Discussion of 'Equivalence'

[David H. Hartman]

But what I've been puzzled with is that some people propose 'exposing to the left' for digital.

If may be forced by the need for DoF or to freeze subject movement or both. One may use a lower ISO out of fear that the well will run dry.

I'm afraid my sorry attempts at comic relief may not be well received so I'll take a brake from this thread.

[simsurace]

These people are generally misapplying the concept of "ISO Invariance" (not close to the topic of this thread)

Some of them. Others are doing it at base ISO in what seem studio or landscape conditions. I don't get it.

[simsurace]

Perhaps a less confusing name would help: perhaps "Sort of Equivalence." I believe the name *implies* that it can give an identical or nearly identical photography but that is not possible.

Things that are not the same, but can be regarded to be the same applying some precisely defined criteria are called 'equivalent'. This is both standard usage in ordinary language as well as mathematics and science.

I'm interested why you think that it is not possible (given the right equipment) to make two nearly identical photograph using different formats. What fundamental limit are you thinking of?

[simsurace]

If may be forced by the need for DoF or to freeze subject movement or both. One may use a lower ISO out of fear that the well will run dry.

I'm afraid my sorry attempts at comic relief may not be well received so I'll take a brake from this thread.

No, on the contrary, some humour is always welcome.
Yes, but that is not the same as recommending 'exposure to the left' as some kind of sweet spot. It is a spot where you would rather not be. Therefore my question.

[simsurace]

There are sites on the Net that clearly show that DoF and Background blurring are not one and the same.

Thanks for pointing this out. They are by definition not the same. One of them is measured as a size in image space, the other as a distance along the optical axis in the object space (the space in front of the lens).
DOF is more complicated to calculate than background blur. This is why I focused on background blur in my little calculation.

[bclaff]

Some of them. Others are doing it at base ISO in what seem studio or landscape conditions. I don't get it.

Those could be the same people. People who think they don't need to raise the ISO setting and can get the same lightening effect in post.
Depending on the camera they may get away with it but it's a poor habit and not something to blindly recommend.

[Roland Vink]

There are sites on the Net that clearly show that DoF and Background blurring are not one and the same.

They are two sides of the coin - related, but mutually exclusive.
- DoF is the part of the image which is regarded as in focus.
- Background blurring or Bokeh relates to the part of the image which is out of focus.
Where you draw the line between the two (sharp vs OOF) depends on the image size, viewing distance, how critical the viewer is etc, there is no hard and fast rule here.

[Les Olson]

With digital sensors, the only sweet spot for exposure is exposing to the right. There is no other sweet spot in my view.

Once again, you are not acknowledging the essential but dubious assumption underlying this claim: that post-processing is fun.  So why wouldn't you commit to post-processing every image?  One reason, apart from not thinking it was fun, is that you want or need to send images directly from the camera.  Another - more important in my view, since I don't use Twitface and I must have been out when New York Vogue called to book me for the spring collections - is that there is a penalty to reducing image brightness (misleadingly called "exposure" - in Lightroom, eg) in post-processing to compensate for over-exposure, which is that changing brightness changes colour relationships.  So, once again, what purports to be a technical issue is a concealed, arbitrary preference - in this case, for shadow detail over colour fidelity. 

ETTR is also a slipperier concept than you are letting on.  The sensor cannot detect clipping, it can only predict it from the fact that a number of pixels reach FWC.  How, precisely, does the camera make that prediction?  And if you have (say) 24MP on the sensor, and (say) 1.3MP on the viewfinder/LCD, (roughly) 20 sensor pixels map to one viewfinder/LCD pixel.  How?  Does a viewfinder pixel blink if all of "its" sensor pixels saturate, or if more than half of them saturate, or if any of them saturate?  Is it's decision to blink influenced by its neighbours?  How many adjacent viewfinder pixels have to blink before you can see them?  So, what you really mean is that you have to not clip the highlights more than some amount the camera designers decided was important but didn't tell you about, for reasons they didn't tell you about either.       

[simsurace]

Once again, you are not acknowledging the essential but dubious assumption underlying this claim: that post-processing is fun.  So why wouldn't you commit to post-processing every image?  One reason, apart from not thinking it was fun, is that you want or need to send images directly from the camera.  Another - more important in my view, since I don't use Twitface and I must have been out when New York Vogue called to book me for the spring collections - is that there is a penalty to reducing image brightness (misleadingly called "exposure" - in Lightroom, eg) in post-processing to compensate for over-exposure, which is that changing brightness changes colour relationships.  So, once again, what purports to be a technical issue is a concealed, arbitrary preference - in this case, for shadow detail over colour fidelity.

I will not disagree on the fun part, but let me keep that emotional reaction to the process separate from a definition of what an optimal exposure is for digital cameras.
Personally, I certainly do not expose every shot to the right, not by far. It requires time both for setting up the shot and time for post processing. Sometimes, multiple shots are required to get it right because the camera does not allow me to see where exactly the clipping point is, despite using tricks such as UniWB etc.
But the exposures that are not to the right are merely convenient, they do not represent a sweet spot of the medium. Anytime I end up with a shot which has several stops to the right of the brightest part of the histogram, I end up with more noise than what I would have needed to put up with, and if my exposure was not constrained by movement, I could have gotten a better image quality by exposing more. But for some shots, the quality is already sufficient and high DR in today's sensors means that I can get away with it most of the time. Still, I would claim, the exposures are not in a sweet spot of the medium in any way.

I hope it is clear that the discussion is about base ISO only. Anytime you are raising ISO from base, you are anyway not using the full capacity of the sensor, reducing dynamic range. It is still good then to have the histogram to the right, but for different reasons.

