Introduction to Aperture – Technical Photography Fundamentals

I’d like to deal with aperture first, as it’s at the front of the camera, and it seems a logical place to start.

Put very simply, aperture is a hole in the lens through which light passes.  On all but the simplest and cheapest lenses, this aperture can be altered, from being as large as possible, to very small. The different apertures are assigned numbers, and just to get you off to a confusing start, the largest apertures have the smallest numbers, and the smallest apertures the biggest numbers. Thanks for that, Mr Inventor of Photography.

The Aperture Scale

f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f45, f64

Small number/wide hole——————————–Large number/small hole

The difference between each aperture is – wait for it – one stop. Remember, that a stop is the basic unit of exposure.  So from f2.8 to f4 is one stop, and from f11 to f16 is one stop, and so on up and down, back and forth. If you make the aperture one stop smaller, say from f5.6 to f8, you will let half as much light through the lens and into the camera. Not surprisingly, if you make the aperture one stop larger, say from f22 to f16, you will let twice as much light into the camera.

I’ll demonstrate this with this very simple picture.  Here’s a correct exposure.

Correct exposure

Now we’ll close the aperture down one stop:

The image is obviously darker, and even more so when we close down by two stops:

Going the other way, if we open up one stop from the correct exposure, it looks this bright:

Even more so when we open up by 2 stops:

Now, if you were paying attention in the previous video about the basics of exposure, you’ll realise that the reason the image gets lighter or darker is because I’m changing only the aperture, and since the camera is in manual mode, it doesn’t do anything to try and correct the mistakes I’m making.  If it was in some form of automatic mode, you wouldn’t see any change in how light or dark the image was.

Right, that’s the basics. Perhaps a little more detail will help you not only understand better, but also make you sound clever in front of your friends. Let’s talk about the numbers a little more. Why f8? Or f16? Knowing this will have no effect whatsoever on your photography, but since you asked, the number is a mathematical expression of the size of the aperture in relation to the focal length of the lens. Exciting, isn’t it! What is useful to know though, is that f8 on one lens, should give you exactly the same exposure as f8 on another lens, and the same is true of any aperture. This means if you’ve got the perfect exposure for a shot, but decide you want to shoot it with a different lens, you don’t need to start all over again, just use the same aperture on the new lens. Clever that.

Actually, one other thing. Don’t panic when you examine the progression of numbers and notice that they don’t actually follow a perfect mathematical sequence. You do not need to know why this is, I’ve lasted my whole photographic career without a satisfactory explanation, so I think we can get through this beginner’s guide without one!

So we know what apertures are, and how they work. What we care about, beyond just getting light through the lens so we can take a picture, is what effect do different apertures have on our image. Let’s take a look

The main effect that changing apertures has on an image is to increase or decrease “depth of field”. Great, another vague sounding photographic term. This simple guide is no place to plunge headlong into the complexities of sharpness, focusing and circles of confusion, all of which can get very confusing (do you see what I did there?) As far as we’re concerned at this stage, depth of field means how much of the image is in focus. If you have a small depth of field, then very little of the image will be in focus, either side of the point you focus on, and if you have a large depth of field, more of the image either side of that focus point will be sharp.

Image with a shallow depth of field, employing a wide aperture
An image with a small depth of field. Notice how the eyes are in focus, and almost everything else is out of focus
Image with large depth of field, employing a small aperture
An image with a large depth of field. The freezing cold journalist is in focus, along with the mountains away in the background

Where aperture fits into this is that a large aperture (a small f number – remember) will give you a small depth of field, and a small aperture (a large f number) will give you a large depth of field. So, shooting something at f2.8 will give you a very narrow area that will be in focus, and shooting at f22 will allow you to get much more in. For the sake of simplicity, think “small aperture number – not much depth of field/large aperture number – lots of depth of field”

The apertures used to take the above images

We’re about to leave aperture behind and move on, but before we do, just let me refresh your memory. In case you’re now thinking “ah, excellent, now all I need to do in future is pick whichever aperture I want for the right exposure and the amount of depth of field I’d like.” Nearly, but not quite. Don’t forget that the three facets of exposure are interlinked, so if you make a change in one area, you’ll need to make a matching change in another. In practice this may mean that the amount of light you’ve got to play with forces you to shoot at certain settings, or the shutter speed you need limits your aperture choice. Don’t start celebrating just yet.


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