3

If I open a raw file in Photoshop without modifying the raw exposure and compare it to the same file with a raw exposure modification of -0.2; when I inspect the linear values it seems like a different mathematical operation has been applied to each pixel. What is it actually doing? I expected a linear scaling. If I then use the adjustments->exposure to +0.2 I don't get back to the same values why?

What are each of these exposure adjustments actually doing?

New contributor
PhotoNoob is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
  • Not sure, but I belive is a multiplication by a factor of 2 ^ x. Where x is the value you give. So, for example, +0.2 EV takes the value of each pixel, and multiplies it by 2 ^ 0.2. – vsis Mar 24 at 18:18
  • This seems to be the case for the adjustments->exposure version. I see r=146,g=5,b=14 go to r=167,g=5,b=14. But if I do the same thing with the raw exposure slider the numbers are r=168,g=6,b=15! – PhotoNoob Mar 24 at 18:45
  • When using the raw exposure slider what are the minimum and maximum values displayed? 0-255 or 1-256? How about when using adjustments → exposure? In your example, each "raw exposure" value is +1 compared to the "adjustments → exposure" value. – Michael C Mar 25 at 21:11
  • Thanks Michael. It does look like that from these numbers but I was only doing a 0.2 adjustment here. If you increase that to 0.7 or -0.7 they are no longer off by 1. – PhotoNoob 2 days ago
0

I found this answer.

It seems the formula is something like: newValue = oldValue * (2 ^ exposureCompensation);

So, for +0.2 EV each pixel is multiplied by 2 ^ 0.2 = 1.1486.

0

While I don't have the code of Photoshop I just can guess.

Exposures are given in logarithmic units, which makes them not so easy to predict as a linear value you may expect. Moreover, Adobe probably applies a gamma curve or preset or allows/prevents posterisation, arithmetic conversion of bits for the screen, etc...

So given log(relative ev)= log(ISO value)+log(aperture)+log(shutter speed)+log(luminance) +other values the software takes into account, you're expecting a value different of calculated. After decreasing/increasing the exp value in photo, some darker/brighter pixels may have clipped, adjacent pixels are changed according to colour gamut, etc... Color also changes in logarithmic values.

Also, supposing you don't have photographic values of shutter speed, aperture, etc... (Some jpg files) when you apply the formula expo=evVal*(2^NewEV) you do it to luminance calculation of the RGB, and all changes are converted back to the pixel in RGB.

Your Answer

PhotoNoob is a new contributor. Be nice, and check out our Code of Conduct.

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.