Bellows Extension
When photographing at normal distances you don't need to make
any exposure corrections. When you get closer to your subject, you need to compensate for
light loss due to bellows extension. To focus at infinity, bellows must be extended to a
length in millimeters equal to the lens focal length. With a 180mm lens for example, the
bellows need to be extended 180mm to focus at infinity. As you get closer to your subject,
you need to more extension to bring your subject into focus. At life-size you need twice
the length of the lens focal length to focus. A 180mm needs 360mm of extension to focus on
a subject at life-size, 270mm at 1/2 life-size, and 225mm at 1/4 life-size.
The longer you
extend bellows, the more exposure you need to add. Generally speaking, you need to add
twice as much exposure at life-size, one stop at 1/2 life size, and half stop at 1/4 life
size. The best way to determine exposure compensation is by running a test. A 180mm lens
needs less extension to focus at life-size while a 300mm needs more. If you take a reading
for close-ups with your 180mm then switch to a 300mm without refocusing, then you don't
need to make any exposure adjustments, but if you extend the bellows to get to life-size
with a 300mm, you must open up to compensate for light loss since the 300mm lens needs
more extension to focus at life-size. Run several tests with all your lenses at different
close focusing distances, and record exposure adjustments. Measure bellows extension with
different focal lengths and refer to your notes when working at close-up range.
My Omega 45E with regular bellows
Keep in
mind that you must also compensate for lighter and darker than medium tone subjects. If
your subject is lighter than medium tone, open up. If it's darker, close down. For
instance, if you decide that opening up 1 stop will give correct exposure for a middle
tone subject at 1/2 life-size, then you should open up an additional one stop if your
subject is much lighter than middle tone or close down 1 stop if it is much darker. How
much you really need to open up or close down, depends on how you want your subject to
appear on film. Open up more to get lighter or close down more to get darker effects.