Understanding the f/stop scale is the key to achieving a correct exposure.

How f/stop work, how they can be changed and what they mean will help you in your search for great photographic imagery.

Our article here gives you all the f/stop know-how. You just have to put it into practice.

What Is a F/Stop?

In photography, a ‘f/stop’ is a measurement of an exposure. We already know that an exposure is made from the settings, making up the exposure triangle. These settings are in fact the infamous shutter speed, ISO or aperture.

To be specific, if you were to increase the exposure by one stop, you would be doubling the exposure. Decreasing the exposure by one stop is having it.

So, for example, suppose 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 settings 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, you will start to shoot in manual more often. As this happens you gain more and more control over how the camera exposes the scene.

Knowing what one stop can do for the shutter speed, ISO and the aperture will affect how you change each one.

Let me make this simple for you:

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.

Perfect! But it will also double the amount of light that’s entering your lens.

You have jumped up one stop with your aperture and made the exposure brighter. You need to counter this with a change in shutter speed or ISO.

To do this, you can halve the ISO to 100 or double the shutter speed from 1/100 to 1/200 of a second.

So you see, this is quite important to know.

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

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.

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.

Sadly, this is not the case, and 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 f/stop 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. 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. In doing so, you’re halving the open area of the aperture in the lens.

The most important thing to know about these f/stop numbers is that, from each number to the next, the aperture decreases to half its size. Thus allowing 50% less light through the lens (1 f/stop).

This is because the f/stop numbers come from an equation used to work out the size of the aperture from the focal length.

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

Say, for example, you have a 50mm lens with the aperture of f/2. To find the width of the aperture, divide the 50 by the 2, giving you a diameter of 25mm.

You then have to take the radius (half the diameter: 12.5), multiply it by itself to create the radius squared (giving 156.25) and multiply that by pi (giving 490.9).

The whole equation looks like this: Area = pi * r².

This isn’t essential for you to know but it may help you to get your head around it.

Here are a couple of f/stop examples:

A 50mm lens with the aperture of f/2 = a lens opening 25mm wide (50mm/2). Half of this is 12.5mm and, using the equation above (pi*12.5mm²) we get an area of 491mm².
A 50mm lens, with the aperture of f/2.8 = a lens opening 17.9mm wide (50mm/2.8). Half of this is 8.93mm and, using the equation above (pi*8.93mm²) we get an area of 250mm².

Now, it doesn’t take a genius to work out that half of 491 is smaller than 250. That’s because the numbers used are rounded to the nearest decimal point. The area of f/2.8 will still be exactly half of f/2.

This is what the aperture scale looks like (not to scale):

Ok, so that’s f/stop for you. With all this new information, you should have a much better understanding of how to control your exposure.

But Wait, There’s More!

You will have noticed with your aperture, shutter speed and ISO, that there are more intervals than just doubling and halving exposures.

These are third stops which give you more control over your exposure.

For example, between f/2.8 and f/4, you will also find f/3.2 and f/3.5.

This doesn’t have to be complicated at all and you shouldn’t think too much of it; just knowing what it does will help you work it out in time.

This video gives you a simple explanation of f/stop photography.

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