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Every photographer will agree with you that aperture and ‘F-numbers’ are confusing at first. Phrases like “open up your lens”, “increase your f-stop”, ‘”use a shallower depth of field”, “use a fast lens” all relate to aperture. Unfortunately, it needs to be understood in order to develop your skills and knowledge of photography.

This is a fairly lengthy article but I’ve written in a way which I wish had been presented to me when I first picked up a DSLR many years ago. Once you get your head around it all, it’s fairly straight-forward and logical and will make a big improvement to your photography. Let’s break it down so that it’s easier to digest.

What is Aperture?

Firstly, if you’re reading this, you’re most likely a new photographer looking to learn more, but may already know that aperture relates to the depth-of-field in photos, i.e. how much of the background (and foreground) is blurred in your shots.

Whilst that is correct, try to remember that aperture changing the depth-of-field in a photo is more of a side effect. The main function of aperture is related to how much light enters camera. This is controlled within the lens via small blades which widen or narrow the lens canal depending on the aperture value you’ve set on your camera.

Of course, as mentioned, it also affects your depth-of-field, where a smaller F-number results in more foreground and background blur.

What do the F-numbers relate to?

Assume we turn on a light bulb and hold up a piece of white paper one meter away from it. Now, let’s say at this point the paper is illuminated 100%. If we were to move that piece of paper to 1.4 meters away from the bulb, the brightness of the paper would halve. Just accept that this is true (as it is) and stick with me. Move it to 2.0 meters away and the brightness of the paper would halve again from when it was 1.4 meters away. The brightness will continue to halve at 2.8, 4, 5.6, 8, 11, 16 and 22 meters from the bulb. Those numbers again? 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Look familiar? Set your DLSR to Aperture Priority mode and scroll through the aperture settings and you’ll see the same numbers appearing.

Your lens works in a similar way!

Every time you increase your aperture setting (or F-number), your lens closes up inside and reduces the amount of light coming through. This can be seen in the following diagram. As with the example of the bulb and paper, when you move your aperture setting from 2 to 2.8, from 4 to 5.6, or from 8 to 11, the amount of light hitting your camera’s sensor (think, piece of paper) will halve due to the area of the lens opening being halved.

Lens Aperture Explained

 

So that explains why a larger F-number means a smaller aperture (and vice-versa)!

Indeed. This also explains why, when you increase your F-number (thus closing up the lens a little), the camera will compensate by selecting a slower shutter speed in order to let more light back in, in order to maintain a correct exposure for the shot.

Therefore, with this in mind, if you find your shots are coming out blurred, try opening the lens right up (i.e. choosing a wider aperture = smaller F-number). This will let more light through the lens enabling the camera to choose a faster shutter speed. This is particularly useful to know when taking photos indoors without a flash or in low-light conditions.

What are f numbers on a lens

Using a wider aperture enables you to shoot at faster shutter speeds in low light conditions.

 

I’ve heard of a ‘fast lens’. What is one?

This relates to lenses which can operate at apertures as wide as f/2.8 right up to even f/1.2. At such wide open apertures as this, the shutter speed of the camera can be set extremely fast with no loss in exposure. Let’s say, for example, a lens at f/4 requires a shutter speed of 1/100s to obtain a correct exposure for a particular photo. The same shot taken at f/1.4 would use a shutter speed of 1/800s.

Ok, so what’s this ‘F-stop’ I keep hearing about?

In a nut-shell, an F-stop is just another way of saying when the exposure (brightness) of a photo is either doubled or halved. For example, adjusting your aperture setting from f/8 to f/11 will (as we’ve already explained) halve the amount of light entering your camera (assuming your shutter speed and ISO don’t change). So you will have decreased your exposure by ‘one stop’.

Similarly, slowing down your shutter speed from ½ second to 1 second (to let twice the amount of light in), or raising your ISO from 200 to 400 (doubling the sensitivity of the sensor) will increase your exposure by one stop (again assuming your other settings remain unchanged).

What is aperture

A wide aperture (here I used f/1.8) is vital when you need to freeze movement even in dark conditions like at a concert.

 

So there you have it. You should now know that aperture is controlled within the lens via blades and that it affects both the amount of light entering the camera, plus your depth-of-field. You should also have learned the significance of those F-numbers and where they originate from, plus, how aperture affects exposure and the knock-on effect it can have on your shutter speed. What’s more, every time somebody talks in terms of ‘F-stops’, you’ll be on the same wavelength.

Author – Oliver Pohlmann

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