I'm not sure I understand what you are talking about regarding color fidelity. Do you talk about individual color channels being clipped? Do you have an example?

ETTR is also a slipperier concept than you are letting on.  The sensor cannot detect clipping, it can only predict it from the fact that a number of pixels reach FWC.  How, precisely, does the camera make that prediction?  And if you have (say) 24MP on the sensor, and (say) 1.3MP on the viewfinder/LCD, (roughly) 20 sensor pixels map to one viewfinder/LCD pixel.  How?  Does a viewfinder pixel blink if all of "its" sensor pixels saturate, or if more than half of them saturate, or if any of them saturate?  Is it's decision to blink influenced by its neighbours?  How many adjacent viewfinder pixels have to blink before you can see them?  So, what you really mean is that you have to not clip the highlights more than some amount the camera designers decided was important but didn't tell you about, for reasons they didn't tell you about either.     

I disagree that the sensor cannot detect clipping. It has access to the RAW data, so why not? The camera manufacturers simply choose not to make that information available to the photographer. This requires to approximate the clipping point by using a UniWB which removes scaling in the color channels, and using a flat profile to prevent clipping by simply increasing contrast on top of the RAW capture.

I do not understand why they will not give us the option of displaying RAW histograms. This has been requested from the very beginning. My suspicion is, that this comes from being too rooted in the way of thinking of the analog times. The user interface and the available metering modes reflects that. The middle gray criterion, for example, is not very adequate in high-contrast situations.

Again, this point is only a practical hurdle that can be solved, in principle. It does not change anything about what the digital sensor's sweet spot is.

EDIT: Just a word of warning for anyone reading along: Do not mistake this discussion for a practical guide to exposure. The topic is not presented in a way that would be suitable for didactic purposes. If you want to push the limits, you are on your own. I do not want to have you botch your exposures on an important shoot.

[Les Olson]

I'm not sure I understand what you are talking about regarding color fidelity. Do you talk about individual color channels being clipped? Do you have an example?

I disagree that the sensor cannot detect clipping. It has access to the RAW data, so why not?

Full is full.  The sensor can only infer clipping, presumably from the number of pixels at FWC.  How many? 

Here is a quick and dirty example of the problem with using brightness to correct over-exposure.  This is the cover of Steve Anchell's The Darkroom Cookbook.  The top image was taken at camera's idea of correct exposure using aperture priority.  The second was made +1 with exposure compensation, and the third was +2.  Then in LR the second was given -1 "exposure" and the third was given -2 "exposure".  There is obvious colour shift.

[simsurace]

Thanks for the example!
In addition to the loss of color in the background, I see lower contrast and motion blur in the 2nd and 3rd shots, which are probably related.
Did you look at RAW histograms to ensure that no channel was clipped?
If no channel was clipped, then the LR exposure slider is not applying the correct transformation (this has been a question of mine for some time: what is the exposure slider exactly doing?)

[Andrea B.]

A quick note about ETTR. Raw Digger has a nice tutorial about how to determine the maximum amount of exposure compensation to apply for good ETTR which will make best use of your camera's dynamic range. (Of course the method involves Raw Digger, but that's ok. It's a great tool. For the record, no affiliation by me or NG.)

Good ETTR is defined as metering for those highlights for which you want to preserve the most detail followed by the appropriate amount of EV+. The appropriate amount of EV+ will vary for different camera models and different ISO settings. But once you have figured it out for your camera and ISO setting, you will have a solid guide for how much to push the exposure to the right. Less trial & error!! Easier to set up the shot.

[THen, yes, you have to recover the photo in the converter.]

[Andrea B.]

Forgot the link!!

https://www.fastrawviewer.com/blog/how-to-use-the-full-dynamic-range-of-your-camera

[Les Olson]

There may be some blue channel clipping in the 2-stop version but not in the 1-stop version. 

The LR "exposure" slider changes brightness, but according to Jeff Schewe's book (The Digital Negative: Raw Image Processing in Lightroom Camera Raw and Photoshop, 2nd ed, 2015) it changes the mid-tones more than the extremes.  That would account for its introducing colour shifts. 

It is possible there is a perceptual effect as well.  The Hunt effect is the phenomenon of greater perceived colourfulness of a stimulus with increased luminance and the Stevens effect is the phenomenon of greater perceived contrast with increased luminance, so an overall reduction of brightness would reduce perceived colourfulness and contrast (the Stevens effect is why old B&W movies often seem to us to have too much contrast: they were designed with high contrast because they were intended for viewing at lower luminance levels than we use today; the Hunt effect is why the stained glass in medieval cathedrals, and slides on a light table, have such brilliant colours). 

[simsurace]

There may be some blue channel clipping in the 2-stop version but not in the 1-stop version. 

The LR "exposure" slider changes brightness, but according to Jeff Schewe's book (The Digital Negative: Raw Image Processing in Lightroom Camera Raw and Photoshop, 2nd ed, 2015) it changes the mid-tones more than the extremes.  That would account for its introducing colour shifts. 

It is possible there is a perceptual effect as well.  The Hunt effect is the phenomenon of greater perceived colourfulness of a stimulus with increased luminance and the Stevens effect is the phenomenon of greater perceived contrast with increased luminance, so an overall reduction of brightness would reduce perceived colourfulness and contrast (the Stevens effect is why old B&W movies often seem to us to have too much contrast: they were designed with high contrast because they were intended for viewing at lower luminance levels than we use today; the Hunt effect is why the stained glass in medieval cathedrals, and slides on a light table, have such brilliant colours).

I think what the exposure slider 'should' do is apply a linear scaling to the RAW levels before the translation to the color space occurs. Can you determine that from the book?

Why would the perceptual effects apply after you have adjusted the brightness?