[Leica] GG physiology

[George Huczek <ghuczek@sk.sympatico.ca>]

Some people have mentioned that they find it difficult to preview the depth
of field on the ground glass after stopping the lens down.  Suppose you
have a reasonably fast lens -- say an f/2 lens -- you are first previewing
the scene at the widest aperture.  That's so that you get the brightest
image on the ground glass, of course, and cameras are made that way.
Stopping the lens down, say to f/16, with stop-down preview, does darken
the image.  You are letting only one-sixtyfourth as much light pass through
the lens.  Not much of a surprise that it gets darker, especially if it was
already somewhat dark to start with.

So why is it that previewing depth of field under these conditions is so
difficult?  The main reason is because your eyes have not switched over to
their night vision mode yet!  Under bright light conditions, your retina's
cones are active.  They provide your color vision.  Once the brightness
levels decrease enough the cones are no longer sensitive to light.  The
rods respond to low light levels.  

It takes time to switch from one type of receptor to the other.  That is
why you get a sudden blinding sensation when brightness levels go from one
extreme to another very suddenly.  For example, when driving at night,
looking into oncoming high beams is very disturbing - almost to the point
of feeling painful.  Similarly, when you switch off the main room lights in
your darkroom, you need a few minutes before you can see again under
safelight illumination.

Another problem is that as your vision switches over to its low-intensity
mode, using the rods of your retinas, you loose all of your color vision as
well.  That is why you only see in black and white late at night.

If you are using depth of field preview, as soon as you stop down the lens
the image gets darker, but you need to give your eyes time to adjust.  Even
so, you may also have lost your color vision if you have stopped down far
enough!  Close the eye that is not looking through the viewfinder.  Keep
looking through the viewfinder for several minutes.  You have to keep
staring onto the ground glass until you can make out the image.  Once you
are able to do so, you will be able to see what will appear in focus.

If you only stop down a few stops, depth of field preview is relatively
easy to use.  It gets more difficult when you are stopping the lens way
down to one of its smallest apertures, but it can be used if you have
patience.  There is no better way of previewing depth of field, short of
having to take test shots on Polaroid film.