Putting it All Together: Exposure Techniques That Work
|Table of Contents:
- Exposure Modes
- TTL Strobe
- Manual Strobe and the Modern Camera
- Manual Strobe and the Manual Camera
Modern electronic cameras usually come with 4 different exposure modes. Different
cameras may call them different things, but they all do basically the same 4 things.
'A' Mode refers to "aperture-preferred mode." What this means is that
you set the aperture, and the camera picks an appropriate shutter speed for you, based on
the ambient light and whether or not the camera detects a strobe attached to your camera.
If you're going to use an auto-exposure mode underwater, this one is probably it.
The aperture is probably the singe most important control underwater. It affects all
the light coming in to the camera, both ambient and strobe. It controls your depth of field. But most important, when you're
using a manually-controlled strobe, the aperture setting is what decides how far you have
to be from the subject for proper strobe exposure.
In all of the auto-exposure modes (A, S and P), the camera has to know the film speed in
order to pick a matching shutter speed. Modern cameras have DX coding, which means
that the camera can read the film speed from information on the outside of the film
canister. Older cameras have a wheel you turn in order to tell the camera what speed
film you're using.
'S' Mode refers to "shutter-priority mode." This mode is the inverse of
A mode. In S mode, you pick the shutter speed and the camera picks a matching
aperture for proper background exposure. You will probably never use S mode
underwater unless you are running your strobe in TTL mode. With manual strobe, S mode will
require you to set up the shot, trigger the camera's light meter, note the aperture
setting, and the change your strobe-to-subject distance or strobe power setting
accordingly. That's just too much trouble. Even if the strobe is in TTL mode,
you will only have a small range of shutter speeds to choose from--usually between /160
and 1/125 or maybe 1/250. 1/60 to 1/250 is only 2 stops worth--hardly enough range
to make it worth using S mode in the first place.
'P' Mode stands for "full program" or just "program." In P
mode, all you do is point, focus, and shoot. The camera picks whatever combination
of aperture and shutter speed it thinks will work, and goes from there. P mode is
useful when you're getting started, because you really ought to concentrate on composition
and framing rather than exposure, but it suffers from limitations similar to those of S
mode. You can't really use manual strobe with P mode, because you won't know what
aperture the camera picks until you trigger the camera's light meter. If you are
shooting in TTL strobe mode, doing fish portraits or macro work, P mode will probably work
OK for you. If you try to do wide angle work, though, P mode is likely to disappoint
'M' stands for 'Manual.' Your camera, your settings, you're in control.
Pick your own aperture and shutter speed. You can still use TTL strobe in manual
mode, but why would you want to? If you're in manual mode, just go for the whole
enchilada and run your strobes on manual, too. Manual exposure is handy when you
think the camera is likely to make a mistake regarding the background exposure. For
instance, if you're shooting out the porthole of a wreck, you don't want the camera to get
fooled by all the black around the porthole. You want the image on the other side of
the porthole to be properly exposed, even if that means making the inside of the wreck
completely black. With auto-exposure, the camera will likely try to balance the
whole scene, resulting in a too-bright porthole. So meter on the open water from
close up to the port hole, lock those settings in on manual mode, and pull back to compose
the shot the way you want it.
TTL strobe is a godsend when it works. There are plenty of situations when it
doesn't work, however. Don't expect TTL to work when the subject is very shiny, or
very dark. Don't expect it to work when the subject occupies only a small portion of
the frame. Also, don't expect it to work if you are very close to your subject
unless you stop down the aperture fairly tight. The problem is that the strobe's intensity
on-subject can get so high that by the time the camera has quenched the strobe, it's too
late. Inside 2 feet, expect to have to set your aperture at f/11 or higher. This
will give the strobe enough time to quench that it won't overexpose your subject.
In general, you can use TTL strobe in all exposure modes discussed above. When
you're just getting started, shooting fish portraits in P mode and with TTL strobe will
get you some results that are sure to please. It's a good way to learn about the
other parts of your camera system before you try to complicate matters with manual
Also, TTL mode works pretty well for macro photography. There you want to use A
mode or M mode, because you want to set the aperture manually in order to control the
depth of field. See the section on macro photography techniques for more details.
Manual Strobe Control and the Modern Camera
Modern cameras have all sorts of nifty exposure modes, but if you're using manual
strobe, only one of them is really useful. If you're taking wide angle shots, especially close-focus wide-angle (CFWA) shots, manual strobe
control will be an important part of taking successful pictures. If you're shooting
macro, that's not as true, but there are still limits.
These days, I do almost all of my wide-angle work in A mode. If you have a good
camera with the ability to work in A mode, I recommend it highly. A mode gives you
almost all of the benefits of manual exposure control, without the hassle of having to
re-meter every shot. Most of the time, what I do is this: set the aperture at f/5.6
or f/8, set the strobe power to the right setting to place my subject at 3-4 feet, and
then just shoot everything from that distance. I can swim around for almost an
entire dive without deviating from that plan. It makes strobe exposure a snap; as
long as the subject is anywhere close to the right distance, I get proper exposure.
If I need to take a picture from a different distance, I have two options: open/close the
aperture a little, or change the strobe power. Usually I change the strobe power.
The only danger with this approach relates to the way the camera picks shutter speeds.
