A note about the digressions: The digressions are intended as
"extra" material that is not really important to understand. They're
here in case you're interested and are willing to slog through some technical
talk on the way to grasping an esoteric concept. If you don't really care, or
you just don't want to spend the time grappling with the math or whatever, then
just skip the digressions. You won't be missing anything important.
Shutter speeds play a sort of strange role when you are using a strobe. There are
a couple of things that you might want to understand. Both of them have to do with
how shutters work in modern 35mm cameras.
What is a "focal plane shutter," how does it work, and why do I care?
Focal Plane Shutter
Figure 1: Camera Layout
Modern 35mm cameras use what's called a "focal plane shutter."
Basically, it's a curtain that blocks light from reaching the film. The curtain comes in
two pieces that overlap a little. This curtain sits directly in front of the film,
almost touching it. Since the film is where the light from the lens is focussed, we
say that the curtain is on the "focal plane." Hence the name "focal
plane shutter." Figure 1 shows a schematic diagram of the arrangement.
In this diagram, the front curtain is colored blue, and the rear curtain is colored
green. I'll be using that color scheme for the rest of this page.
How it works
Before you shoot, both pieces of the curtain are at the top of the film frame.
The front curtain is usually the one that blocks the path to the film in this state; the
rear curtain is retracted all the way off the top of the film frame. When you trip
the shutter, the front curtain drops from the top of the film window to the bottom,
allowing the light from the lens to reach the film. At some time later, the rear
curtain drops, cutting off the light from the lens. The length of time between when
the front curtain drops and when the rear curtain drops is the shutter speed you
select. For instance, if you chose 1/60 as your shutter speed, the front curtain
would drop and then 1/60 of a second later the rear curtain would drop. Light from
the lens would be able to reach the film unimpeded for 1/60 of a second.
Figure 2 shows this in action. Remember, the front curtain is blue, and the rear
curtain is green. The white areas are open to the film.
Sounds simple, doesn't it? Well, it's not quite that easy.
Each curtain takes a short period of time to traverse the distance from "top"
to "bottom" on the film frame. "Electronic" shutters, powered by
motors, take less time than "mechanical" shutters, which are powered by springs.
Modern SLRs have electronic shutters; older SLRs have mechanical shutters. In
the Nikonos line of cameras, the Nikonos I, II, and III all have mechanical shutters; the
IV, IVa and V all have electronic shutters with mechanical backups in case of battery
Anyway, back to the story here. Mechanical shutters typically take as much as
1/60 of a second to cross the film frame. Some electronic shutters can do it in as
little time as 1/300 of a second. Either way, you have to account for that time.
Or do you? If a mechanical shutter takes 1/60 of a second to cross the film
frame, how can you have shutter speeds faster than 1/60? Even my first camera, a
completely mechanical Pentax K-1000, had shutter speeds as fast as 1/1000. So how
did that old clunker pull off that fast a shutter speed?
Think about this: does it really matter how long the window is completely open, as long
as the rear curtain starts moving at the right interval after the front curtain?
That is, if both curtains move at the same speed, as long as the rear curtain starts
dropping 1/1000 of a second after the front curtain, won't you get a 1/1000 second
Yes. That's the point. What happens for shutter speeds faster than the
curtain speed is this: the front curtain starts to drop, and before it has crossed the
film frame, the rear curtain starts to drop, as well. Then you get a small moving
rectangle of "open curtain" that moves across the film frame. The height
of the open section is the speed of the curtain (in mm/sec) multiplied by the shutter
duration. Effectively, the film gets a 1/1000 second exposure (to follow our
example), but different parts of the film get exposed at slightly different times.
That last phrase is really important. Be sure you understand it thoroughly
before you proceed.
Step 1: Front curtain closed.
Step 2: Front curtain starts to drop.
Step 3: Rear curtain starts to drop.
Step 4: Both curtains are dropping.
Step 5: Both curtains continue dropping.
Step 6: Front curtain all the way open, rear curtain still dropping.
