16-bit per channel Brightness Adjustment Bug?

CB
Posted By
Carl_Bahor
May 5, 2005
Views
444
Replies
7
Status
Closed
Hi,

I am trying to make very fine brightness adjustments to a 16-bit per channel image, but photoshop CS2 seems incapable of doing that. It seems that even in 16-bit per channel mode when using the Brightness/Contrast adjustment a -1 change in brightness equates to an 8-bit per channel change. For example a -1 brightness change makes a RGB value change from 27493 to 27365. 27493-27365 is 128. 32768 (white) divided by 128 equals 256 steps (8-bit). Therefore even though I am in 16-bit per channel mode I can only make 8-bit per channel adjustments. Is there anyway to make one step brightness/contrast changes in 16-bit per channel mode(ex. 27493 to 27492)? Any help would be appreciated.

Carl
Related Tags
16-bit adjust bit bright bug channel

Master Retouching Hair

Learn how to rescue details, remove flyaways, add volume, and enhance the definition of hair in any photo. We break down every tool and technique in Photoshop to get picture-perfect hair, every time.

PF
Peter_Figen
May 5, 2005
The Brightness/Contrast tool should have been excised from the years ago. Not only is it not very precise as you can see, it’s also a linear adjustment, which can wreak havoc on your images. Far better to use Curves, which gives you infinitely better control over all parts of your image, and can be manipulated via numeric inputs below the standard dialog box when desired.
TR
The_Rock
May 5, 2005
Peter is absolutely correct … Curves manipulation is the way to go regarding your issue …

Vern

wrote in message
The Brightness/Contrast tool should have been excised from the years ago.
Not only is it not very precise as you can see, it’s also a linear adjustment, which can wreak havoc on your images. Far better to use Curves, which gives you infinitely better control over all parts of your image, and can be manipulated via numeric inputs below the standard dialog box when desired.
CC
Chris_Cox
May 6, 2005
Peter – can’t get rid of it. Ilustrators (and comic book artists) make heavy use of Brightness/Contrast.

Carl – no, we don’t have anything with that fine a control in Photoshop currently.
CB
Carl_Bahor
May 6, 2005
Hi,

Thank you both for your responses!

Peter, I am actually having some luck with your suggestion of using curves instead of brightness/contrast. It allows me some fine contrast adjustments by using it multiple times. First time to get it in the ball park and then again to start zeroing it in. Thanks!

Chris, thank you for responding as well. While I’ve got you, so to speak, I am curious why in the Photoshop info tab it shows 16-bit images as having a value range of 32768-0 which is only 15-bit (actually shouldn’t 15-bit be 32767-0, 32768 values?) rather than 65535-0 (16-bit). Are the values just interpolated or are they actual values?

Thanks again,
Carl
CC
Chris_Cox
May 6, 2005
15 bit would be 0..32767

They show as 0..32768 because they are represented in Photoshop as 0..32768. Using a power of 2 representation greatly simplifies the math (and speeds up 16 bit operations), plus improves a number of lookup table operations because the representation has a real middle value.
Yes, you are seeing the actual values.
CB
Carl_Bahor
May 7, 2005
Ah, the balance between mathmatical correctness and speed. That makes more sense now. Thanks for your insight.

Have a good weekend!

Carl
PF
Peter_Figen
May 7, 2005
Hadn’t consider the comic book think, but it makes some sense I suppose.

How to Improve Photoshop Performance

Learn how to optimize Photoshop for maximum speed, troubleshoot common issues, and keep your projects organized so that you can work faster than ever before!

Related Discussion Topics

Nice and short text about related topics in discussion sections