Notice that the two scales run in opposite directions.
As you read from
left to right, the shutter speeds progressively reduce exposure times in
half, and the f/numbers progressively double the light intensities. Each of
the above combinations of f/number and shutter speed provides exactly the
same amount of exposure on the film. That is, an exposure of 1 second at
f/32 will have the same darkening effect on the film as an exposure of 1/250
second at f/2, and so will all the others in between.
But what if this particular combination doesn't give the correct
If you are in a dim room with a slow film, 1 second at f/32
probably won't be enough and the film will be underexposed. But perhaps an
exposure of 1 second at f/5.6 would be enough to do the job. All you need
to do is slide the two scales past each other until the f/5.6 is opposite
the 1 second mark. Then all other shutter speed and f/number combinations
will give the equivalent exposure.
You can make an "Equivalent Exposure
Calculator" by copying the two scales as they are shown here and then
cutting the paper down the middle between them. Then you can actually slide
the scales back and forth as described. The calculator, once you have found
one good shutter and lens combination, will give you all the other
equivalent exposure combinations available to you.
Another useful method is the "count on your fingers" method.
works well when you don't have your calculator handy. If you wish to change
either the f/number or the shutter speed, you recite the number sequence
while counting them on your fingers until you arrive at the number you want.
Then you recite the other sequence exactly the same number of times to
determine what your new value will be.
That's quite a complicated
explanation of a simple act.
Here's an example.
Suppose your camera is
currently set at 1/500 second at f/5.6.
You decide, however, that you'd
rather shoot the picture at f/16. All you do is mentally count the f/number
sequence up to f/16, "eight, eleven, sixteen".
(Three stops, closing the
lens.) Then count down in shutter speeds, "1/250, 1/125, 1/60"; also three
stops, increasing the time. So your shutter speed at f/16 is 1/60 second.
Just make sure that as you count up in f/numbers you count down in shutter
speeds, and vice versa.
Earlier we said that mechanical shutters could not be set to an
intermediate setting. With the iris diaphragm, you can select intermediate
settings for the lens openings. It is with the iris diaphragm that you fine
tune your exposures to the degree of precision you need. You will often see
intermediate exposures written such as "1/2 second between f/16 and 22," or
"1/2 sec @ f/16-22." You rarely ever see a precise intermediate value given
for an f/number, such as "f/27". (There really is an f/27, of course. You
just can't be that precise when setting a lens to intermediate positions.)