[Leica] Tip: More local constrast control in PS

[Martin Howard <mvhoward@mac.com>]

I tried to go to sleep last night, but was abruptly awoken when my 
brain figured out yet another way to do local contrast control in 
Photoshop.  It seems to work relatively well, so I thought I'd share it 
with the group.

The problem is this.  You have a scanned picture in which some region 
needs greater contrast between the tones.  A face, for example.  What 
you want to do is make the light parts lighter and the dark parts 
darker -- rather than just straight dodging or burning.

Again, "Soft Light" comes to our rescue.  Blending a layer by "Soft 
Light" to an underlying layer dodges the underlying layer where pixel 
tone is more than 50% in the blend layer, and burns the underlying 
layer where pixel tone in the blend layer is less than 50%.

It occurred to me that rather than use a paint brush to create the 
blending layer, you could use a copy of the image layer itself.  In 
order for this to work, you'd need to "recenter" the tonal values 
around 50% grey, so that you use the relative tonal differences to 
control dodging and burning.

For example: If the tones in your face span, say, 66% black to 83% 
black, it's going to look flat.  However, even the "shadows" in the 
face (at the 66% level) are above 50%, so a copy of the image cannot be 
used directly as a blending layer -- because it would only lighten the 
tones in the face.

Enter "Levels" adjustment layers.  The output sliders are the key to 
"recentering" the tonal values of an image around 50% grey (level 128). 
  With those, and the input sliders, you can exert very fine control 
over how much you increase the local contrast.

Step by step, it looks like this:

1) Using the Lasso tool, select the region of the Background layer that 
you wish to modify contrast in.  Feather the selection a bit (I used 20 
pixels for a 300dpi 8.5x11" image). Save the selection to a new 
channel.  Copy the (image) selection to the clipboard.

2) Create a new (standard) layer.  Paste the clipboard to this layer.  
Load the selection from the channel you just created, invert, hit 
delete (i.e., make background in this layer transparent), invert.  Lock 
the movement and transparency of this channel.  Set blend mode to 
"Normal" and opacity and fill to 100%.

3) Load the selection mask (if ncecessary).  Create a new Adjustments 
Layer->Levels.  Group this with the previous layer  (the one you 
created in step 2).  Set blend mode to "Normal" opacity and fill to 
100%.  For now, just click "OK" in the Layers dialog box and leave this 
layer alone.

4) Create a new layer set from the two previous layers.  Set blend mode 
to "Soft Light" and opacity to 100%.

5) Go back to the Adjustments Layer you created.  Double-click on the 
half-filled circle icon (the layer thumbnail).  This brings up the 
Layers dialog box.  Since you're using a portion of the image, you'll 
probably not see a full histogram, but it will be scewed towards on 
end.  Pull the input sliders so that they are centered around the peak 
of the histogram.

6) Now move the output sliders of the levels dialog.  Center them 
around 128 (I use [128-30] 98 and [128+30] 158 as starting points).  
Your image copy, along with the levels adjustment curve, is now a 
doding and burning mask.  Play with the input/output sliders in the 
dialog box until the image you've got has the local contrast you want.  
Moving the shadow output triangle darkens or lightens the shadows.  
Moving the highlight output triangle darkens or lightens the highlights.

Voila!  You can fine tune by playing with the opacity in the layers set.

You can also use the gamma slider (middle triangle in Levels dialog) to 
change the balance between highlights and shadows.

You don't have to create a layers set (although it's convenient), in 
which case you need to change the blend mode of the image copy layer to 
"Soft Light" (and possibly also the adjustments layer... though I don't 
think so).

Also, as step 2.5, you might want to blur the image copy a little.  It 
seems to smooth out the tonal range and limit the amount of 
posterization that takes place.  Use Gaussian blur.  you'll have to 
play around with the values, but you're aiming for a level of blur 
where you can make out the image (i.e., some detail remains), but it 
has soft transitions between highlight and shadow.

Together, there are a lot of controls you can manipulate to affect the 
image, but the neat thing is that your original image is unaffected: 
you're just using the image copy and levels adjustment layer as dodging 
and burning masks.  As such, it is always possible to go back to what 
you had before.  By clicking on and off the layer set, you can make 
before/after comparisons with your original image.

M.

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