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I would like to know how the Photoshop Exposure tool works (In CS4 it's Image -> Adjustments -> Exposure...)

There are three sliders: Exposure, Offset and Gamma Correction. I am interested only in the Exposure slider, ignoring the other two. It seems like it tries to simulate the behavior of exposure value of a camera: http://en.wikipedia.org/wiki/Exposure_value

Does it take a value x > 1 and the EV from the slider, and multiplies all color channels by x^EV? I am making a program that adjusts exposure and this approach seems close enough. But isn't there something else?

Thanks for your answers.

Michal Ferko
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  • It looks to be explained and defined in the Wikipedia article you linked to... – Cody Gray - on strike Dec 20 '11 at 09:17
  • the wiki article keeps talking about camera settings, but the photo to be modified in photoshop does not need to have any information about camera settings. Increasing EV by one doubles the exposure time, but does that mean that all pixel values are multiplied by 2? I don't think so. – Michal Ferko Dec 20 '11 at 09:40

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Exposure slider will change the brightness (luma channel) of the image relative to RGB = 0,0,0 by altering - the darkest point - brightest point - and all the points in between

If the darkest point in the image is black (RGB = 0,0,0) then that will nto change but every other point will.

The latitude (range between darkest and lightest) will also change, increasing latitude as you brighten, decreasing latitude as you darken.

The best way to see this is to create a simple jpeg with a black band a white band, a dark grey band and a light grey band. Make sure there is no color information (Command-shift-u). Make the Histograms pane visible, add an exposure adjustment layer, move the slider and watch the histo values change.

PrimeLens
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