"Undo" auto-iso to equalize exposure

New Here ,
Aug 09, 2018 Aug 09, 2018

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Hello all,

As part of a photogrammetry project, I took a number of photos of an object using manual settings on a D800.  However, I forgot to turn off auto-iso (argh!).  So, all my photos have the same aperature and shutter speed, but they differ in their ISO.  Is there some way I can calculate the exposure adjustment needed in each photo to remove the effect of the changes in ISO between photos,  such that they will have the same exposures across photos as I originally intended?

I think the adjustment in EV units would go something like this: exposure_adjustment = -log2(ISO/baseISO).  Any thoughts about how to carry that out?

Thanks,

Allie

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LEGEND , Jan 12, 2021 Jan 12, 2021
[This post contains formatting and embedded images that don't appear in email. View the post in your Web browser.] [This is an old thread, but since search digs up old threads, it's worthwhile to have a correct answer.] "...all my photos have the same aperature and shutter speed, but they differ in their ISO. ... I think the adjustment in EV units would go something like this: exposure_adjustment = -log2(ISO/baseISO)." LR's Photo > Develop Settings > Match Total Exposures command makes pre...

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Adobe Community Professional ,
Aug 10, 2018 Aug 10, 2018

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You can not undo the effect it had while shooting.

Doubling the ISO (i.e. from 100 to 200) adds a step to the EV (i.e. 12 to 13). So, it's easy to calculate

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New Here ,
Aug 10, 2018 Aug 10, 2018

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I would count "equalizing exposure" as "undoing" the effect while shooting.  I know there are other ancillary effects (noise, etc), but as the thread title indicated, my main goal is to equalize exposure.

I have a couple hundred photos.  The question is, is there any way to automate this calculation and apply the appropriate exposure compensation to all of them in lightroom?  For example, standardize everything to ISO 1000, and thus apply an exposure compensation of -log2(ISO/1000) to each photo?

Thanks,

Allie

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Adobe Community Professional ,
Aug 10, 2018 Aug 10, 2018

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AFAIK, there is no automatic for this ... there's no need actually.

I still do not understand what you expect to achieve.

Even with Auto-ISO, the camera takes every shot perfectly set. This is like when you change the shutter speed and the aperture accordingly (i.e. Program-Shift) - it will result in the same picture taken, in terms of EV. ISO only adds a third variable and the camera would still use the very same EV.

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New Here ,
Aug 10, 2018 Aug 10, 2018

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Appreciate your reply, though I must emphasize that I do have a need for this, even if you don't understand it.  Too bad there's not a way to achieve it in lightroom, alas.  Would be nice to have some sort of programmatic way to adjust exposure based on metadata.  Maybe I'll have to turn to ImageMagick or something like that.  Is there no plugin that might allow this?

I am using this for photogrammetry, and hence have hundreds of photos of an object from multiple angles.  I am trying to have every picture taken with the exact same settings.  At this point, the best I can do is to correct the photos after the fact.  If the scene from one angle is dark... then the photo should be dark.  If it is light from another angle, then it should be light.    I want dark sides to be dark, light sides to be light, etc.

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Adobe Community Professional ,
Aug 10, 2018 Aug 10, 2018

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Got it. You need consistent exposure and the ISO-Auto acted like it is in P mode, adjusting EV.

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New Here ,
Aug 10, 2018 Aug 10, 2018

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exactly.

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Adobe Community Professional ,
Aug 10, 2018 Aug 10, 2018

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I have been caught by this error in shooting before myself. Unfortunately, there doesn't seem to be a way to override it. The camera exposed the image the way it was set to do so. It's one of those "lesson learned" errors that we all have to grow from.

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New Here ,
Aug 10, 2018 Aug 10, 2018

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uhh, a change in iso essentially just changes the exposure.  undoing that change in post-processing by adjusting exposure (brightness) is the way to "undo" it.  i don't understand what the big mystery is here.

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Adobe Community Professional ,
Aug 10, 2018 Aug 10, 2018

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Yes, you can adjust the exposure. However, you cannot change the ISO at which the image was taken. Even if the camera wasn't set to auto ISO it is possible to adjust exposure. I'm just saying that if you wanted ISO 100 but the camera was on auto and took the image at ISO 250, there isn't any way for you to change that ISO setting. You can adjust the exposure, of course. But the ISO setting obviously isn't going to change. I'm not sure what your question is, really. Initially it seemed you were inquiring about undoing auto ISO. The image has already been taken, and I don't see any way to change that aspect of an image. I'm not sure what you expect to be able to do.

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New Here ,
Aug 10, 2018 Aug 10, 2018

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Hi... yes, of course you can't actually change an exposure setting post-hoc, save for something like white balance in a RAW photo.  But aperture, shutter speed, iso... those are all baked in, i understand that.  The closest one can come to "undoing" an iso setting, given my needs, are to adjust exposure based on the -log2(iso/base iso) formula i believe.  anyway, thanks all for the attention, i think this has become more of a distraction than anything else for all involved...

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New Here ,
Jan 12, 2021 Jan 12, 2021

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I understand what you mean. I've tried to find a program that has an auto function to "equalize" ISO's across multiple images (like the human eye sees it) but I don't think there is one. You can come close to having images equalized by using that formula you mentioned but it has to be done manually. I tried doing a time lapse of a sunset once and made the same mistake. Because the ISO's were all auto adjusted for each of 400 shots, the sunset lost all its natural luster. The best reason I can figure there isn't an auto adjust is because once the image is burnt into digital code, getting the computer to figure out a way to recognize how each pixel's ISO should be adjusted would be like filming a dog and cat run across a screen, showing the computer the cat part and getting it to figure out just from that that a dog had just run across the screen. If that makes sense? In essence, when it comes to ISO settings, you can't change the past.

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LEGEND ,
Jan 12, 2021 Jan 12, 2021

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[This post contains formatting and embedded images that don't appear in email. View the post in your Web browser.]

 

[This is an old thread, but since search digs up old threads, it's worthwhile to have a correct answer.]

 

"...all my photos have the same aperature and shutter speed, but they differ in their ISO. ... I think the adjustment in EV units would go something like this: exposure_adjustment = -log2(ISO/baseISO)."

 

LR's Photo > Develop Settings > Match Total Exposures command makes precisely this adjustment -- given a selection of photos, it adjusts the Exposure slider so that the effective exposure value of all the photos matches that of the most-selected photo. If the photos' original exposures differed only by ISO, then the adjustment to the Exposure slider will be given by the expression - log2 (ISO / most-selected ISO).

 

As an example, I set my Sony RX100 V to "M" and Auto ISO and took four photos (ordered here by increasing exposure value):

 

Untitled1.png

 

Then I selected those photos, with the last (the one with the highest EV) as the most-selected, and did Match Total Exposures:

 

Untitled2.png

 

The first three photos got darker, as expected.

 

This table shows that LR adjusted the Exposure slider for each photo by - log2 (ISO / 2000):

Untitled4.png

"Exposure 2012" is LR's internal name for the Exposure slider in later process versions. (You can use Any Filter's Sort command to extract these fields into a spreadsheet, if you want to experiment for yourself.)

 

[Use the blue reply button under the first post to ensure replies sort properly.]

 

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