Adjusting exposure in ACR/PS

D Fosse and Stephon,

I did have another technical thought  you can  comment on.  I have always been unsuer about was how photoshop was handling 16 bit data.  In this exposre thing used Python scripts to generate  files, either tif or JPEG.  That way I could do experiments writing different bit lengths, I could put linear or gammed data in. I could also  assign profiles or not. Then I could read them in PS, view them, change or assign profiles, read the pixels values in PS and then save the files and re-read them in Python. 

OK, that confirmed for me what the slider was do exactly as you two were explaining.  It appears to me that  hotoshop applies gamma when you read the file into a color manage color space.  If you leave unmanage or use custom space with gamma=1 it stays linear. 

The problem sensors and the raw is usually less than 16 bits. Was twelve and now I think most are 14. I think my R5 is fourteen but not even sure. 

The problem is if you you apply gamma you loose information and add noise when you don't need to. If that isn't clear I can give you two lines of simple arithmatic example that shows.

Now converting from 16 to 8 for JPEG you do want gamma. Your are throwing away huge amounts of information  and gamma takes advantage of human percept to shape that loos to minimize  visual impact. 

The anser is not that you are converting raw into a color space that has a gamma like virtually all do. It is trivial to apply transorm to convert that space to a tempory linear space that you  worrk in.  You can then work in that space all you want.  Then at only last possible instant convert that linear space back to the gamma space. 

I have one thought on why may want to pay the price of going straight to a gamma space at the  price of adding some noise up front.  But I would like your thoughts first..

Jay