Auto Levels and auto contrast in CS2

Chicagophotoshop · Feb 9, 2007

would people agree or disagree about this doing the trick?

I have been doing and it seems ok. not for all photos, but most.

thoughts?

Arch · Feb 9, 2007

Some people frown upon using auto levels... if you need to learn how to use levels, i would say try and not use it. The more you practice with using levels the better. However once you are a confident user, there's nothing wrong in seeing what PS thinks the levels should be set at.... it can save time in the long run.

I would never use auto contrast on the other hand.... because i dont like using the contrast bar period. You get far more control by using a curves adjustment instead.

Chicagophotoshop · Feb 9, 2007

yea I agree about the auto contrast. that didnt seem to work well. but the auto levels seemed ok.

what/where is the curves adjustments?

Arch · Feb 9, 2007

you can go to Image > Adjustments > Curves.... or alternativly click the Adjustment layer button on the bottom of the layers pallet and choose curves to make the adjustment on a seperate layer.
Its best to find a 'Curves' tutorial via google to learn how to use it, as its hard to explain.... but basically inserting a point near the top of the diagonal line changes the highlights.... one near the bottom changes the shadows. Making an 'S' shape is the best way to improve all round contrast.

ksmattfish · Feb 9, 2007

Auto levels is the same as going into the individual RGB channels in levels, and sliding the highlight and shadow sliders to the ends of the histogram in each channel. It's making the darkest pixels 0, and the lightest pixels 255 in each channel.

Auto contrast deals with a luminosity histogram (all three RGB channels averaged together). It's the same as sliding the shadow and highlight sliders in a little past the outside edges of the luminosity histogram. It makes the almost darkest pixels 0, and the almost lightest pixels 255.

Levels allows you to adjust tones by setting 3 points (shadow, middle, and highlight), and then it averages out the histogram/tones between where you set the points. Curves allows you to set and adjust as many points as you want. With this you can have more control how PS is averaging the histogram between the shadows, and the midtones, and the highlights.

I set my curves display up so the shadows are in the lower left corner, and highlights in the upper right corner. I switch the grid to 10 x 10 squares (press alt and click on the grid, or something, to do this) so it's like the 10 zones of the zone system tonal scale.

When you start out the line runs straight diagonally from shadows to highlights; every point on the line corresponds to the same tone on the tonal scales running on the left side vertically, and the bottom side horizontally. If you take the cornermost point in the shadow corner, and slide it to the right horizontally, that's the same as sliding the shadow pointer to the right in levels. If you take the cornermost point in the highlight corner and slide it to the left horizontally, that's the same as sliding the highlight pointer to the left in levels. You can put a point, or points, anywhere on the line. Moving the point up makes that tone lighter. Moving the point down makes that tone darker. Leaving the point in the middle on the center diagonal is leaving the tone as it was when curves was opened. The curving line is a graphic representation of the computer averaging the tones between the set points.