Numbers. Yuk. I don’t know about you but I’d be lost without a calculator. Okay, I occasionally resort to working things out on paper, but only as a last resort if my phone isn’t handy. (Or even my 20-year pocket calculator that otherwise sits in a drawer for months and gives an audible squeak of surprise when it sees daylight.) When I left school I was so relieved that I’d never have to do long division ever again that I almost wept.
And then I took up photography and numbers came back with a vengeance. Numbers are everywhere in photography. Admittedly no long division is required (phew!) but occasionally you have to indulge in a spot of mental arithmetic. Fortunately, shutter speed was fairly easy to understand. Shutter speed is just a division of time. And even I could understand that 1/500 was twice as fast as 1/250 or half as fast as 1/1000. No need to dig out a calculator for that. Well, not often.
However, when it came to aperture values (or f-stops), crikey, I thought, what on earth is this all about? An aperture is a variable iris inside a lens. Make the iris bigger and you let more light inside the camera, make it smaller and less light reaches the film. (Yes, it was film in those days, but the concept of aperture values is exactly the same with a digital camera.) It took a while but that sunk in.
And then it came to the aperture values on the aperture ring of my lenses. (Yes, my lenses had aperture rings, I really am that old.) At first (and second) glance they made no sense whatsoever. Who on earth came up with a system that uses values like f/2.8 or f/5.6? And then I started to whimper softly when I discovered that an aperture value of f/4 created a bigger hole in the aperture than f/16. But, I thought, f/4 is a much smaller number than f/16. That’s completely illogical. Are camera manufacturers doing this deliberately?
Fortunately, I had a very patient photography teacher who carefully explained what was going on. An aperture value is actually a fraction. They start to make a wacky sort of sense if you put a 1 in front of the / – So 1/4 or 1/16. Suddenly it was easier to see that 1/4 is a much bigger number that 1/16, and why f/4 is a much larger aperture than f/16.
Okay, so an aperture is a fraction but what’s it a fraction of?
This is where you can zone out for a paragraph. This is useful stuff but you don’t necessarily need to know this to enjoy your photography. Right then, here goes. An aperture value will tell you the width of the aperture opening in millimetres when the focal length is divided by the aperture value. So, for example, if the focal length of a lens is 50mm, at f/4 the lens aperture will be 12.5mm across (50/4) but at f/16, the aperture will only be 3.125mm across (50/16). (It’s not long division, but yes, I did use a calculator to work those figures out…)
Aperture values work in the similar – if less apparently logical – way to shutter speeds. An aperture of f/4 is twice as ‘fast’ as an aperture of f/5.6. It there lets twice as much light reach the film/sensor. Or, to look at it another way, f/5.6 is half as ‘slow’ as f/4 and lets in half as much light.
Other than controlling how much light reaches the film/sensor, what else does the aperture do? Well, I’m glad you asked. It also controls depth of field. This determines how sharp a photo is. The less depth of field there is, the less sharp a photo will be. Now, there are a number of factors that determine depth of field so it’s not entirely straightforward. However, the simple rule to remember is that a large aperture (f/2.8 say) will produce less depth of field/sharpness than a smaller aperture (like f/8).
Another factor that affects depth of fields is subject-to-camera distance. The closer the camera is to the subject the less depth of field there will be at any aperture.
So, numbers are important. Even I can see that now. Though I’ll still not going anywhere near a long division if I can help it.