I’m contributing this article to explain exposure for those beginning your exploration of photography. It is key to understand some very fundamental principles in order to produce an optimum quality image. One typical reference is constantly made to a time honored device, The Exposure Triangle. That seems to introduce a bit of confusion. Let’s simplify this entire concept with something with which almost everyone has, at least, a passing acquaintance, something that is dynamic in nature. However, before we start, let’s get a couple of definitions established. So we are all discussing the same concepts across the board, let’s set a couple of descriptions for the purpose of this article. These are not my arbitrarily made up definitions. They are definitions accepted throughout the photographic world.

Exposure
In photography, it’s how much light is allowed through an opening and for how long. That’s it. And, it’s that simple.

Photographic Exposure
This involves the above definition, but includes a light sensitive material (film or sensor) upon which the light coming through a lens, or pinhole, is allowed to fall.

It is understood that the reader of this article is familiar enough with the settings of their camera where it comes to ISO, Aperture, and Shutter Speed. The idea is to balance the amount of light with a duration to effectively expose the light sensitive material optimally. That is all we are trying to accomplish.

So, with balance being brought into the discussion, let’s imagine a scale, very much like the Scales of Justice. We have a beam from which is suspended two platforms, one from either end. The idea is that the ideal exposure will occur when the balance is perfectly level.

We have a set of weights that go on the left side of the balance, and only on the left side. These weights are labeled ISO. They are in values of 50, 100, 200, 400, 800, 1600, etc. Weights for the other side of the scale are labeled f/1.4, f/2.8, f/3.5, f/4.6, f/5.6, etc. and 1, 1/2, 1/4, 1/8, etc. The weights on this side are designed to be used together in some combination.

We now have all the tools to determine a means to associate the three weights. If the beam is perfectly horizontal, we have the correct settings for an optimum exposure. For example, let’s say we have a scene that requires a setting of f/11 at 1/125 of a second to balance an ISO of 100. If we choose to change the f/11 weight to f/5.6, we are either going to have to change the 1/125 weight to another value to make the scale balance again. In this case, the 1/125 weight would have to be replaced by one of value 1/500.

There is a one to one ratio in the manner in which the right hand weights work. As we saw in the above example, we changed the f/stop weight 2 units, f/11 -> f/8 ->f/5.6, we had to change the associated weight by two units, 1/125 -> 1/250 -> 1/500, as well.

If we change the left hand side of the scale, we are going to need to change one or both of the right hand scale to keep things in balance. It’s all that simple. Once this simple concept is understood one can see that exposure becomes a system management situation. Understanding the systems completely will aid in making consistently better exposures.

I really like the image of a balance. Here's a little added sophistication. Of course the explanation would be even clearer if accompanied by a graphic.

I think of the right side as a two-line scale, almost like an old slide rule, with one-stop increments of shutter speed (decreasing from left to right) above and one-stop increments of aperture (increasing from L to R) below.

Top would read, in part, something like:
25,50,100,200, etc.
(i.e., 1/25, 1/50, 1/100, 1/200 seconds.)
Bottom might read:
32,16,8,4,2, etc.
(i.e., f/32, f/16, f/8, f/4, f/2)

The idea is that you slide the top scale over the bottom one until their combined "weight" balances the ISO "weight" on the left side. Then you lock the two scales on the right together. Now, any pair of shutter speed and aperture numbers (any two that are over and under each other) will balance the ISO.

(In practice I think this graphic of the right side would be in half-stop increments.)