The terminology and concepts of photographic exposure control often confuse people and
frighten them away from cameras. But it doesn't have to be that way; the conceprs
are basically straightforward. But the words might foreign to you, so...
Here's an analogy for you: let's say we're trying to produce an accurate model of the
caribbean, with a bucket to hold the water, and a set of stone "mountains" to
represent the islands. If everything is done right, and the model is perfect, the water
level will rise to a point where just the right amount of each "mountain" is
seen above the water as an island. If you have too much water, you'll get only the tips of
the islands, rather than the whole islands. If there's not enough water, you'll see too
much of the islands. Either way, your model will be wrong. Figure 1 shows the
Figure 1: The "Islands" model
Too little water in the bucket
Too much water in the bucket
Fig. 2: The "Islands" Experiment in Action
So your goal is to get the right amount of water in the
bucket. But how do you get water into the bucket? With a hose, of course. At the end of
the hose is a nozzle with a valve on it. The valve is either 100% "on," or 100%
off. That is, if you open the valve, water shoots out of the nozzle at full speed; if it's
closed, no water comes out. There is nothing in between.
Obviously, the longer the valve is open, the more water shoots through the nozzle. But
there's a control on the nozzle that sets how wide open the nozzle is. The nozzle is
calibrated with click-stops that tell you it's all the way open (1/1) half open (1/2), 1/4
open, 1/8 open, etc. You can control this by tightening the nozzle (allowing less water
per second through the nozzle) or loosening it (i.e. allowing more water per second) But
still, unless the valve is open, no water gets through.
At the other end of the hose is a pump. The pump has infinite capacity, but you can't
control the pressure. Whatever pressure the pump is set for on that day, that's what you
See figure 2 for a sketch of this gizmo in action.
So how do you fill the bucket? Obviously, you set the nozzle to some reasonable value,
open the valve, and wait until the water reaches the right level.
But did I mention that the bucket has a lid on it, and you can't tell where the water is
until you shut off the valve and open the lid, and that each time you do this the water
will give you just enough time to see the level, and then drain out instantly, forcing you
to start over? It gets a bit harder now, doesn't it? :)
So what do you do?
Well, you approach it "scientifically." You start by setting the nozzle width,
and use a stopwatch to open the valve for a fixed period of time. Then you look in your
bucket to see if the level was right. Too little water? Well, try again, but this time let
the valve stay open a little longer. Too much? Same deal, but leave the valve open for
Eventually, by trial and error, you will come to a right length of time, and you will
record your numbers: "at 500 PSI, I had my nozzle set to 1/4, and I opened the valve
for 10 seconds, and got the right amount of water in the bucket." Then you
might perform an experiment: at the same 500 PSI, if you set your nozzle to 1/2, would 5
seconds be the right duration to fill the bucket properly? Sure it would.
So you might make a notation in your lab notebook that there's an inverse
relationship: doubling the width of the nozzle lets you open the valve for half the time
(and vice versa). In photography, this is called "reciprocity."
What happens if you come back on day 2 and found the pump was at 250 psi? You have two
choices. You can open the valve for 20 seconds, but that's boring. You don't want to
wait that long (or maybe your bucket is a moving target, and it's hard to keep the hose on
target long enough). Your other choice is to open the nozzle to twice the area it had been
(i.e. 1/2 instead of 1/4), allowing twice the water per second through the nozzle. Twice
the area at half the pressure means that you'll get the same gallons per second on day 2
that you got on day 1.
So you set the nozzle to 1/2, and open the valve for 10 seconds, and -presto!-
you get the right amount of water in the bucket, and your model is perfect! How long
do you think you'd have to open the valve if you set the nozzle to "wide open"
(i.e. 1/1) ?
Where is this stupid analogy going?
Figure 3: Schematic Diagram of a Camera
If you hadn't figured it out yet, the "Islands"
experiment is an analogy for the way your camera works. In the analogy, the valve
represents your camera's shutter, the nozzle represents your aperture, and the pump
pressure represents how much ambient light you have. The brighter the day, the higher the
Just like your water experiment, you have an on/off valve
(the shutter) and a stop watch (the camera has a built-in stop watch that opens the
shutter for the length of time you ask for). You also have a nozzle (the aperture
setting on your lens) that is calibrated in pre-set "stops" that represent how
much of the total area is open. The nozzle was marked 1/2, 1/4, 1/8 etc., where
each "stop" represents twice as much water per second, and your lens aperture is
marked f/2.8, f/4, f/5.6 -- but each aperture "stop" still represents twice as
much light per second. Click here if you want to know
why. Your pump had a meter that told you how much pressure was on the water--and
your camera has a meter that tells you how much light is all around you. Finally,
your "islands" gizmo had a way of telling you if the water level was right: you
just look inside the bucket and see for yourself. Similarly, your camera has a
similar way: you get your film processed, and see for yourself!
But where does film speed factor in to all of this? Think of film speed as how quickly the
film reacts to light. The higher the speed, the faster it reacts to light. In our example,
the film speed would correspond to the scale of the model (as in 1:100 scale, where 100
feet in "real life" would be modeled as 1 foot in the model). The higher the
film speed, the higher the ratio--so 200 speed film might be 1:200 scale. That means the
model mountains would be half the height at 1:200 as they were at 1:100---and they'd
require half the water to reach the "right" level! Figure 4 shows this
graphically. The required water level indicates how much light is required for proper
So now when you show up at the bucket and discover the pump is at half of yesterday's
pressure, you have another option. You can use a 'faster' model, and leave your nozzle and
valve settings alone!
So let's take all of this in context of film an photography again. Your goal is to get the
'right' exposure. That is, you want the right amount of light to strike the film. You use
a light meter to find out how bright the light is on your subject. Once you know how
bright the light is (i.e. what pressure the pump is set at), you can calculate the rest.
Given enough experience and data points, you'll be able to figure out exactly the right
combination of shutter speed, aperture, and film speed. Or if you have a modern camera
that has a little computer in it, you can let the camera figure out all of this stuff for
Of course, in the real world you have to pick your film speed when you load the camera,
and it's tough to change afterwards. Then the meter will tell you the right combination of
shutter speed an aperture (nozzle diameter and valve duration) to use for the light
conditions and film speed (pump pressure and model scale).
Next Chapter | Back to Exposure 101