What is aperture? Photography tutorial for DSLR beginners

This clip explores aperture and how it's used to control depth of focus, as well as how wide aperture can be used to shoot in low light with low ISOs. Tips on choosing lenses according to aperture, balancing ISO and shutter speed plus using AV and M modes.

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What is aperture?

Well, let’s look at some images:
This is a shot by the legendary landscape photographer Ansel Adams. I’d like you to consider the plant at the base of the central rock and the rugged mountain skyline. Think about focus and sharpness. Notice both the plant – just a few feet from the camera – and the mountain, many miles away are sharp – in focus.

Now, this is a shot by the not-quiet-so legendary Tom Greenwood. Notice that the opposite effect is in play here. The tent in the background and the child in the foreground are both nicely blurred.

What is responsible for these huge extremes in depth of focus? Yes, aperture.

Put simply, aperture refers to the size of the hole through which light travels before it hits our camera sensor or film and burns an image. Clearly the bigger the hole, the more light is allowed in; the smaller the hole, the less light is allowed in. VIDEO – lens varying aperture

It’s important to understand that aperture is a function of the lens NOT the camera itself (although on today’s digital SLRs we usually control aperture through dials on the camera).

Each lens is identified by its f number – in other words the size of its aperture when wide open. We can see that the lens on the left has a much smaller aperture than the lens on the right. Width of aperture is a major indicator of the quality of a lens.

Aperture – like ISO and shutter speed – is measured in stops, a measure that represents the halving or doubling of the light absorbed by the camera’s sensor. Unlike ISO and shutter speed, the sequence of numbers, or f-stops, seem a bit confusing.

But here are the important ones that you should really try to memorise:

F2, f2.8, f4, f5.6, f8, f11, f16, f22

Let’s look at how aperture works in practice. We are going to shoot some portraits. I’m using a 50mm lens with a wide - f1.4 – aperture. I suggest every photographer has a 50mm lens – a good f1.8 50mm can be bought for a very affordable $100. This really allows you to experience wide aperture in a way that an f4 lens really doesn’t.

So we’ll start shooting at f5.6. The background is reasonably sharp but let’s see what happens when we widen the aperture to f4, f2.8, f2 and f1.6. You can see the background soften and blur nicely.

Notice too that the closer we get to the subject the narrower the depth of focus.

Narrow depth of focus is a beautiful tool for highlighting the important features – such as eyes – in an image.

But it does require some caution. In these shots I’m focusing on the girl in front. Here at f5.6 both children are sharp but as we widen the aperture to f4, f2.8 and f2, the boy who is only a few inches behind gets out of focus.

So for group shots try to maintain a broad depth of focus: f5.6, f8 or f11 depending on how far from the camera – and each other - each person is.

Assuming the light remains the same, any changes to aperture require compensation in either ISO or shutter speed.

Let’s go back to the portraits:

Here we’re shooting at a ISO 800. The aperture is f5.6 and shutter speed is 1/125. Opening the aperture to f4 – one stop – requires a doubling of shutter speed to 1/250 or a halving of ISO. Changing shutter speed won’t have any effect on the image but since ISO 800 is quite high we can get a less ‘noisy’ image, by lowering it to ISO400. And opening the aperture another stop to f2.8, allows us to reduce the ISO again to 200.

This illuminates another fantastic benefit of wide aperture: it’s great for shooting in low light.

Since a wide aperture allows more light into the camera you can shoot at lower ISOs and therefore avoid nasty ‘noisy’ images.

Ultimately you should be aiming to shoot in manual or M mode but to begin experimenting with aperture, you might want to start by using AV – aperture priority – mode. NEED VIDEO – switch to AV mode

In this mode, you set the aperture and the camera adjusts the shutter speed to get a correct exposure. Note though, it doesn’t change ISO for you.

To sum up, aperture is great for two things: exploring depth of field and, assuming your lens is wide enough, shooting in low light. So, grab a 50mm lens and get shooting…

If you enjoyed this video, I recommend you also look at my videos on using DSLR modes, ISO and shutter speed.

Thanks and see you next time!