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Jargon buster: Stops

Bus stop lit with an off camera flash
Bus stop lit with an off camera flash
I didn’t know how to take a photo of the concept of a stop, so I took a photo of a bus stop instead. The bus stop has it’s own light inside, but it’s nowhere near powerful enough to illuminate it against even a modest amount of daylight, so I’ve cheated and hidden a flash inside. The warm tone comes from a ½ CTO gel over the flash. CTO (Colour Temperature Orange) is a warming filter. Where the flash has illuminated is correctly exposed, giving a bright image. Where the flash hasn’t lit I’ve deliberately allowed it to become underexposed, giving a dusky look.

Photographers are always talking about stops. We stop up and we stop down. We have f-stops. (and bus stops). The question a lot of new photographers will be wondering is what exactly is a stop?

Exposure

In order to understand stops we first need to understand what exposure is. Exposure in it’s simplest form can be thought of as how light or dark the image recorded by the camera is. There are three factors that determine this: shutter speed, aperture and ISO (how light-sensitive the digital sensor/film is). These three things are sometimes referred to as the “exposure triangle”.

As a quick recap:

  • Shutter speed is the length of time that light is allowed to enter the camera for.
  • Aperture is the size of the hole through which light enters the camera.
  • ISO is the light-sensitivity of the surface the light falls on to (i.e. the digital image sensor or the film).

The key to a correct exposure is balancing out these three settings.

Stops

So with exposure out of the way we’re on to the big question: What is a stop? Put simply, a stop is a relative unit of light. If I double the amount of light recorded I’ve increased my exposure by a stop. If I halve the amount of light recorded I’ve decreased my exposure by a stop. It really is that simple… almost!

The confusion comes when we start to look at the numbers:

Aperture Shutter Speed ISO
2.8 1/15 100
4 1/30 200
5.6 1/60 400
8 1/125 800
11 1/250 1600
16 1/500 3200

They’re all different! How do you know what doubling and halving any of this is? Let’s take them one at a time, starting with the easiest and working our way up to those weird aperture “f-numbers”.

ISO is the easiest to understand. If we’re set on ISO 200 and double the amount of light we get ISO 400:

200 × 2 = 400

That makes sense. It works the other way too. If we have ISO 1600 and we want half as much light we can choose ISO 800:

1600 ÷ 2 = 800

Again, very sensible.

Shutter speed is pretty easy to get to grips with too. It’s complicated a little bit by the fact that we’re often working in fractions rather than the friendly whole numbers we had with ISO. Lets start off with an easy whole-number example. Suppose we’re using a shutter speed of 4 seconds. To double the amount of light we would choose an 8 second shutter speed:

4 × 2 = 8

It’s almost too easy isn’t it. Going the other way, if we have a 2 second shutter speed and we want half as much light:

 2 ÷ 2 = 1

Lets make this a little more challenging now. Lets say we have a 1/250th second shutter speed and we want twice as much light (this will show who remembers those high school maths lessons!):

1/250 × 2

= 1/250 × 2/1

= 1 × 2 / 250 × 1

= 2 / 250

= 1 / 125

So, even with fractions involved, the maths still works – if we want double the exposure of 1/250th we would choose 1/125. Lets go the other way. If we’ve got 1/30th and we want half as much light:

1 / 30 ÷ 2

= 1 / 30 ÷ 2 / 1

= 1 × 1 / 30 × 2

= 1 / 60

Again, the maths is a little more complicated but still holds true, and we’re left with 1/60th.

At this point you’re probably thinking that I forgot about leaving the most complicated to last. But sorry – we’re not nearly there with mathematical complexities until we dive in to the strange world of f numbers – the world of the aperture.

2.8, 4, 5.6, 8, 11, 16  . . . who came up with that one?! That doesn’t make any sense at all right? While the numbers don’t seem to make a lot of sense they do actually serve a very important purpose. The f number (sometimes called an f-stop) is the ratio of the diameter of the aperture to the focal length of the lens. For example, at f/4 a 50 mm lens will have an aperture diameter of 50 / 4 = 12.5mm. So why not use 12.5 instead then? Suppose we have a 100mm lens with a 12.5mm diameter. Due to the longer focal length, the lens gathers light from a narrower field of view when compared with the 50mm lens. This means that the 100mm lens will give a darker image than the 50mm at the same diameter. It’s going to get confusing if every time I change lens the numbers all change with it. That’s why instead of using the diameter, camera manufacturers have standardised on the f-number system. Our 100mm lens at 12.5mm aperture diameter will have an f-number of 100 / 12.5 = 8.

There isn’t really any easy way to work out f-numbers on the fly, so what most photographers do is accept the sequence of 2.8, 4, 5.6, 8, 11, 16, 22 as being “whole” f-number values. This means we only need to remember a short sequence of numbers.

Clicking the dial

The good news with all three settings on most cameras are they’re handled in 1/3rd steps, so for every click of a the dial your camera changes the setting by a third of a stop (this may be indicated as -0.7 / -0.3 /+0.3 / + 0.7 on some cameras). With this knowledge we know that if we want to make the image darker by a stop all we need to do is click any of the settings down by 3 clicks. If we want to make the image one stop brighter we just need to go three clicks the other way.