Photography: f sops and exposure

What is a stop?

In photography, a ‘stop’ is a measurement of an exposure, depending on either the shutter speed, the ISO or the aperture.

To be specific, if you were to increase the exposure by one stop, you would be doubling the exposure. So, for example, your aperture is f/4, shutter speed is 1/100 and ISO is 100. If you keep the aperture at f/4 and the shutter speed at 1/100 but you increased the ISO to 200, you have increased the exposure by one stop.

Doubling the ISO makes the exposure twice as sensitive, hence the jump in single stops.

It can get a little bit confusing but it’s really important that you learn this, and here’s why…

As your skills as a photographer improve and you start to shoot in manual more often, you’re become more in charge of looking after how the camera exposes. Knowing what one stop is for the shutter speed, ISO and, most importantly, aperture will affect how you change each.

For e.g.

You’re shooting at f/2.8, at 1/100 of a second, with an ISO of 200 but you want a shallower depth of field. You know that widening your aperture to f/2 will produce a shallower depth of field but it will also double the amount of light that’s entering your lens.

You have jumped one stop with your aperture and made the exposure too bright so you have to counter this with shutter speed or ISO. To do this, you can halve the ISO to 100 or double the shutter speed to 1/200 of a second.

To briefly summarize, increasing the exposure by a stop will double the exposure and decreasing the exposure by a stop will halve it.

If only it were that simple…

ISO Stops

Lets start with the easiest to understand: ISO. One stop up from ISO 100 is 200 but one stop up from ISO 200 is 400. The intervals aren’t equal but, instead, the ISO doubles between stops. Easy enough to understand, so I’ll leave it at that.

Shutter Speed Stops

The majority of the time when you use your camera, you’re shooting at a fraction of a second. If you shoot at speeds of 1 second or longer, the same principle as above applies: you simply double the time from 1 second to 2, then from 2 seconds to 4. Simple.

When shooting at a fraction of a second, such as 1/200, to double this number, halve the denominator (the number on the bottom of the fraction, in this case 200). If you’re new to photography, don’t worry; this will soon become second nature.

1/100 is twice the length of 1/200 so that’s one stop and the exposure is doubled. 1/50 is twice the length as 1/100 and so on.

Aperture

I’m afraid this is where things get a little bit complicated and somewhat mathematical.

If you use the logic that I’ve explained above, you would probably assume that f/2 is twice the exposure of f/4 but, sadly, this is not the case. f/2 actually allows in four times as much light as f/4.

You might be scratching your head at this but, I promise, it will all become clear if you can just stick with it.

The aperture scale does not take on the same principles as shutter speed or ISO because of how the measurement is taken. If you’ve read my tutorial on Aperture, you should be familiar with how this works but, for everyone else, stick with it.

Aperture is measured using something called the f-stop scale.

On your camera, you’ll see ‘f/’ or just ‘f’ followed by a number. The number denotes how wide the aperture is which, in turn, affects the exposure and depth of field: the lower the number, the wider the aperture.

This may seem confusing: why a low number for a high aperture?

The answer is simple and mathematical but, first, you need to know the f-stop scale.

The scale is as follows: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22.

Before we go any further, lets recap on what the aperture is.

The aperture is the hole in the lens through which light passes and it controls both the exposure and the depth of field. We’re only looking at exposure here though. If you are changing from f/2 to f/2.8, you are halving the exposure but, to do so, you’re halving the area of the open aperture in the lens.

The most important thing to know about these numbers is that, from each number to the next, the aperture decreases to half its size, allowing 50% less light through the lens (1 stop). This is because the numbers come from the equation used to work out the size of the aperture from the focal length.

The ‘f’ stands for focal length and the number is a fraction of the focal length which tell you the size of the aperture.