Auto Contrast Setting

GB
Posted By
George_Butch
Nov 21, 2006
Views
542
Replies
6
Status
Closed
Is there a way within PS-CS2 to apply Auto Contrast to an entire image but have the calculation based on a selected area? I often use Auto Contrast as the first step when working on an image. There are times I would like to ignore areas of the image that are already at brightest white.

I know that when using the levels dialog, for example, it is a simple matter to to select an area, apply an automatic adjustment, save that level setting, and then apply it to the entire image. I would like to be able to likewise with Auto Contrast.

Any ideas?

Thank you.

George Butch
Plantation, FL

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Mathias_Vejerslev
Nov 21, 2006
George,

In the Levels dialog, click the Options button and choose the first option, Enhance Monochromatic Contrast. This is Auto contrast. Save your settings as you would with any other Levels adjustment, and apply.

But you might want to play with the clipping percentages instead of applying Levels to a selection.

This is in the help file, BTW.
GB
George_Butch
Nov 21, 2006
Thank You Mathias,

Every time I come to this forum looking for help, someone points the way.

Be well.

George…
C
chrisjbirchall
Nov 21, 2006
I often use Auto Contrast as the first step when working on an image.

Hi George.

I wonder if you’ve ever considered another approach to your workflow.

It seems a same to carry out a "destructive" adjustment so early on in the editing process. If you just want to rush through some small images, then fine. But if you are seeking the very best quality, there is another way.

First let me explain what I mean by "destructive".

Any of the adjustments, Levels, Curves, Contrast, Hue/Sat, etc., actually harm the image as they work. To illustrate this, open an 8 bit file, do a Levels adjustment and then open the Levels dialogue a second time. See how there are now "Gaps" in the histogram? These, more or less, represent data which has been lost during the previous adjustment. Clearly, if you make repeated adjustments to the image, these losses will be accumulative.

So how can we get around it?

One way is to work on 16 bit images. The losses still occur, but are less problematic because you are starting out with significantly more data.

Another way – and arguably the very best method, of course, is to shoot RAW files and do all your white balance, and tonal adjustments before you ever get the file into Photoshop.

Other than that you could use "Adjustment Layers". These leave the original pixel data unchanged in the image layer(s) beneath them – and you can safely keep going back to tweak the settings.

You will have observed by now that there is no such thing as an "Auto Contrast" adjustment layer. In fact, some question why any of these "auto" adjustments have been included in a professional imaging app such as Photoshop in the first place. (It’s a bit like having a super hi-tec kitchen and only cooking ready meals in the microwave!) But there are some high volume workflows in which the can be of use.

If you still need to make your tonal adjustment at the start of the editing process, simply create a Levels adjustment layer and drag the highlight slider in to the top edge of the histo and the shadow slider in to the bottom end. This will give you an initial "contrast" setting almost identical to that achieved with "auto contrast". The difference is, of course, you can keep adjusting it as you work to obtain the very best result.

I do hope this will be of some use to you.

Chris.
GB
George_Butch
Nov 22, 2006
Thanks Chris,

In fact I do shoot Raw and do nearly all of my editing at 16 bit. I also never make any changes to the original file. I am always working with a copy. Further I make almost all of my global adjustments with adjustment layers.

The rationale for the auto contrast adjustment is to simply start with the image filling the entire dynamic range. Even that is nearly always done on separate layer.

I realize that PS CS2 offers a lot of new options for doing things. Most of my working habits were developed over several years of using PS 6.

I am most appreciative of any and all advice.

George…
C
chrisjbirchall
Nov 22, 2006
The rationale for the auto contrast adjustment is to simply start with the image filling the entire dynamic range.

By shooting RAW and opening the files in 16 bit mode, you are getting the most out of the 12 bits per channel captured by the CCD.

Adjusting the dynamic range in ACR will do a far better job than auto contrast for setting your starting point (make sure "Auto Correct" is OFF by default).

The correct procedure is to carry out the Highlight adjustment first followed by the Shadow slider, to set the upper and lower clipping points. Then adjust the Brightness and Contrast sliders for the desired tonality.

Also, explore the "Curves" setting in ACR for fine tuning the contrast.

I find that the bulk of my image optimization is now done in ACR, before moving into Photoshop for retouching etc.

This is also the philosophy behind Adobe’s soon to be released Lightroom.

Have fun!
MD
Michael_D_Sullivan
Nov 22, 2006
Or, if you have a powerful enough computer, dowload the beta of Lightroom from labs.adobe.com and have a blast!

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Learn how to rescue details, remove flyaways, add volume, and enhance the definition of hair in any photo. We break down every tool and technique in Photoshop to get picture-perfect hair, every time.

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