Ember Razement ________________________________ The Fine Art Photography of Malcolm Smith

 

NOISE CONTROL AND SHARPENING

PART C – SOME OTHER USEFUL TOOLS AND POINTERS

LINK TO LINK TO
PART A - Background Theory
PART B - Sharpening Tools Available
PART D - Edge Masks
PART E - Real World Sharpening Examples

In Part C we look at additional techniques for enhancing local contrast and texture,  “brushing in” these changes and some ideas and tests to help control sharpening.

CONTRAST ADJUSTMENT WITH SHARPENING TOOLS

When the sharpening radius gets bigger the effect is to increase the local contrast of the image.  By local contrast I mean that, for example the contrast of blades of grass, twigs, bark texture, hair etc. can be increased and suitable use of this effect can be magic.  You can look on the effect as increasing local contrast or enhancing texture.  This process can be used with either Unsharp Mask or High Pass sharpening.  Try the following settings

  • With Unsharp Mask: set Radius 20 to 50 pixels, Amount 20% and threshold 0.(Use Luminosity Blend Mode if colour changes a problem)
  • Alternatively use the High Pass filter.  Create a new layer, set the Blend Mode to Soft Light or Hard Light and run high Pass filter with radius 20 to 50 pixels.

Some articles describe this as when the Radius gets bigger the halos eventually merge but this is doublespeak.  Here is my explanation (you may need to review my notes on how Unsharp Mask and Gaussian blur works).  Consider the picture image as a three dimensional surface with height being image intensity - hills for the brighter bits and valleys as the darker areas.  As in the real landscape the average height of the land is higher above sea level in some areas and not so high above sea level in others so we can imagine for our picture an average local height of the image surface.  This average height of the image surface is calculated in Photoshop by Gaussian Blur with a wide Radius (a Radius of 50 pixels looks at the image values 3 x 50 = 150 pixels on all sides of the pixel being currently blurred.).

Then the Unsharp Mask is calculated by subtracting the heavily blurred image from the original. This Unsharp Mask will be positive where the image surface is above the average and negative when below.  It is in fact, the variation above and below the average.  If this Unsharp Mask is added at low amount (eg Amount := 20% or K := 0.2) to the image surface we get a contrast enhanced image.  Where a pixel is brighter than the local average it becomes a bit brighter and conversely when darker than the local average it gets a bit darker.  This is probably not an easy concept to understand. But don’t worry, you really don’t need to understand how it works - just try it to see the effect.

When I compare the three options in tests I find Unsharp Mask with Radius 50 and Amount 30% to be around the same as High Pass Radius 50 and Soft Light blend Mode.  With Hard Light Blend Mode the effect is like Unsharp Mask with Amount around 100%.  I usually use High Pass with Radius 50 pixels and Hard Light Blend Mode to get a very aggressive contrast enhancement and brush in the enhanced effect with a soft low opacity black brush as described later in this section to control the amount.

This technique is also useful for combating haze and lens flare.  If these contrast adjustments on a layer cause blocked up highlights or darks they can be retrieved by using the blend If sliders,

LUMINOSITY BLEND MODE

Another problem sometimes encountered with sharpening is colour artifacts (changes) resulting from the sharpening process.  This can be fixed by sharpening on a layer and setting the Blend Mode to Luminosity which will prevent sharpening colour changes.  This can also be done in the Layer Style Dialog box (double click on the Layer thumbnail to bring up) but also can usefully be set with the Edit>Fade “filter” command

L*a*b SHARPENING

A method I once used which is often described in books and magazines etc.and which is widely used is to change the image Mode to the L*a*b colour space and sharpen the L or Luminance channel only, then convert the sharpened image back to RGB mode.  I now use the change Blend Mode to Luminosity approach as going back and forth to L*a*b mode slightly degrades the image each time it is done (rounding errors) and is often inconvenient with many layers (which I like to keep so I can go back and redo).

SHARPENING A CHANNEL

As I described in L*a*b Sharpening if you are in RGB Mode it is possible to sharpen an individual Channel.  To do this go into the Channels Palette and select a Channel (R or G or B) which has the best contrast for sharpening, target it by clicking on the Thumbnail and Sharpen it with USM.  You should also set the Layer Blend Mode to Luminosity or you will get colour changes in the Halos.  The overall effect is subtler than sharpening all channels.  You could also try sharpening more than one channel separately.  This is not something I generally do but keep it in mind.

