The primary unit of measurement in photography is the “stop”. In its simplest terms, this means twice as much or half as much. If I brighten my exposure by a stop I am allowing in twice as much light as before. If I darken the shot by a stop it’s half as much. And so on. We measure each part of our exposure triangle – our aperture, shutter, and ISO – in stops.

This is most obvious when examining shutter speed. A selection of the most common major stops would be 1/30, 1/60, 1/125, 1/250, and 1/500. As you can see, depending on the direction you read this list, it is a constant doubling or halving of light. It’s plain that 1/125 of a second is twice as much light as 1/250. But this is the way each aspect of the exposure system works.

ISO 100 is half as sensitive to light as 200. Let’s imagine an example. You and I are in a haunted house with our cameras and we can shoot a photograph at 1/60 at ISO 100. Then the house shakes and the lights dim. They’re half as bright as they were before. And in case you’re wondering, yes, I could find an example without the haunted angle, but I like ghost stories.

Anyway, the lights, or the ambient amount of light in the environment, are now half as bright as they were before. My previous exposure is now underexposed by one stop. Now a ghost is coming through the walls. So if I want to shoot a picture of it I need to find a stop of light. From the perspective of overall photographic brightness I don’t care where I get it. I could slow my shutter speed to 1/30 but that would make his arm waving motions blurred. So instead I can increase my ISO to 200. In other words, if the light is half its original brightness I need to double my sensitivity to it or expose for light for twice as long.

We were shooting that photo at f/4 and now we notice writing on the wall behind our ghost. But when I focused on the ghost the writing was out of focus due to the depth of field that my aperture gave me. If I want to stop down my aperture I need yet another stop of light.

For a reminder of the major stops in aperture please refer to the aperture installment in this essentials series. The light doubles or halves every stop, just like shutter and ISO, though the number doubles every two stops. So f/4 lets half as much light through the lens as f/2.8 but is, in turn, letting in twice as much light as f/5.6. So if we shot our ghost picture at f/5.6 we need our ISO to increase, this time to 400. Alternatively we could slow our shutter speed, but that has aesthetic consequences we aren’t willing to accept.

At its core, the stop system is essentially a series of trade offs. We control light through three mechanisms in the camera. I like to imagine baking. I can pour a cup of flour from a measuring cup to a bowl and it’s still a cup of flour. Or maybe I just think of that because I like making chocolate chip cookies.

Let’s take another example. Assume ISO 100, 1/30, f/8 is a properly exposed photograph.

But let’s say we are shooting spots or some other kind of action. By the way, we escaped the ghosts, good for us.

This is exactly the same amount of light. So is this.

We are gaining light through one mechanism and using it elsewhere. Now let us assume the lens we are using is f/4. It cannot open up further. We change our ISO if we still need a faster shutter speed.

The nature of exposure theory is to make a creative decision about one exposure mechanism (aperture or shutter) and then move the light around in order to accomplish this creative goal. The creative goal will be depth of field or the crispness of the movement. We then change the ISO if we still need assistance. When we shoot in manual we can control the combination of exposure elements in order to achieve the brightness level we desire for the photograph.

Now I'm off to bake some cookies.