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.
Aperture 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.
So let’s look at shutter speed. On most cameras, where this is adjustable by the photographer, it is operated by a knob, a wheel or a dial. Sometimes this has a digital display and sometimes the speeds are just marked on the knob. Shutter speeds are always marked as reciprocals, that is to say that 1/125 of a second is marked as ’125′. These will be marked like this: 1, 2, 4, 8 15, 30, 60, 125, 250, 500, 1000, 2000 and so on. You can see that essentially each step in this series is HALF as long as the preceding one, since these are fractions. So 500=1/500, which is half as much as 250=1/250. All the rest work the same way.
Since the aperture f-stop scale is marked in steps that admit half or twice as much light as the next, and shutter speed is marked in the same way, then closing the aperture by one stop and reducing the shutter speed by one step will allow the same amount of light through. So f4 at 125 is the same as f5.8 at 60, f11 at 500 is the same as f8 at 1000, etc.
However we have already seen that stopping down will increase depth of field. So if we want depth of field, then we should set a small aperture (higher f-number) and slower shutter speed (lower number).
This is not a simple trade-off though. All photography has to take into account motion. Some of this motion is in the subject. Obviously, the world is moving and very few things are really still. Some things, like mountains, change so slowly it can’t be seen, whereas other things, like people and animals, move very quickly. The lower the shutter speed, the more likely is this motion to be seen, and the higher, the more likely it is to be ‘frozen’. So shutter speed has to be chosen with care, since there is no point in having huge depth of field if everything in the image suffers from motion blur.
There is another motion which is also important, and that is camera motion, or ‘camera shake’. This is the single most common technical flaw I have seen in my career as a photographer and editor. Camera motion is affected by the focal length of the lens, so the general rule when hand-holding is: never use a shutter speed that is lower than the focal length in millimetres. So if you are using a 50mm lens, never use a shutter speed less than 1/60 (60) when holding the camera.
Now experienced pros and capable amateurs can increase the safety factor by good technique in other areas, but this basic rule presumes you can hold the camera still. Even with great technique, it’s not possible to reliably get beyond a step or two slower than the rule suggests.
While the problem of camera movement can often be taken care of by using a support for the camera, like a mono-pod or tripod, this is not always practical and in any case does not help with subject movement. So you should work on holding the camera as still as you can.
On a side point here, though small digital compacts are wonderful and useful, it is practically impossible to hold them still, since to use them you have to hold them at arm’s length to see the monitor screen. Cameras like this which have an integral viewfinder are much better, though unfortunately usually also bigger.
To freeze subject movement, then , using 35mm or equivalent digital as our base, slow movement like boats need 1/125 or faster, crowds and children 1/250, slower sports like golf 1/500, field sports 1/1000. A similar scale is true in nature photography, dance, theatre and so on. The faster the subject is moving, the faster the shutter speed will have to be to stop it, and the longer the lens, the faster the shutter speed too. The more the subject fills the frame, the faster the shutter needs to be; a general view of a football match where the players are small in the image might be fine at 1/125, but using a long lens so they fill the frame may need 1/1000 or even more. This is because the effect of motion blur is dependent on angular movement. Some technical subjects may need even higher shutter speeds. Always be aware that longer focal length lenses demand faster shutter speeds.
We may not always want to freeze motion, however. For example, when photographing running water, using too high a shutter speed will make the water look glassy and unnatural. Some blur is quite acceptable, say in the wheels of a racing car, which enhances the feeling of speed. Sometimes the gentle motion of trees in the wind is best allowed to show. So don’t get the idea that motion must always be slavishly frozen. In many images it is an important part of the picture. Photographers are artists and the decision about how much motion to allow to be seen has to be thought about in the context of the image as a whole, and there is no one ‘right’ answer, because every photographer has a unique vision. The important point is that you consider this parameter and make an informed decision.
Finally, a high shutter speed may simply be required when we want to use a larger aperture, for example to isolate the subject from the background as discussed earlier.