How Layers Serve
and Protect Photos
Step back a while to high school biology. Remember that frog illustration in the biology textbook? It was
made of several sheets of clear acetate. The top sheet showed the belly of the frog. Lift that sheet and the
skin went with it to reveal the muscles lying just beneath. Lift that and you could see the frog’s guts. One
more sheet and you were down to the bones. If you’re familiar with ranunculus acetatus, then you’ll have
no trouble at all understanding layers, Photoshop’s primary tool for changing an image while at the
same time protecting it.
Ideally, in Photoshop no editing is ever done directly to a digital image.
Instead it is done on a layer—a copy of all or part of the image that is
virtually suspended over the original. It’s as if you lifted the first sheet of
acetate, with froggy’s tummy on it, only to find a second sheet with an
identical frog belly. You could scribble on the first sheet until the bell
rang, and the image beneath would be unscarred.A Photoshop layer, however, is not always a dupli-
cate of the original image. We can start with one
photo, such as this one of a lily pad and over that
place a layer displaying another image, one of,
say, a frog.2
In this instance, the original full-frame frog image com-
pletely obscures the lily pad on the bottom layer. But the
frog is on virtual acetate. We can erase everything in
the frog layer that’s not the frog to reveal the lily pad in
the layer below. Or better yet, make a duplicate of the
frog image on the second layer and do the erasing on
that new, third layer.3
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160 PART 3 HOW THE DIGITAL DARKROOM WORKS