ADVANCED COMPOSITING & RETOUCHING - Chapter 5, Exercise 2
What Goes Wrong When the Blur Radius Is Set Too Large
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QUESTION:
Explain what goes wrong if the Gaussian Blur radius used to build the Low
Frequency layer is set far too large for the texture you're actually trying
to preserve.

SOLUTION / EXPLANATION:

The Gaussian Blur radius determines the boundary between what counts as
"tone" (goes into the Low Frequency layer) and what counts as "texture"
(goes into the High Frequency layer, since it's whatever the blur removed).
Setting the radius far too large means the blur smooths away far more than
just the fine texture - it also erases genuine, larger-scale tonal
transitions that were never meant to count as texture at all, such as a
real shadow edge, a natural highlight, or a gradual color transition across
a cheek or a piece of fabric.

Once the blur has removed those genuine tonal transitions, they don't
vanish from the image - they get pushed into the High Frequency layer
instead, since the High Frequency layer is mathematically defined as
"everything the blur took out." The practical result: real tonal
information that should have stayed on the Low Frequency layer - and should
have been available to smooth or adjust as tone - is now sitting on the
High Frequency layer, mixed in with the actual fine texture detail.

This causes two concrete problems during retouching:

  - Retouching the Low Frequency layer for tone no longer has full access
    to all the tone that's actually present in the image, since some of it
    has been misclassified as texture.
  - Retouching or removing texture on the High Frequency layer risks
    accidentally erasing real tonal transitions that got mixed in there by
    mistake - for example, smoothing out what looks like "texture noise" on
    the High layer might actually delete a real shadow edge, leaving a flat,
    physically implausible result once the two layers are viewed together
    again.

The fix is choosing a blur radius closer to the actual physical size of the
fine detail you want to preserve as texture, so the split boundary falls in
the right place - between real texture and real tone - rather than
accidentally sweeping genuine tonal information into the texture layer.

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WHY THIS WORKS AS AN ANSWER:
It explains the blur radius's actual role (defining the tone/texture
boundary) rather than treating it as an arbitrary setting, traces
specifically where the misclassified tonal information ends up (the High
Frequency layer) and why, and names the concrete retouching problems that
follow from getting the boundary wrong.