CONTROLLING THE LIGHT AND DARK HALOS SEPERATELY

To do this duplicate the Sharpen Layer and set the Blend Mode of one to Lighten (which only allows the lighter halos to be seen) and the other to Darken (which allows the darker halos to be seen).  Then to play off the lighter halos against the darker is only necessary to adjust the appropriate layer Opacity.  Keep in mind that the object of sharpening is to have these halos so don’t reduce the opacity too much.  Again this is not something I do often as I can usually target and tone down a bright halo by graying the edge mask (see edge masks below).

BRUSHING SHARPENING AND SMOOTHING INTO PARTS OF AN IMAGE
 
A great way to apply some of these tools (Some sharpening, smoothing and contrast expansion for example but also intensity changes using curve layers etc) is to create the effect on a new layer with a hide all mask and “brush in” the amount and in the areas required with a soft white low opacity brush on the mask.  To do this:

  • First create a new layer and apply contrast enhancement, Sharpening or Smoothing (blur) as required – you can afford to be a bit heavy handed at this stage.
  • Add a Hide All layer mask (Black) ( <ALT Click new mask icon>.  The new mask icon can be found at the bottom of the Layers Palette)
  • Using a low opacity (say 20% to 50%) soft brush and gradually paint in the effect with white on the mask (click on the Layer Palette mask to make it active but paint on it over the large image).  This can be reversed by brushing the mask with black brush.  Use keyboard shortcut “D” to get the white/black colours set up and “X” to swap them

You can also try with this technique:

  • Setting the Blending Mode to Luminosity, reduce the Opacity and use Blend If sliders to protect the highlights and shadows
  • For smoothing add some Gaussian Noise (clear the monochrome check box  for a colour image) with settings around 2 to 5%.
  • An edge protecting mask can be used but needs to be applied before the brush mask can be added.  To do this first create the edge mask then use LAYER>LAYER_MASK>APPLY which applies the edge mask to the image layer and then deletes the applied mask leaving the image free to have another mask (the “brush in” mask) added.

ADDING EXTRA TEXTURE

A layer can be created with repeated application of Unsharp Mask  that builds up mild texture into something very spectacular and which can be brushed in with care for creative effects.  Bruce has some suggestions in his book (and which was apparently suggested to him by Jeff Schewe) and other references have variations.  Read Bruce’s book or try:

  • On a new layer with the image first apply Unsharp Mask with amount 100, Radius 0.5 Threshold 0 then Edit Fade 100% and Luminosity Blend Mode.
  • Apply Unsharp Mask with amount 500 Radius 1 Threshold 0 then Edit Fade 50% and Luminosity Blend Mode.
  • Again apply Unsharp Mask amount 500 Radius 2 Threshold 0 then Edit Fade 25% and Luminosity Blend Mode.
  • Unsharp Mask amount 200 Radius 4 Threshold 0 then Edit Fade 12% and Luminosity Blend Mode.

I do not use this technique very often – I usually find the contrast increasing technique with wide radius and low amount mentioned previously to give a satisfactory texture enhancement.

THRESHOLD TEST

A useful technique for examining the highlights and how close they are to clipping is to insert a temporary reduced opacity threshold adjustment layer at the top of the Layer Stack as in the following illustration.

Threshold Test Demonstration

Threshold Adjustment Layer Test

Here the threshold has been set to 240 and the white lines are those with values greater than 240.  The non white lines are the parts of the image below 240 which are seen because the Opacity of the Threshold Layer is 55%.  The Sharpen was Radius 2, Amount 300 and Threshold 0.

There are two main areas where I use this technique:

  • I examin the image for clipping as illustrated above; and
  • To enable the highlights to be tuned for the final print.  By setting up a threshold layer with reduced opacity you can use a curves layer and the image tuned to, say, get the highlights just hitting 245.

 

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LINK TO LINK TO NEXT
PART A - Background Theory
PART B - Sharpening Tools Available
PART D - Edge Masks
PART E - Real World Sharpening Examples