Step 7: Rear curtain closes.
Figure 3: Shutter speeds faster than the curtain speed
Why do I care?
If you're just shooting with ambient light, you don't care. If you're shooting
with a strobe, you care. Why? Because the strobe's light output has a
duration. It's not on all the time, but rather just for a short while. That
duration has to be synchronized with the opening of the shutter, and not just the time the
front curtain starts moving. The strobe has to trigger and complete its business
while the shutter curtain is completely open. That is, the front curtain has to
reach the bottom of the film frame, then the strobe fires, then the strobe finishes, then
the rear curtain starts moving. Figure 4 shows this in action. The
yellow frame represents a strobe at full brightness.
Why does it have to be this way? Imagine this situation. You have set your
shutter speed to 1/125, and the actual time it takes the curtains to cross the film frame
is 1/60. Then imagine that the strobe were to fire just when the front curtain
started moving. Further imagine that the strobe's duration was only 1/1000 of a
second. So 1/125 of a second after the front curtain starts moving, the rear curtain
starts moving. Meanwhile, the part of the film frame that is "open" has
been collecting light from the strobe. But before the front curtain can reach the
bottom of the film frame, the strobe has shut off (remember, it take the curtains 1/60
second to cross the film plane, but the strobe burst lasts only 1/1000 sec.). So the
top portion of the film frame got strobe light, and the bottom portion didn't.
Ooops. The top of your photo will have good exposure, and the bottom will have bad
exposure. That's definitely not the goal here. Is this situation
hypothetical? Not at all; all of these numbers are within the range of equipment you
might be using. The interesting thing about this situation is that the whole frame
will have the same ambient lighting; only the strobe lighting will be
So how do you get around this? Well, cameras are designed not to trigger the
strobe until the front curtain is all the way down. Then, while the frame is
completely open, the strobe does its job, illuminating the entire frame. Then the
rear curtain falls, and you're done. The whole frame got both ambient light and
The upshot of all this is that there is a limit to how fast a shutter speed you can use
and still sync with your strobe. For mechanical cameras, 1/60 and 1/90 are common
"sync speeds." For electronic cameras, 1/125 is common, and a few even
have 1/250 sync speeds. You can always synchronize at slower
shutter speeds than the max, but you can't go faster than the max. If you're not
sure about your camera, consult the owner's manual. Whatever the maximum strobe sync
speed is for your camera, chances are that the camera will not trigger the strobe at all
if you select a shutter speed too fast for proper synchronization.
The implications for how you take pictures are sweeping. This effect limits the
number of shutter speed settings you can use when you are using a strobe, which in turn
limits the number of aperture settings you can use for proper background exposure.
Limiting the range of available apertures then limits the range of subject distances available to
you, due to the guide number equation.
There are certain types of cameras that do not use focal-plane shutters; these cameras
essentially use the lens diaphragm to act as a shutter. Cameras with lens diaphragm
shutters can synchronize at much higher speeds than cameras that use focal plane shutters.
This is because the diaphragm opens evenly, allowing the same amount of light to
reach all points on the film. They do not need to wait for the shutter to open fully
before firing the strobe, and they do not need the strobe to finish before the shutter
starts to close. Obviously, the most light will reach the film if the strobe is on
while the shutter is all the way open, but that's another story for another day.
Why does the guide number equation not take into
account the shutter speed?
As we just discussed, focal-plane-shutter cameras trigger the strobe only when the
shutter is completely open, and the strobe has to do its thing before the rear curtain
starts to move. Thus, as long as you're at a shutter speed equal to or slower than
the maximum sync speed, the film will see all of the light from the strobe, regardless of
how long the shutter is actually open. Then the only thing that can affect the
amount of light reaching the film from your strobe is the lens aperture, which affects all
light reaching the film.
So the guide number equation only takes into account the aperture and distance, because those are
the only two things it has to account for. Do you want to know how
the guide number equation accounts for distance? If so, click
here to read digression #3.