Camera College
Controlling the size of the hole in your lens might be crucial to making a good exposure, but it also allows you to get creative with sharpness and blur
What’s in an aperture? Read all about it...
Changing the aperture of your lens is one of the fundamentals of photography. It allows you to alter both the exposure of a picture and how much of it appears sharply focused. But aperture can be a confusing control to get to grips with, thanks largely to its strange f/number scale, which is seemingly arranged in the wrong order. The aperture itself requires little explanation: it’s simply a physical hole in the lens that light passes through to reach your camera’s imaging sensor. In most lenses, this hole is formed by overlapping blades that create a diaphragm, which regulates how much light is let into the camera.
Large apertures let more light into the camera; small apertures reduce the amount of light. That bit’s easy enough to understand. What’s more confusing is the set of numbers that represent the size of the aperture. Why does it jump from f/4 to f/5.6 to f/8 and so on? And why does a low f-number such as f/2.8 represent a large aperture, and a high f-number such as f/22 represent a small aperture? Surely high f‑numbers should give you more light, not less!
The first thing to consider is that f-numbers aren’t a physical measurement of the diameter of the aperture. Instead, they represent the ratio between the diameter and the focal length. An aperture of f/4 actually means the focal length (f) divided by four, and an aperture of f/16 means the focal length divided by 16. Using this rule, you know that when a 400mm lens is set to f/8, for example, the diameter of the aperture is 50mm.
The same aperture on a 100mm lens has a diameter of 12.5mm, but it lets in the same amount of light.
This sequence of f-numbers might seem arbitrary, but there is a logical reason why the scale jumps in such a strange fashion. It’s because the amount of light let in by the aperture is determined by the area of the opening, so it’s the square of the f-number that counts: f/2.8 squared is about 8; f/4 squared is 16; f/5.6 square is about 32. So as you can see, each f-number gives twice the level of exposure as the previous f-number in the sequence. An easy way to remember that lower f-numbers represent larger apertures is to treat the f/numbers as fractions: so 1/4 is larger than 1/8, and an 1/8 is larger than 1/11.
The size of the aperture isn’t the only factor that controls the brightness of a picture, however. The shutter speed sets the length of the exposure, and the ISO sensitivity determines how much light is required overall to record an image. You can let the camera adjust the aperture, shutter speed and ISO, or just one or two of these settings – or you can take
An easy way to remember that lower f-numbers represent larger apertures is to treat the f-numbers as fractions
control of all three yourself. There are many times when you’ll want to set the aperture manually, as it can have a significant impact on the depth of field as well as the brightness.
Although you can only focus the lens at one specific distance at a time, sharpness doesn’t start and stop at this point. There’s an area that extends in front of and beyond the point of focus where things still look acceptably sharp – this area is known as the depth of field. Small apertures (such as f/16 and f/22) increase the depth of field, while large apertures (such as f/1.8 and f/2.8) reduce it. Using a large aperture can help to blur background details and make the main subject of a picture stand out, so it can be a good choice for portraits and wildlife shots. Larger apertures also allow you to use faster shutter speeds, because the increase in light means the sensor only needs to be exposed for a shorter period of time. The downside to large apertures is that the shallow depth of field will expose any focusing errors.
Smaller apertures are more forgiving in this respect, as the increased depth of field means that more of the image will appear sharp. They’re useful for landscape and macro photography, where you’re typically looking to squeeze as much detail out of the scene as you can. The trade-off is that smaller apertures require slower shutter speeds to make an exposure, which means that you may need to use a tripod or increase the ISO to get sharp shots.
The downside to using a large aperture is that the shallow depth of field will expose any focusing errors, particularly when you’re working in close-up