exposure to -0.5 EV gives an sRGB value of 96, very close to the predicted result of 94:

[attachment=15431:ACR_Line...inusHalf.png]

Using ACR defaults gives an image which is much too light. Mid gray is 159. If you didn't know better, you would think that the camera was overexposing. Such is a common complaint on the Nikon forums. Using an exposure value of -0.5 EV in ACR, I get an sRGB value of 126 which is more reasonable (mid gray is 118 in sRGB).

[attachment=15432:ACR_Default.png]

[Guillermo Luijk]

 

What worse is the fact, that it is not only automatic, but clandestine.

hahaha clandestine, I really love how you use the language sometimes. Still, Gabor and Bill, I don't think there is a clear way to speak about the _actual_ exposure achieved in the camera, since the white balance means a change in exposure in at least 2 of the three channels.
If we want to be strict about the _actual_ achieved exposure and the 18% gray story (which is basically useless, but interesting from a technical point of view), white balance should be entered into the equation.

However I always considered the discussion about the 18% and the actual exposure quite useless for several reasons:
- Camera exposure settings are not fine tuned, they are rounded to 1/3EV (this means an average error of at least 1/12EV=0.08EV, and max error of at least 1/6EV=0.17EV).
- Any RAW development and/or processing will involve tonal shifts (any level adjustment means a tonal shift), that can both corrrect or ruin the 18% criteria. In fact most RAW developers like ACR calculate the initial settings to obtain a pleasant image, not using any 18% criteria.
- The precise point from saturation at which each digital camera locates a light metered area is irrelevant. Cameras with a large gap from metering to sat can be used with +EC to compensate for this. Cameras with a reduced highlight headroom from metering to sat can be used with a systematic -EC compensation.

At least for RAW shooters, the only important thing to know is how your camera behaves: i.e. how many EV it allows from metering to sat, and act accordingly depending on your needs and situation.

These are the RAW log2 histograms from shots made over a tungsten uniformly lighted surface, according to camera metering for 350D and 5D:

350D


5D



Both cameras in their 3 RAW channels produce a RAW level well below (more than 1EV) the expected [18% -> -2.5EV] or the [128 in a gamma 2.2 encoding -> -2.2EV], so even after a positive white balance it won't reach the expected middle gray (128) in a 2.2 gamma encoding. But it's unimportant.

Regards

[bjanes]

Still, Gabor and Bill, I don't think there is a clear way to speak about the _actual_ exposure achieved in the camera, since the white balance means a change in exposure in at least 2 of the three channels.
If we want to be strict about the _actual_ achieved exposure and the 18% gray story (which is basically useless, but interesting from a technical point of view), white balance should be entered into the equation.

I must disagree with our esteemed colleague. For exposure, one usually takes the value of the green channel for which the white balance multiplier is unity. With daylight exposure, the red and blue channels are underexposed, but the WB multipliers bring them up to the level of the green channels.

However I always considered the discussion about the 18% and the actual exposure quite useless for several reasons:
- Camera exposure settings are not fine tuned, they are rounded to 1/3EV (this means an average error of at least 1/12EV=0.08EV, and max error of at least 1/6EV=0.17EV).
- Any RAW development and/or processing will involve tonal shifts (any level adjustment means a tonal shift), that can both corrrect or ruin the 18% criteria. In fact most RAW developers like ACR calculate the initial settings to obtain a pleasant image, not using any 18% criteria.
- The precise point from saturation at which each digital camera locates a light metered area is irrelevant. Cameras with a large gap from metering to sat can be used with +EC to compensate for this. Cameras with a reduced highlight headroom from metering to sat can be used with a systematic -EC compensation.

At least for RAW shooters, the only important thing to know is how your camera behaves: i.e. how many EV it allows from metering to sat, and act accordingly depending on your needs and situation.

These are the RAW log2 histograms from shots made over a tungsten uniformly lighted surface, according to camera metering for 350D and 5D:

Both cameras in their 3 RAW channels produce a RAW level well below (more than 1EV) the expected [18% -> -2.5EV] or the [128 in a gamma 2.2 encoding -> -2.2EV], so even after a positive white balance it won't reach the expected middle gray (128) in a 2.2 gamma encoding. But it's unimportant.

If you use an automated exposure mode such as aperture priority, the camera shutter speed is adjusted to the exact value needed and not in the 1/3 stop increments indicated on the camera LCD. To test this, I took exposures with my Nikon D3 of a white wall which was illuminated unevenly, using a 300 mm lens to select small areas which were evenly illuminated over a small area. I used ACR with a linear tone curve and rendered into sRGB using the required exposure offset of -0.5 EV and read a small central area of the images using a mask in Photoshop so I would be reading the same area in each shot.

