Archived posting to the Leica Users Group, 2003/03/11

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Subject: Re: [Leica] Help: Contrast management in Photoshop
From: Johnny Deadman <lists@johnbrownlow.com>
Date: Tue, 11 Mar 2003 15:14:28 -0500

On Tuesday, March 11, 2003, at 02:44 PM, Martin Howard wrote:

> Occasionally, I need to increase contrast locally in an image.  For 
> example, the face in an environmental portrait may be a bit flat, i.e. 
> it only spans a few levels from the darkest to the lightest pixels.  
> In a darkroom, you'd fiddle this with dodging and burning typically 
> combined with a different grade contrast filter on VC paper.
>
> However, in Photoshop, I'm lost.  What I'd ideally like to do is get 
> Photoshop to do is two things: pull apart the end points in a local 
> region (masked off by a selection) and then "fill in the blanks" in 
> the resulting histogram so that I get a smooth tonal gradation and no 
> posterization.
>
> Is there any way of accomplishing this?

Well, sort of. Here's one way:

1.	select the area that you want to affect using the lasso tool

2.	Do select->feather selection with a radius of 50-200 pixels 
depending on how gradual you want the edges of the effect to be

3. 	Layer -> add adjustment layer -> curves (or levels). This will add 
an adjustment layer correctly masked so that only the bit of image you 
selected is adjusted.

4.	Monkey with the tonality and apply the changes

5.	If you suffer posterization there is really no way to smooth it out 
again without losing detail. The solution is to convert to (or work in) 
16-bit mode, which means rather than using an adjustment layer you have 
to skip step 3 and apply the changes using levels or curves directly.

There is another way of doing this which is more subtle:

1.	Roughly select the area you are interested in

2.	Apply tonal changes with the levels or curves tool (don't worry 
about the sharp edge that results, we'll fix that).

3.	Click on the previous state in the history palette, which reverses 
your changes

4.	Click on the little box next to the modified state in the history 
palette to set this as the source for the history brush

5.	Use the History Brush to paint in your tonal changes. Use 25-50% 
opacity and an airbrush or something similar. This is a very powerful 
technique. You are painting with a FUTURE state, so the 'history' brush 
is a bit of a misnomer, but hey.

> I suspect there is -- because I've noticed something else interesting. 
>  I've taken to do a quick preview of my scans using "Preview" (the 
> Apple application bundled with OS X).  It manages to do something 
> remarkable: When you first load an image, it looks grainy and harsh, 
> with semi-posterized areas.  But after about a second of calculations, 
> it smooths out the tonal transitions, seemingly without loosing 
> detail.  The result is something that looks a hell of a lot more 
> photographic than it did originally.  Since this is step two of the 
> process outlined above -- and something that I'd love to be able to do 
> in PS -- how is it accomplished?

This is just anti-aliasing (in Preview's preferences there is the 
option to turn it on or off). The nearest thing in Photoshop is a 
Gaussian Blur with a very small radius of say 1-2 pixels. It actually 
destroys fine detail but removes jaggies in things like rendered text.

Hope this helps.
- --
John Brownlow

http://www.pinkheadedbug.com

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