Unsharp Masking at 50%?

Dan Margulis <76270.1033@...>

Russell writes,

<<I realize that it's generally recommended to perform Unsharp Masking with
an image viewed at 100%. But if the image's resolution is 2x the screen
frequency (for offset) then what's wrong with making USM decisions based on
viewing at 50%?>>

You would then be looking at a crude RGB downsample to predict a very
complicated CMYK averaging process.

When you ask for a screen preview at 50% you ask Photoshop to present a
preview where one screen pixel is based on an average of the RGB values of
four actual pixels. This is loosely similar to what would happen in print,
where each dot is produced from, approximately, an average of four pixels.
However, it's quite inaccurate, because,

1) In print the averaging is done per channel whereas on screen it's done
in composite mode;

2) On screen we are likely to see brighter colors as a result of the RGB
averaging, causing us to believe that the image has been oversharpened;

3) On screen the RGB averaging is likely to suppress mild patterns of noise
that would be visible in print, especially if they occur in the black
channel, causing us to believe that an oversharpened image is OK;

4) Any kind of monitor preview is going to be deceptive because it hits our
eyes with continuous color, as opposed to print, where the white space
between the dots has a significant softening effect. That deceptiveness is
magnified at a lower resolution.

While 50% is probably better than 66.7%, 100% is much better, but far from

The above goes for sharpening/graininess issues only, obviously, not for
overall color evaluation.

Dan Margulis

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