Welcome to another entry in our series of jargon-busting posts, aimed at those of you just starting out in your photography journeys. In this series, we’ll be attempting to demystify some terms that you’ll hear being bandied about in photography circles that might leave you scratching your head.

These are concepts it will help you to be familiar with and which we’ll be explaining in broad terms in order to develop a general understanding, without getting bogged down in the technicalities.

This time, we’re exploring what is meant by a “stop” of light, how this helps us set exposure, and why it’s important to understand in pet photography.

The general gist:

• Shutter speed, aperture, and ISO all change how bright your image is.
• Stops of light are relative, not absolute.
• A stop increase doubles the brightness of an image.
• The numbers used to designate stops of light are not consistent across exposure settings

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You may already know that there are three settings which govern the exposure of your image - or how bright or dark it is. These are: aperture, shutter speed, and ISO. Together, these control how much light the sensor of your camera collects: the more light, the brighter the image.

These settings affect the image in other ways too, but in this post we’re just going to be looking at how each element affects how much light is gathered by the sensor.

Aperture

The aperture of the lens is the size of the roughly-circular opening created by overlapping “leaves”. This can be made bigger to let more light in, or smaller to allow less light in. The aperture is measured in f-stops, and expressed as a fraction with f on the top, and a number on the bottom. We’ll get a little more into this in another post, but for now, you just need to know that the larger the number on the bottom, the smaller the size of the aperture.

This may sound confusing, but it’s just the same as normal fractions: we all know that ½ is larger than ¼ and equally f/2 is larger than f/4. When it comes to how the size of the aperture affects the brightness of the image, you can think of standing out in the rain with an umbrella over your head. The umbrella, unfortunately, has a small hole in it, and so some raindrops get through onto your head. If that hole is made larger, more rain is allowed through and so you get wetter.

That’s the same with light passing through an aperture: as the aperture (hole) becomes larger, more light can pass through and onto the sensor in any moment. The more rain passes through, the wetter you get; the more light passes through, the brighter your image becomes.

Shutter Speed

The shutter speed of your camera controls how long the shutter is open, and therefore how long the light is able to pass through the aperture onto your sensor.

You can think of the shutter as a hand over that unfortunate hole in your umbrella. All the time the hand is covering the hole, you stay nice and dry, however large the hole is. But as soon as the hand is removed, the rain starts to come through. The longer the hand stays off, the more rain comes through the opening. Equally, the longer your camera’s shutter is open, the more light can come through and hit the sensor. As before, the more rain passes through, the wetter you get; the more light passes through, the brighter your image.

Considering the hole in the umbrella, we can imagine that there are many different ways we can combine the size of the hole and the length of time it is uncovered to vary how wet we get. A larger hole exposed to the elements for a shorter time might well allow you to get equally wet as a smaller hole uncovered for much longer. In terms of light, these scenarios would relate to a larger aperture with a fast shutter speed compared to a small aperture with a slow shutter speed.

ISO

The ISO is a little more complicated but can be thought of as how sensitive the sensor is to the light that’s hitting it. Essentially, a higher ISO tells your camera to boost the signal coming in from the light hitting the sensor to make your image brighter.

So, there we have it: the three things that govern the brightness of our image. You should be able to see that, since each one has a way of making the image brighter or darker, then the three together can be mixed in a multitude of ways to achieve the same overall brightness.

So what is a “stop” of light?

A stop of light is a relative measure of the amount of light, rather than an absolute one. This is an important concept to grasp, so it’s worth repeating.

A “stop” of light doesn’t have a value on its own.

It’s not like that half kilo of dog poop swinging in a bag on your belt, or the three hundred metres you find yourself chasing Fluffy to retrieve the pair of underpants he’s just run off with in the park after they fell out of your trouser leg. Nope. On its own, a stop of light is not measurable.

What we can say, though, is that increasing an amount of light by a stop doubles the amount of light there is. Conversely, decreasing the amount of light by a stop will halve the amount of light.

Correct Exposure

One stop over

One stop under

How do stops relate to exposure settings?

Each of the exposure settings has a different scale or range of numbers that designate the value of a stop.

Shutter speed makes stops pretty easy to understand just by virtue of how they are numbered. As you can probably imagine, if your shutter is open for half the amount of time, it lets in half the amount of light. So you already know that a shutter speed of 1/1000 second lets in a stop less light than a shutter speed of 1/500 second.

Knowing this, it’s easy to create a scale of stops when it comes to shutter speed settings. Each of these is one stop difference:

Similarly, an ISO of 100 is half as sensitive to light as an ISO of 200, which equals a stop less light.

Aperture doesn’t play neatly in the same way, unfortunately, but here’s a list of apertures that are exactly one stop apart:

Now you’ll probably notice that the numbers listed above do not correspond to the options that your camera gives you when choosing exposure settings. Most modern cameras are set to provide you with settings in ⅓ stop increments.

So for shutter speed, instead of having to go from 1/500 second straight to 1/1000 second, you can stop off ⅓ of the way along at 1/640 second and 1/800 second.

For ISO, you can set it to ISO 1000, then ISO 1250, then ISO 1600 - on the way to the next full stop of ISO 2000.

Aperture, with its weird increments, allows you to go from f2 to f2.8, stopping off at f2.2 and f2.4

This gives you finer control over your exposures.

In conclusion, a stop of light is a relative measurement of light, where a full stop increase means that double the light reaches the sensor to form the image. There are different scales for the three different exposure settings, and while you don’t need to learn the exact values off by heart (especially the tricky one, aperture) it does help to have a good working knowledge of how that light is increased or reduced depending on the settings you choose.

Hopefully this has helped you wrap your head around what a stop actually is - and isn’t! Understanding stops of light is a fundamental aspect of photography that allows you to take control of your camera's exposure settings.

If you have any questions, feel free to pop over to our Facebook group and ask!