Aperture and Depth of Field

How to adjust the aperture

Most digital SLRs and some compacts have at least the following three shooting modes: Manual (denoted by a large M); Shutter Priority (a large S or Tv) and Aperture Priority (you've guessed it, a large A or Av). The last two of these will allow you to manually adjust the aperture. In Manual mode you will have to set both the shutter speed and aperture for an accurate exposure, while in Aperture Priority mode you only have to set the f-stop, the camera will set the shutter speed for you.
So let's assume you are in Aperture Priority mode. Where you will adjust the f-stop that is selected will depend on your camera and lens combination. For some cameras you will have to turn a ring which is right at the base of the lens, where it meets the camera's body. For others the setting on this ring will have to be at the largest number, and the actual f-stop selected via a dial or button on the camera itself. Finding the dial that adjusts the f-stop will often entail hunting around for something that changes a set of numbers on the cameras display in a seemingly irregular fashion. These will most often contain, f5.6, f8, f11, f16, f22, with one or two stops at either end of the scale.
If you are shooting in Aperture Priority mode and the selected f-stop is too large or small for the ISO setting you are on, the camera will display an error message such as "lo" or "hi". Heed this warning and adjust accordingly, or your pictures may be too light or dark. (Of course, the camera's built in light meter is fallible and in a future issue we will look at how to override it completely).

The aperture explained

In photography, aperture refers to the size of the opening in the lens of the camera through which light can pass. The size of this aperture is adjustable in nearly all the lenses that fit digital cameras. By adjusting the size of the aperture, the photographer can ensure that the correct amount of light reaches the digital sensor during any given exposure.
As such, it is one of the three elements used to provide a correctly exposed image. The others are the length of the exposure, called the shutter speed, and the light sensitivity of the sensor, called the ISO.
The aperture can be adjusted either manually or, in most cameras, automatically by the camera. When the diameter of the aperture is changed, a set of blades inside the lens narrow down or open up to allow more or less light to pass through the lens.
The act of narrowing down the aperture is often referred to as 'stopping down' while opening it up is called 'stopping up'.

Novices often find the word 'stop' connfusing, but it is easier understood when one realises that the aperture is, by convention, adjusted not over a continuum, but rather is set steps, or stops. One stop is equal to half the area of the one preceding it. So when you close down one stop, you are halving the area of the aperture, and when you open up by one stop, you are doubling it.
The stop selection dial (which selects the size of the aperture) is marked with an odd set of numbers as noted above, often starting at f5.6, followed by f8, then f11, f16 and f22. There may be one or two stops on either end of the scale. The concept works like this. The number indicates the inverted size of the aperture as it relates to the focal length of the lens. This is quite a mouthful, and not easily understood, but the point is that the larger the number, the smaller the hole, and therefore, the less light is allowed to pass through the lens. The smaller the number, the larger the hole, and the more light is allowed to be transmitted.
So, other things being equal, the brighter the light in which the picture is taken, the less light would be needed for an accurate exposure and the bigger the f-number one must use. Inversely, the dimmer the light, the bigger the hole that is needed, and the smaller the f-number that is selected.
Once you understand this, you are halfway to grasping all that is needed to become a master of the aperture.

Depth of field and the aperture

I did say halfway...
The aperture is more than just a tool used to adjust the size of the opening in a lens. It is perhaps the strongest compositional tool in the arsenal of the
photographer.
Something interesting happens when the size of the aperture is changed. When it is opened up, and

more light passes through the lens, the area which appears in focus on either side of

the distance for which the lens is focused becomes smaller.
Picture this: you are taking a portrait picture. You focus the lens on the subject's eyes. Behind him is a tree. If you set the lens at a large aperture (small number) the tree behind him will not be in focus. If you use a small aperture (large number) the tree will be in focus.
This byproduct of adjusting the size of the aperture is referred to as 'depth of field' and translates into the depth (or distance) of the area which will remain in focus for a given aperture and focus distance.
The creative utility of adjusting the depth of field should immediately be apparent. By changing how big a part of the photograph is in focus, you can control exactly which details show up, and which do not, allowing you to lead the viewer's eye anywhere you wish.

More about depth of field

The depth of field becomes greater with smaller apertures, and vice versa, that much is true, but things are a little more complicated than that.
Depth of field decreases the shorter the focusing distance, so if you are focusing on a subject that is very close to the lens, you will have less depth of field than if you are focusing on a subject which is far away.
This has tremendous consequences for macro photography and it is often very hard to even get a tiny part of the image in focus.
Further more, the depth of field for a longer lens is always less than for a shorter lens, even with the same f-stop setting and distance from the subject. So if you are shooting with a very long lens, you will be struggling for enough depth of field, more often than not.
Finally, the depth of field on either side of the point on which the lens is focused are not equal. The area further from the camera which will remain in focus for any given f-stop is larger than the area for objects nearer to the camera than this point.
There is an old photographer's maxim which says: 'Focus a third of the way in' but this is merely a guide, and as you come closer to the object, the area that remains in focus on either side of the distance for which you have set the lens, tend to converge, meaning that the area that remains in focus in front of and behind this point are closer in size.
It is also important to remember that in most cameras, the size of the aperture is not actually adjusted until the moment the photograph is taken. At the moment the shutter is released, the blades which make up the aperture swing closed to the desired setting, and the sensor is exposed to the correct amount of light.
This means that the image you see when you look in the viewfinder does not correspond to the final image. What you normally see, is what the photograph would be like if the aperture was left at its largest opening (smallest number). Most digital SLRs have a 'depth of field preview' button, which when depressed, adjusts the aperture to the required setting, which, provided that the setting is smaller than the lens' maximum, would mean that the image in the viewfinder would dim down a little and that a larger part of the image will come into focus, allowing the photographer a live preview of what the final image will look like.