Brightness/Contrast Control in LR3 and PS CS5

Started Jan 4, 2011 | Discussions thread
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Hugowolf Forum Pro • Posts: 12,671
Re: Brightness/Contrast Control in LR3 and PS CS5

Jsimmons wrote:

Thanks for the support. I don't use "Auto" either (but I wouldn't be ashamed if I did...) but I am really interested to understand what the relationship is between Brightness and Contrast on the one hand and Black Point, Clarity, Tone Curve, etc. on the other.

Most of the ‘don’t use contrast and brightness’ advice was for those controls in Photoshop prior to CS3.

In CS2 and earlier versions both these algorithms were very simple. Reducing contrast increased the value of any pixel with a value less than 128 and decreased the value of any pixel with a value of more than 128, If you decreased the contrast to zero, you ended up with the whole image a uniform mid gray. If you pushed contrast to 100, then all the pixels ended up as pure black, white, red, green, blue, yellow, cyan, or magenta.

Brightness was an even simpler algorithm. It added the slider value to all three color components. So if you had RGB(0, 128, 255) and pushed the slider all the way to the right (100), then you ended up with RGB(100,228,255) {can’t exceed 255 with 8 bit values]. If you pushed the slide all the way to the left (–100) you would have ended up with (0,28,155).

You can still use these older algorithms, by checking the Legacy box in the brightness and contrast dialog.

The newer algorithms that both PS and LR use, are more complex and much less linear. Brightness adds less to higher values than it does to lower values; in effect reducing the onset of clipping.

You can see the effects of the LR basic controls and the tone curve control, by watching the tonal histogram as you make adjustments. For example, black point clipping does just that, it increases the dark tones that get clipped to pure black.

Brian A

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