Rod Fleming's World

Four key factors in photography are linked: aperture, shutter speed, film or sensor sensitivity and subject brightness. These are linked in such a way that if any one is changed, then one or more of the others must also be changed to compensate. The first two, aperture and shutter speed, are easily controlled on camera, and now we will look at the third factor, which is also under the photographer’s control: film or sensor sensitivity.

In pre-digital days this was always referred to as ‘film speed’ and this is convenient; we continue to use it to describe digital sensor sensitivity — sensor speed. A number of indices or scales were established to measure film speed, but the one that became the most widely used was the American Standards Association, or ASA, scale, which was eventually adopted by the International Standards Organisation or ISO. This is an arithmetical scale, so that a doubling of the ISO value indicates a doubling of sensitivity or speed, and a halving the opposite. (This is as opposed to the more complex DIN scale which was logarithmic.) Because the ISO values are arranged in this way, they conform exactly to the conventions we have already met for aperture and shutter speeds, where each stop or step is a doubling or halving of the one before. So it is easy to apply the same logic to film/sensor speed.

Shutter speed is one of four interdependent parameters that are central to photography. These are : Aperture, Shutter Speed, subject brightness, and film or sensor sensitivity. IF any ONE of these is changed then AT LEAST one other must also change to maintain the same exposure. The most common example of this is that if aperture is altered, then shutter speed must also be altered, reciprocally. So if aperture admits less light, then shutter speed must admit more.

We have already discussed aperture, and seen how the f-stops marked on the aperture ring are calculated and what they are. To recap, the focal length divided by the aperture gives the f-stop, so a 50mm lens with a 25mm aperture is f2, and the they progress f2, f2.8, f4, f5.6, f8 and so on. Each of these allows half as much light to pass as the f-stop before, counting from low to high.

In the last article we looked at the optical principles that govern focus and depth of field. Now let’s explore how these are applied practically in the camera. For the moment we will assume that we are using a single-lens-reflex (slr) or rangefinder type camera, either digital or film.

If you look at the lens barrel you will probably see a ring which controls the aperture diaphragm. This will be numbered in f-stops, which on modern cameras will run f1.4,2, 2.8, 4, 5.6, 8, 11, 16, 22, (32, 45, 64). I put the last three in brackets because you’re pretty unlikely to have them but you might see them on view cameras. Note that the numbers double on every second step—2,4,8 etc and 2.8, 5.6, 11 etc. The f-stops are determined by dividing the effective aperture into the focal length, so a 50mm lens with an aperture of 25mm is f2, and with 12.5mm is f4. Each f-stop, going from low number to high, admits half as much light as the one before, so f2.8 is half of f2 etc. It’s easy to work out why if you’re mathematically inclined, but if not, just remember it.