Talk about
f-stops and shutter speeds can confuse beginning photographers. Let's make it simple.
Too many camera store associates and photo
instructors forget that once they didn't know the difference between an f-stop and a bus
stop. The jargon can be intimidating to newcomers, but almost everybody has a pretty good
understanding about a few plumbing basics - so I make comparisons:
F-stops are the hole in the lens that lets the light in. The
number is just the bottom (ok, the denominator) of a fraction.
- When the lens is set to f2, the hole in the lens is
1/2 as big as the lens is long.
- When the lens is set to f8, the hole in the lens is
1/8th as big as the lens is long.
- Which hole is bigger, 1/2 or 1/8?
- Which hose would let more water through, one that's a
half inch in diameter or one that's one-eighth in diameter? (See, there's the first plumbing
parallel.)
- Which f-stop lets in more light, f2 or f8?
Shutter speeds control how long the lens lets the light in.
The number is just the bottom (ok, the denominator) of a fraction.
- When the shutter speed is set to 125, light comes in
for 1/125th of a second.
- When the shutter speed is set to 8, light comes in for
1/8th of a second.
- Which is longer, 1/125th or 1/8th?
- If you leave the water turned on for 1/8th of an hour
(7.5 minutes), does more water flow than if you leave it turned on for 1/125th of an hour
(less than 30 seconds)? (Another plumbing parallel.)
- Which shutter speed lets in more light, 1/8th or
1/125th?
Shutter speeds also control the action-stopping ability of
the camera.
- If you aim a garden hose at your girlfriend and she is
running, how much of her gets wet if you only open the nozzle for 1/1000th of a second?
- How much gets wet if you open the nozzle for 1/4 of a
second?
- How much will you like it when she hits you with the
bucket because you turned the hose on her?
- Here's the plumbing parallel - moving subjects move
farther the longer the shutter is open (the longer the hose is turned on.) The more a
subject moves during the exposure time (the length of time the shutter is open), the more
the image of the subject moves across the film. The more the image moves across the film,
the more blurred the image becomes. Short exposure times "freeze" action.
- Which shutter speed freezes action better, 1/8th or
1/500th?
A sunny day is like a high pressure hose
- If the light is bright, it's like strong water
pressure.
- It takes less time for just enough light to get to the
film.
- High shutter speeds are more suitable for bright
light.
- Longer shutter speeds are suitable for dim light, just
as you'd leave the faucet turned on longer to fill up a glass if the water pressure is
low.
Film speeds can be compared to measuring cups
- Let's think of the film speed rating (ISO or ASA or
DIN) as a standardized measuring cup.
- Fast films are like small measures. Let's call ISO 800
1 pint.
- We'll call ISO 100
8 pints (that's a gallon.)
- Which takes longer to fill up, a gallon or a
pint?
- Which takes longer to properly expose, ISO 100 film or
ISO 800?
- Plumbing and photography: Slow films (large
containers) require bigger f-stops (larger diameter hoses) and/or longer exposure times
(leaving the hose turned on longer) for proper exposure.
Automatic Electronic Flash is like a toilet tank…
- A toilet tank is supposed to stop filling when just
enough water has flowed into it.
- Automatic flashguns stop making light when just enough
has gone out to the subject and bounced back to make a proper exposure.
- An even better analogy, which doesn't involve
bathroom fixtures: The automatic flash gun is like a flood light. You turn on the flood
light and measure the light with a meter. You leave it turned on until the meter tells you
that enough light has come back for a proper exposure, and then you turn it off - whether
it takes 10 seconds or 5 minutes. The electronic flash is the same, but it performs this
routine in as little as 1/50,000th of a second!
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