The Basics of Exposure 2 - Aperture

[This is the second article in a series on the Basics of Exposure. Before reading this one you should read, or at least have looked at, Basics of Exposure 1 – Overview so that this one will make sense.]

You can go along happily with your digital camera, DSLR or otherwise, set on fully automatic, without ever even running into the word APERTURE and still end up with some decent snapshots. But sooner or later, if you are interested in photography to any degree beyond the very superficial, you are going to need to understand the subject of aperture setting – what it is, what it means, what you can do with it and how to use it. It forms an important part of correct exposure, but that’s not all.

What is “Aperture”?

As we covered in the first article in this series, the aperture is simply the opening in the camera through which the light comes in when you press the shutter release to take a photo. It is at the center of a diaphragm, a device made up of a number of leaves or plates, usually metal, that can be controlled so as to make a larger or smaller hole or aperture.

The diaphragm is built into the lens in an interchangeable lens camera. If you have several lenses, each one contains a diaphragm. In a fixed lens camera you have a single diaphragm which is usually part of the lens.


Wide aperture	Narrow aperture	Diaphragm

Numbers are assigned to different widths of the aperture – f2.0, f3.5, f5.6, f22 and so on. There is a method to this numbering, but all you need know at this point is that the smaller the number, the wider the aperture. So an aperture of f2.0 will allow a lot more light in than an aperture of f22. Don’t worry about the “f”. At this point just understand that if you see “f” followed by a number (e.g. f8) then the number is a designation of the width of the aperture for a particular lens.

The numbering system is a bit odd and varies from lens to lens. But here is an example of the f-numbers for a particular lens. Each whole f stop represents half the amount of light as the previous one:

So at f2 this lens will let twice as much light through as at f2.8 and four times as much light through as at f4 and so on.

And that is all you need to know at this point about what the aperture is: it’s a variable hole through which light is allowed into the camera when the shutter is opened. The wider the aperture, the smaller the number used to designate it.

We can go into the technicalities of how the numbers are arrived at and so on in a later article, but you’re not going to be building a diaphragm. You just need to know it’s there and that it controls the aperture which can be adjusted to allow more or less light into the camera per unit of time to record the photo. And on most cameras, you can control the aperture yourself for every shot you take.

In fully automatic mode the cameras sets the ISO, the aperture and the shutter.

In aperture priority mode (a possible shooting mode on most modern cameras), you choose the aperture you want for the shot and the camera sets the appropriate shutter speed. If the camera has an automatic ISO setting (not so common) then it will set both the ISO and the shutter speed when you set the aperture.

In full manual mode you choose everything and set everything yourself – ISO, aperture, shutter speed.

Most cameras except for the most elementary ones will give several choices of shooting modes.

Choosing the shooting mode will be the subject of another article. But you need to understand what you are setting and why before you can make use of these features.

Controlling the Exposure

If you had a hose that was made so that you could increase or decrease its diameter, a tap to turn it on or off, and water flowing in it, then the water could represent the light, the variable width hose would be the diaphragm, the diameter of the hose at any given time would be the aperture, and the tap, which you can open for long or short periods of time to allow much or little water through, would be the shutter. You can see right away that a wide hose would allow more water to go through the tap in a given amount of time than a narrow hose.

The same amount of water will go through a wide hose in a short period of time as will go through a narrow hose in a longer period of time.

How does this work with the camera? Well, if a certain amount of light needs to pass through the lens to the sensor in order to get a correctly exposed photo at a given ISO setting, then you have a choice of a combination of shutter speeds and apertures each of which will allow that exact amount of light through and thus give you the same exposure.

For example, let’s say the correct exposure for a certain landscape you want to capture would be 1/500^th of a second at f8 at an ISO setting of 100. OK. Well you can get the exact same exposure if you set the aperture at f11 (f11 will let in half the amount of light as f8 – it is one whole f stop less than f8) at 1/250^th of a second at the same ISO setting. You change the aperture from f8 to f11 which means you allow half the amount of light in so you have to double the amount of time you allow it in for, in other words 1/250^th of a second which is twice as long as 1/500^th of a second. The result will be the same exposure.

Or you could double the length of the exposure again, 1/125^th of a second. You have to also halve the aperture again. The next full stop smaller than f11 is f16. So an exposure of 1/125^th of a second is the same as one at 1/250^th of a second at f11 which is the same as 1/500^th at f8 as long as you keep the ISO setting the same.

This next diagram shows the relationship between shutter speed and aperture.

f/2	f/2.8	f/3.5	f/5.6	f/8	f/11
•	•	•	•	•	•
1/500^th	1/250^th	1/125^th	1/60^th	1/30^th	1/15th

(Each of the above combinations of aperture and shutter speed will give the same exposure as long as the ISO setting is not changed.)

You can figure it out from there. Basically, if you halve the aperture you must double the shutter speed to get the same exposure. If you double the aperture width, you must halve the shutter speed to keep the same exposure. Don’t worry about the actual numbers. Just grasp the principle.

If you haven’t totally grasped this, don’t worry. It will make sense as we put it into practice.

But, if you have followed so far, you should be asking yourself, What does it matter? If you can get the same exposure at f2.8 at 1/1000^th of a second as you could at f3.5 at 1/500^th, f5.6 at 1/250^th, f8 at 1/125^th, f11 at 1/60^th or f16 at 1/30^th, then why should I even pay attention as long as the camera chooses one of those settings? Good question.