The indicated exposures were 1/15, 1/13, and 1/10 sec. The sRGB readings were 107.41, 107.95 and 107.29, indicating fine control of exposure. Rawnalize gave a gamma 2.2 value of 109 and a 14 bit raw value for green of 2418. The camera was giving a saturation of 15%, about 2.75 stops below 100% saturation. The ISO saturation standard leaves 0.5 EV of headroom, so a saturation of 12.5% would be expected. ACR with the required exposure offset and a linear tone curve gave an accurate result. However, I do agree that you do need to know your camera and how it exposes. In addition, you need to know your raw converter and the tone curve used by the camera JPEG engine. The Nikon D3 leaves slightly more than 0.5 EV of headroom and then uses a hot tone curve to bring up the value. With Capture NX and the Standard Picture Control, I got an sRGB value of 161. NX closely approximates the JPEG engine of the camera. With ACR defaults and the Adobe Standard camera profile, the sRGB value was 153.

BTW, mid gray in gamma 2.2 is 118, not 128. See Bruce Lindbloom's companding calculator for L* = 50. The normalized pixel value is 0.4635, or 118.2 in 8 bit notation.

Best regards,

Bill

[Panopeeper]

I don't think there is a clear way to speak about the _actual_ exposure achieved in the camera, since the white balance means a change in exposure in at least 2 of the three channels

At the moment we are inspecting the raw data, the term "exposure" as the combination of aperture and shutter time plays no role any more. From here "exposure" is the pixel value. The only important aspect is if the pixel value is reliable, i.e. not clipped and not in the very noisy range; everything else is raw processing. I personally don't care the very least for the "correct exposure", I am looking at the raw-like histograms and adjust the exposure as I see fit (assumed that the setting allows for the required adjustment).

The clandestine adjustment of the "exposure" ruins that world by falsifying the result. Extreme example: the Phase One P45+. Its ISO steps are supported by analog gain (it plays no role in this aspect, how useful the increased analog gain is). Consequently, increasing the ISO and reducing the exposure yields quasy the same pixel values. Still, ACR adds one full stop to ISO 400 and two stops to ISO 800. The result is, that you believe you have horrendeously overexposed the shot. You reduce the exposure and get a "correct" shot - in fact you are heavily underexposing. The result is, that this rubbish (the clandestine adjustment) costs the photographer a lot.

Or take Canon's Highlight Tone Priority - the vast majority of phographers do not know, that the only effect of HTP when shooting raw and using ACR is, that they get more noise; this too is the result of BaselineExposure.

If you use an automated exposure mode such as aperture priority, the camera shutter speed is adjusted to the exact value needed and not in the 1/3 stop increments indicated on the camera LCD

I have a hard time to believe, that the camera sets the shutter speed to the exact value required to the correct exposure. Though I saw some indication, that at least some of the Nikon cameras can set exposure in 1/6 EV implicitely.

mid gray in gamma 2.2 is 118, not 128. See Bruce Lindbloom's companding calculator for L* = 50. The normalized pixel value is 0.4635, or 118.2 in 8 bit notation

Why would one define mid-grey as L*=50, instead of B=50%? Some people are not confronted with Lab in their entire photographic life.

[bjanes]

The clandestine adjustment of the "exposure" ruins that world by falsifying the result.

I agree entirely and think that the baseline exposure that Adobe uses is counterproductive for ETTR exposure, because it hides the actual exposure.

I have a hard time to believe, that the camera sets the shutter speed to the exact value required to the correct exposure. Though I saw some indication, that at least some of the Nikon cameras can set exposure in 1/6 EV implicitely.

My assumption was that the shutter speeds in aperture priority mode remain in the analogue domain and do not increment in 0.33 EV units. My testing was not extensive, but my results are consistent with this assumption. What if you have the exposure increment set to 0.5 EV via a custom function? One could perform more testing and determine the granularity of the shutter speed, using the raw value as a measure of the exposure as you indicated previously.

Why would one define mid-grey as L*=50, instead of B=50%? Some people are not confronted with Lab in their entire photographic life.

Because L* = 50 is mid gray on the perceptual scale. A few tests in Photoshop indicate that B = 50 is L* = 54 in sRGB and 61 in ProPhotoRGB.

Wikipedia: "Because HSL and HSV are simple transformations of device-dependent RGB models, the physical colors defined by (h, s, l) or (h, s, v) triplets depend on the colors of the red, green, and blue primaries of the device or of the particular RGB space, and on the gamma compression used to represent the amounts of those primaries."