Exposure and histogram stretching

PE
Posted By
phoney.email
May 26, 2004
Views
986
Replies
0
Status
Closed
PS 6. Somewhat theoretical questions but I hope someone responds.

Using Photoshop, how do I emulate image (histogram) changes resulting from scanner exposure boost? This is not as simple as it may appear:

I ran some tests with my film scanner by increasing exposure by a fixed amount (0.1 ev increments). I then plotted the pixel value increase against ev increments and the result, instead of a straight line, was a slightly upward sloping curve. In other words the histogram did not just move to the right from scan to scan but was actually "stretched" (as expected). Pixels on the left "moved" more slowly than pixels on the right i.e. the distance between darkest and brightest pixels increased with exposure boost (more dynamic range).

Taking the baseline scan (before exposure increases) and applying brightness to it in Photoshop resulted in a different histogram. This time the whole histogram just shifted to the right, but the distance between the darkest and brightest pixels remained the same (same dynamic range).

So, how do I emulate this non-linear exposure histogram "stretching" in PS? I know, "use curves", but how exactly?

And while we’re at it, given an arbitrary starting pixel value and a specific EV exposure increase applied to it, does anyone happen to know the formula to calculate the resulting pixel value?

Don.

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