Well, for one thing, you can see that if the aperture changes, the shutter speed changes. So in the example shown in the diagram above, if you decided to shoot at f22, you can see that you would have to set the shutter speed at 1/4^th of a second to get the right exposure. Well, that would present a problem. If you are hand-holding the camera or if the subject is moving, you’re going to get a blurred image. We’ll go into this in more detail in the article about shutter speed. For now you can take my word for it. That shutter speed, 1/4^th of a second is not going to stop rapid motion or prevent camera shake even if you have a steady hand. So if you want to have your photo come out really sharp, you can forget f22. Now, most modern digital cameras will adjust this automatically so that the shutter speed is adequate at least to deal with camera shake and make sure the shutter speed is not too slow, but the camera won’t know if you’re photographing something moving fast and it won’t necessarily know what to do if there isn’t enough light to shoot at a sensible shutter speed at the ISO setting you’ve chosen.

Perhaps you can begin to see the value of having some knowledge and control of these settings.

You should have a pretty good idea by now that for any given photo situation you have a choice of aperture and shutter speed. You can choose different combinations of aperture and shutter speed and still get a correct exposure.

But there is more to the subject of APERTURE than just how much light it lets into the camera at a given f stop (aperture) and how to set it to make sure you also have an acceptable shutter speed so that your photo is not blurry.

Aperture and Depth of Field

In order to understand a key reason why you might choose one aperture over another, we have to look at a subject called Depth of Field or Depth of Focus (same thing – depth of field is probably more common though depth of focus is more descriptive). You have run into Depth of Field already, perhaps without knowing it. You have seen photos where everything is in focus from the near foreground all the way to the horizon. And you have seen others where a face perhaps fills the frame and the eyes are sharply in focus but the tip of the nose is out of focus and soft and so are the ears and the hair.

This can be used to advantage if you know how to control it. But it can also work against you if you don’t.

So you might as well know how to control it, or at least one way to control it. It has much to do with aperture settings.

Depth of Field refers the distance in front and behind the point of sharp focus in which objects are still in acceptable focus. A narrow depth of field means that things are in focus only a little bit closer to the camera than the point you focus on and only a short distance behind this point. The photos here will show you what I mean. A wide depth of field means that quite a lot of the scene in front and behind the point you are focusing on is still acceptably sharp.


f/2.8	f/8	f/32

The point of focus is the pen in the middle in all three photos. You can see that the narrower the aperture, the more is in focus beyond and in front of that point of focus.

It can be very annoying when you have a beautiful landscape in front of you with flowers in the foreground, trees and maybe some horses or people in the mid ground and a striking mountain range or dramatic sky in the background and you want it all in focus, only to look at your resulting photo and find that the flowers have turned into blurry blobs, the horses are OK (or at least two of them) and the mountains and clouds are hardly discernible they’re so out of focus.

On the other hand, it can be just as frustrating when you are trying to take a photo of your five-year-old and you really don’t want the wire fence behind to stand out and you would like to blur the leaves of the flowers you are shooting through, but you end up with everything in perfect focus, ugly fence behind, and flowers in front, all distracting the viewer from the image of your child who you wanted to be the center of the photo.

In both cases, understanding and controlling the depth of field would have given you the photo you wanted.

This is where aperture comes in. The basic rule is, the wider the aperture, the narrower the depth of field. We should note that aperture is not the only thing that influences depth of field. The lens you choose or the zoom setting you use (wide angle or telephoto) and your distance from the point that you are focusing on also play a part. We’ll cover that in a separate article so you get the full picture. But the aperture setting is key. If you want everything in focus from foreground to distant background, you want to use as narrow an aperture as you can – f22 if your lens goes to that, or even narrower.

If you want only a small band of your subject matter to be in focus, with an out-of-focus foreground and background, then you want to use as wide an aperture as you can, say f2 or f1.8 or even f1.4 if your lens will go that wide.


In this photo the background is completely out of focus, emphasizing the subject – f/3.5.	Here the focus is on the house but even the daisies in the foreground are in acceptable focus – f/16.

As with many things in life, there is compromise. In order to shoot your landscape at f22 you may have to reduce the shutter speed to 1/15^th or 1/8^th of a second. Time to mount your camera on a sturdy tripod so that you won’t get camera shake. And if your subject is moving fast – wind in the trees, horses galloping, etc. – you will not be able to stop the motion even if you can eliminate the camera shake by use of the tripod. This is when your third factor, ISO setting, may save you. It’s the third corner of the triangle. So if at ISO 50 your choice of f22 forces you to shoot at 1/8^th of a second, you can change your ISO setting to 400 and, hey presto! you can now set your shutter speed to 1/125^th of a second and your photo will be in focus all the way and sharp. BUT, the increased ISO will cost you in terms of image quality. Compromise.

So you see, you have to have an understanding of these basics so that you can use your judgment to decide on the best combination of aperture, shutter speed and ISO setting to get the image you want or at least as close to it as you can.

Well, we are getting a bit more into the thick of it, even though we are still only scraping the surface. You can always back out now, trust to luck and get some good shots (and ignore the disappointing ones). Or, you can bite the bullet and read the next article on Shutter Speed, and then read these first three again and start putting all this information together, learn how to adjust all these controls on your camera and begin to really learn the subject…and be on the way to greater photography. It’s not that hard to learn. Easier than learning how to play a musical instrument for example. You just need to decide to do it.

David © Phillips is a professional writer and photographer living in Seattle, WA. You can find out more about him and his work at www.dcpcom.com