Photography A to Z: A is for aperture

January 21, 2017  •  Leave a Comment

This is the beginning of an occasional series of blogs on the meanings of various photographic terms. I've wrapped it up in an A to Z format but will no doubt go round the alphabet several times, as there are a few options under each letter.

So what the heck is aperture? Well, it is 'a space through which light passes in an optical or photographic instrument, especially the variable opening by which light enters the camera'. Mmm, much clearer, eh! No, perhaps not.

Aperture is one of photography's trinity of exposure - the other 2 being ISO and shutter speed (these will be covered in a later blog). So if the hole, or aperture, allowing light into the camera is wide, then more light will hit the sensor, and vice versa. In photography the size of the aperture is referred to as the f-number or f-stop. And if you are interested this is the ratio of the lens's focal length and the size of the aperture.

Here is a graphic showing some of the f-numbers:

These would continue from f8 to f11 to f16 to f22 and further if your lens could achieve it.

It can be seen that the smaller the f-number, the larger the aperture and this would seem rather counter-intuitive. It might also be less obvious that for each 'stop' of aperture there is a halving or doubling of the light entering through that aperture. So for example, going from f5.6 to f8 is halving the amount of light reaching the sensor. Conversely, going from f5.6 to f4 will be doubling the amount of available light. Still not convinced?

This next bit is a bit technical and may not 'float your boat' but I think it's worth sticking with as it proves a point...eventually.

The thing to remember from the definition at the start is that f-number is a ratio of focal length and the diameter of the aperture. For example, if we used a 50mm lens then the diameter of the aperture at f2 would be 50/2 = 25mm. So far, so good. It would make more sense to think of the size of the aperture in terms of its total area rather than just a diameter in order for us to understand how much light might be getting through. This means we have to get into calculating the area of a circle. Honestly, it's not as bad as it seems. We would use that old calculation from school days of πr2. π is a constant, call it 3.142, r2 is the radius (half the diameter) of the aperture, squared, i.e. multiplied by itself. Stay with me ... this means in our example above that the area of f2 would be 3.142 x (12.5 x 12.5) = 490 square mm.

So what was the point of that! Well, if you let me finish it will help show why the next f-number will have a half or double sized aperture area. So if we now go to f2.8 the calculations are the same as above i.e. 50/2.8 = 17.8 mm; πr2 = 3.142 x (8.9 x 8.9) = 249 square mm (8.9 being half (radius) of diameter). Therefore 249 is pretty much half of 490 thus proving that each f-stop either halves or doubles the amount of light passing through it depending which way you are moving through the stops.

Phew that wasn't so bad, was it?

This, of course, means that the amount of light entering the camera is the same for any given f-stop irrespective of focal length of the lens.

Well that's about it for aperture for the moment. There are lots of other relationships for aperture, such as its effect on exposure, depth of field etc and these will be covered in later blogs.

One last thing though. A common term used by photographers is that of 'stopping down'. This just means that they are going to a smaller physical aperture which is of course is represented by the larger f-number e.g. going from f5.6 to f8. On the flip side, 'opening up' means the opposite; a wider aperture but smaller f-number e.g. f5.6 to f4.

Hope that makes some sense of it all for you. Until the next time!


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