| In general, for macro, the rule of highest
order governing f-stop selection dictates that you choose
the smallest aperture usable under the conditions. Depth of
field is usually your prime concern, since at high magnification
there is precious little of it. When shooting macro—especially
at somewhere between 1:2 and 1:1 and higher magnifications,
effective f-stop starts to increase to the point that at 1:1,
the actual or effective f-stop number is twice that marked
on your lens. This is due to the "bellows effect"
as the distance from the rear element of the lens and the
film plane is increase. At "life size", the dimensions
of your image on film are the same as the subject’s
actual dimensions, the f-stop number as read on your lens
has doubled from say twenty-two to forty-five, and you have
lost two stops of light. If your strobe produces enough light
to do f22 at a strobe-to-subject distance of one foot, which
would be reasonable for a smaller underwater strobe and 100
ISO film, you will have to move your strobe in to about six
inches to get the advertised f22 at 1:1. Probably 90 percent
of all macro is shot at f22. Since that is all you’ve
got with Nikonos and Nikonos RS systems, you are probably
going to use this stop for virtually all macro.
Many housed systems offer macro lenses with f32. More depth
of field! But alas, at what cost? Enter diffraction. It entered
at around f8 to f11, but now it’s here with a vengeance.
Diffraction is the bending and scattering of light rays ricocheting
increasingly off the blades of the lens iris as the aperture
is closed down: they scatter, then hit the film in places
they shouldn’t, causing a loss of sharpness in the image.
So as the aperture is diminished, depth of field is increased,
thus increasing sharpness. At the same time, diffraction increases,
causing loss of sharpness. Diffraction causes loss of sharpness
over the entire image. Loss of depth of field does not affect
sharpness at the plane of focus at all and progressively degrades
image sharpness as the outside of the zone of depth of field
are reached. So what to do?
More of your image will be sharp with a smaller aperture.
You r sharpest point, (an eyeball, maybe) will be sharper
in the photo taken with the larger aperture, but less in the
frame will be sharp. Which is better? The fact is that sometimes
the shot will look better (to most people, I might add) at
f32, and some photographs will look better at f22. God help
us all, but some will actually look better at f16. Take both
shots, one at f22 and one at f32—i.e., bracket. When
you get your film back, look at both images and keep the better
one. It’s a helluva lot easier to figure out what’s
best when you’re looking at it. Even this may take some
serious deciding; but then, you do have the time.
Let’s put this strategy into the context of a photo
dive. We’ll make the safe assumption that you’re
not one of the dozen or so photographers on the planet using
a 250-exposure back on your camera underwater. You’ve
got thirty-eight or so frames. You’re making a dive
to—say—sixty feet. You’ve got roughly one
hour of bottom time—maybe forty-five to fifty minutes
to be spent looking for subjects, setting up, and photographing.
You find your first subject. It’s an inch-long goby
on some brain coral. You set the aperture at f22, your default
setting for 1:1 or so, and look through the finder. The little
critter that was the size of a postage stamp a second ago
now fills your entire field of view. Now comes the string
of questions initiating the decision process which will result
in a shot or two all the way up to all the frames left in
your camera. Is this an outstanding subject or is it just
okay? Do you already have him? Do you have him in this good
a setting? He doesn’t look too squirrely, so you’ll
turn your light on (if, in fact, you didn’t already
have it on). Now, the color dimension enters the decision-making
process. How much film do you want to use? How much air do
you have left? How much bottom time do you have at this depth?
Will this site have a few more subjects—or a lot more
subjects—maybe better than this? On and on it goes until
you tentatively decide how much film to use on this subject.
Now what angles to shoot from, to light from? Two strobes
or one? How about three? You might end up with two or three
shooting angles, then have to decide if you want all the lighting
angles from each shooting perspective. Should you shoot each
angle and lighting at f32 or more depth of field (less overall
sharpness)? How about f16 for a sharper eyeball? It goes on
and on. All it costs it bottom time and frames of film. Will
the subject sit there while you shoot the whole roll? No?
Then what to shoot first, Let’s see: three angles at
three exposures, three times three is nine frames at f22,
and nine more at f32—that’s eighteen. At f16,
oops, that’s getting close to a whole roll!
So you decide that, since you have plenty of bottom left
to find other subjects, and you’ve got similar shots
already, you’ll just take a quarter-front perspective,
kiss him on the lip with one strobe from the left, and just
try an f32 to see what it looks like next to the frame at
f22. So goes the decision process with this subject. It could
have been way different. You really don’t know exactly
what the subject and negative space (fancy term referring
to the stuff around the subject) will look like with the lighting
you selected, your arbitrary placement of the plane of focus,
etc.
Next, you find a porcelain crab in an anemone. Fish, whatever
size they are, have the common decency to maintain relatively
the same proportion: about 20 to 25 percent as wide as they
are long. Just about any fish filling the frame in the same
manner will include about the same body parts within the accepted
depth of filed at given f-stop. This is a function of the
inverse square law. Crabs are different. Crabs are square.
This is governed by the perverse square law. You’ve
got a few choices. Shoot down on him, keeping all of his top
in the same plane, thereby keeping the whole animal sharp—the
venerable and generally boring aerial shot. Get just the front
or just a side of the critter in the plane of focus and let
the rest go. Try an angle which compresses the subject’s
depth in the shot—maybe from a lower point, off to the
side about forty-five degrees: f32 can certainly help here
to pick up some of what would be out at f22. You might even
want to take most of your shots at f32 and vary other parameters
for the number of frames allotted to this subject.
As always, you are optimizing—borrowing from Peter
to pay Paul. Whether you’re paying with frames to bracket,
giving up depth of field to stop motion, backing off and sacrificing
image size in the frame to get depth of field, or whatever,
you don’t get something for nothing. If your strobe
will put out the light for f64 (the real effective f—stop
when shooting at 1:1 at f32), you can trade something else
for more depth of field. Another ploy in your bag of tricks.
Simply put, there are no rules in underwater photography,
there are only consequences.
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