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Technical (beginner to intermediate) - Understanding APERTURE

A lot of people new to photography - and even some who have been in photography for a long time - struggle to understand the terminology we use when talking about apertures. It really is not complicated or something to fear. It just needs some proper explanation, one step at a time.

First, let's just be clear that "aperture" refers to a hole which allows light to pass through the lens. In lenses with variable aperture settings (i.e. most lenses) it is roughly circular and is controlled by a diaphragm made up of several blades. These blades move in unison to make the hole bigger or smaller by specific measured amounts. The blades have curved edges and although they don't create a perfectly circular hole, it's an approximation which works well enough.

If you look into your lens, you can often watch these blades moving to create bigger or smaller apertures. On SLR cameras, lenses will leave the aperture wide open (i.e. the biggest hole) until the shutter button is pressed, wthen the blades quickly move to the appropriate position during the taking of the photograph. This is to let the maximum amount of light pass through the lens and into the camera's viewfinder. There is usually a button you can press to let you see the effect of reducing the aperture. If you press that while looking into the lens, you should see the aperture changing. That button is usually called the "depth of field preview". (We'll come back to this "depth of field" phrase later.)

What confuses many people is the numbering system, with smaller numbers used for larger apertures - and even those numbers seem bizarre!


Before discussing any numbers, let me first explain another term frequently used in connection with apertures and exposure, namely "stop". One stop is an adjustment to the camera settings which doubles (or halves) the amount of light landing on the light-sensitive material (the sensor in digital cameras). So, we might talk about going up a stop (doubling the amount of light) or going down a stop (halving the amount of light). The amount of light getting through the lens is controlled by two camera settings: the size of the hole (aperture) and the length of time it is open (shutter speed). When we talk about adjustments to the aperture setting, we refer to f-stops.

Cleverly, cameras and lenses are designed so that the adjustments we make are accurately controlled, and evenly spaced. You really don't need to understand WHY f2.8 allows twice as much light through as f4, just that it does. Interestingly, though, and for those who enjoy mathematical progressions, alternate stop numbers are doubled (sometimes rounded to the nearest whole number), so, for example, they go f2.8, f4, f5.6, f8, and so on. (f5.6 is double f2.8, f8 is double f4.) As I said, don't worry about why this is. It's enough just to accept that these are not the purely random numbers they might appear to be at first sight.


As if the numbers themselves aren't confusing enough, the sequence seems to go in the wrong direction! It is important to understand that bigger numbers actually mean smaller apertures. So f2 is a large aperture, while f16 is a small aperture. I'll come on to an easy trick to help you understand how to use these numbers in a moment.

We briefly touched on "depth of field" earlier. This is simply a range of distances over which our image is acceptably sharp. We can adjust this by using bigger or smaller apertures. The smaller the aperture, the longer the depth of field. This allows us to exercise control over our images in a very creative way. If we want to have our subject in focus and the background blurred, we use a large aperture to do this. Alternatively, we might want to have as much in focus as we can possibly get, and a small aperture will achieve this for us. Maximising our depth of field is a subject for another day.

Now, here's that trick I mentioned for remembering how to use apertures if the numbers get you confused. A bigger number means a bigger depth of field, and conversely, a smaller number means a smaller depth of field. It's that simple.

© David McHutchison (Deemacphotos)
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