The camera will not pick a shutter speed too high for proper strobe sync. If the
scene is very bright and you have the aperture set too wide, the camera can easily reach
the max sync speed, in which case your background will come out too bright. To avoid
this, be sure to re-meter the scene at the start of every dive, and every time you change
depth by 30 feet or so. My camera (Nikon N90s) has a meter in the viewfinder that
tells you the shutter and aperture combination, and then whether that's too much or too
little light. When I re-meter, I just point the camera at blue water, and press the
shutter release halfway. Then the meter in the viewfinder tells me what I need to
know. If the scene is too bright, the camera will show me whatever aperture I
picked, 1/250 shutter speed (that's the max for the N90s), and then +/- meter will read
way over to the + side, telling me that I've reached the fastest shutter speed and there's
still too much light. Then I have to stop down the lens a bit, and try again.
When I finally get the meter to agree that it can get a good exposure for the background,
I have to set my strobe power to match 3-4 feet at whatever aperture made the camera's
Similarly, there's a danger in too-slow shutter speeds. Depending on your camera
model, the auto-exposure system may not want to set the shutter speed below 1/60 or 1/30.
In this case, if the ambient light is too low, you can easily pick a 'normal'
aperture like f/8 and not have enough light for proper background exposure at the minimum
shutter speed. If this happens, you can still get a proper strobe exposure but the
background will be too dark. The procedure for avoiding this is just the same as the
one for avoiding too-birght backgrounds. Every time you start a dive or change
depths by 30 feet or more, meter the background water and set the aperture to a value that
lets the camera pick a good shutter. Then be sure to remember to set the strobe
power for proper exposure at whatever distance you've picked.
It is entirely possible that you will not be able to set a wide-enough aperture for
good background exposures. In other words, you're at f/2.8 and the meter is still
telling you "too little." This mostly happens when you're diving deep
and/or with slow film. If you find yourself in this situation, there's not much you
can do about it on that dive. So think about what sort of dives you're going to
make, and pick a film speed accordingly. If you plan to go deep or are diving in
low-vis water that eats up the sunlight, don't load the camera with ISO 50 film.
Pick something higher, like ISO 100, 200 or even 400. There are 3 stops between 50
and 400. That means that f/2.8 at ISO 50 would translate to f/8 at ISO 400...which
in turn means you have three more stops (f/5.6, f/4, f/2.8) to play with in order to
satisfy your camera's meter.
There is one more thing you can try. If your camera allows it,
you can blow through the minimum shutter speed and shoot really slow. My N90s, for
instance, has a "slow sync"
mode available that tells the camera's computer "don't stop at 1/30 seconds."
The camera will then pick whatever shutter speed it wants, even if "5
seconds" is the right answer. That's great as long as you think you only need
an extra stop or two, but I forgot I had it on that mode in Devil's Throat once, and the
shutter stayed open for almost the entire time I was in the swimthrough. I thought
the camera had broken, until it finally closed the shutter. I then immediately took
it off slow sync, and I haven't used it since. :)
If you're shooting macro, you'll still want to use A mode most of the time. The
reason for this is depth of field. You can read up on
depth of field in the macro section, so I don't want to cover it in depth here. But
basically, depth of field is the range of lens-to-subject distances that will be in focus
on a single picture. DOF changes depending on what lens you use and how far away the
subject is, but there is one rule that is always true: the higher your aperture number,
the greater depth of field. As a result, you will shoot most of your macro shots at
extremely small aperture, often the smallest aperture your lens can manage: f/22, f/32, or
even f/64! Because of this, you can't afford to have the camera pick some random
aperture/shutter combination. You have to be in control of it
yourself. Thus, if you are going to use an auto mode for your macro shots, A mode is
going to be it.
So that's it for manual strobe and the modern camera: use A mode when you can, and
you'll have a great time of it.
Manual Strobe and Manual Camera
So you have a camera that doesn't have any auto-exposure modes, or you have one with
auto modes but you want to shoot manual for some reason. What do you do?
First off, you need a light meter. If your camera has one in it, great. If
not, there are several dedicated light meters you can buy to do the job. The old
standby is the Sekonic Marine Meter II, an amphibious light meter that you'll see in use
all over the world. Nikon used to make a housing for a different Sekonic meter, and
Ikelite makes a digital light meter for use underwater. I'm sure there are others.
So you have a meter. Great. What do you do with it? First off, you're
going to simulate A mode here. Pick an aperture that works for your strobe and
strobe-to-subject distance. Then pick
up the meter, dial in your film speed, and take a reading on the background light.
The meter will then give you a range of aperture / shutter pairs that will work.
Find the aperture you selected, and then the shutter speed that matches it is the one you
want. If your aperture is not on the list, something is wrong: either the scene is
too bright or too dark for that aperture. If you have a small aperture (like f/22)
set, pick a larger one (f/8 is a good place to start, usually). Similarly, if you
picked f/2.8 and your aperture isn't on the list of acceptable aperture/shutter
combinations, try again at f/8.
You've got an aperture, you've got a shutter. Now the only thing left to do is
dial in the strobe power setting you want. You're ready to go!
Just like with A mode, any time the ambient light changes due to depth, clouds, etc.,
you'll want to re-meter. In truth, you want to re-meter for every shot, but that's a
real pain, so I'd just stick with the same shutter until you suspect there's reason to
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