What does the Gradient Map actually do?

[pflower]

I have been scanning a whole range of old negatives.  These come out of the scanner somewhat flat so as to capture all the information.  I have stumbled across a technique in PS - adding an adjustment layer of a gradient map with a black to white gradient.  The result is greatly improved contrast.  What I can't find out is exactly what the gradient map is doing and how.

Can anyone give a concise explanation and point me in the direction of editing the map to particular images.

Thanks

[walter.sk]

As far as I can tell, the Gradient Map takes the foreground color and background color, forms a smooth gradient, and maps it to the brightness levels  of the image.  While black and white works well, I have fun with complementary colors.  You can play with the brightness and saturation of the colors you select for foreground and background.

I suppose the reason your relatively flat-looking scans become more contrasty is that the lowest value of your image gets mapped to 0,0,0, and the highest, to 255, 255, 255.  Try scanning at even lower contrast, and use the Info panel to find the brightest and darkest values.  Then do the same after applying the Gradient Map.

You can control the ultimate range of levels by using the color picker to limit the white to less than 255 and/or limit the black as well.

[rovanpera]

If you're not tinting the BW photos, you could just use curves instead of gradient map...

[cyeh01]

Here are some links to gradient maps that I discovered when doing my research on them

http://www.peachpit.com/articles/article.aspx?p=708297
http://www.devco.net/archives/2003/04/20/color_to_black_and_white_using_gradient_maps.php

Cheers!

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Charles Lucima
www.lucima.com
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