My camera sets the exposure for me automatically. Why should I need to worry about things like “shutter speed” and “F numbers”?
In order to answer that question, we first need to understand the principles behind exposure, which are exactly the same for both film and digital cameras.
There are three factors that affect the exposure of a photograph:
The shutter speed. This is the amount of time that the film (or sensor in the case of a digital camera) is exposed to light. The slower the shutter speed, the more light gets in and the brighter the image will be. Shutter speeds are usually very fast – typically somewhere between 1/60th and 1/1000th of a second.
The aperture of the lens. The wider the opening through which the light enters the camera, the more light will get in during a given time.
The “speed” of the film. This is represented by the ASA/ISO number of the film. The higher the number, the faster the film. “Fast” films have fewer, larger particles making up the emulsion and react to light more quickly. This means they need less light and therefore need faster shutter speeds.
”Slower” films have a larger number of smaller particles. This means they need more light and therefore longer shutter speeds, but the smaller particles mean that the film will capture finer detail with less “grain”.
Digital cameras work in a similar manner as many models allow you to change the ISO setting. This is equivalent to changing the film speed, but with the advantage that it can be changed on a shot by shot basis! A faster ISO means the sensor will collect light more quickly than with a slower ISO, resulting in faster shutter speeds. However, as with film, there is a trade off between speed and quality, as the faster settings can result in more “noise” in the image.
When considering the exposure of a photograph, what matters is the total amount of light that reaches the film or sensor. This depends on a combination of shutter speed and aperture. It may help to use an analogy of water being poured from a tap into a bucket. If the tap is pouring water at the rate of a pint a second and you put the bucket underneath it for two seconds, you will collect two pints of water. If you then increase the flow so that the tap is pouring two pints a second and put the bucket underneath it for one second, you will also collect two pints of water.
In this analogy, the aperture of the camera lens is the flow of the tap, only in this case we are talking about how much light passes through the lens in a given time rather than how much water passes through the nozzle of the tap. The time that the bucket is held under the tap is equivalent to the shutter speed – the time that the film or sensor is exposed to light.
As with the water analogy, if you increase the aperture so that the amount of light passing through the lens doubles and then halve the shutter speed, the overall effect on the exposure is neutral as the same amount of light reaches the film or sensor.
Traditionally, cameras have been designed so that the shutter and aperture settings double (or halve) with each step. Typical shutter speeds are 1/60th, 1/125th, 1/250th, 1/500th & 1/1000th of a second – each setting being twice the speed of the previous one. Aperture settings work the same way, although the numbering isn’t quite as obvious. F22 is quite a narrow aperture. The next setting is F16 which lets in twice as much light as F22. F11 lets in twice as much light as F16 and F8 lets in twice as much light as F11.
Therefore, on a nice sunny day with ASA200 film, a shutter speed of 1/250th of a second with the aperture set to F16 may give the correct exposure. However, this would not be the only “correct” answer as any of the following combinations would allow the same amount of light into the camera:
Film speed obviously affects this. ASA400 film will be twice as “fast” as ASA200 film so the following shutter/aperture combinations would be needed:
Okay, I understand that, but if it doesn’t matter which combination you use, what’s wrong with letting the camera decide? My pictures all seem to be exposed properly doing it that way.
For the majority of photographs, that is certainly true. However, there may be occasions when you want a particular effect in your photograph and the camera’s choice of shutter/aperture may not give you what you want.
Varying Shutter Speed
If the subject of your photograph is moving, the shutter speed used will change how your photograph appears. Suppose you are taking a picture of someone going past on a bicycle. If you use a fast shutter speed, the bike and its rider will appear “frozen” as if they are not moving. Alternatively, a slower shutter speed would mean that there was an amount of “motion blur” on the image. How much blur would of course depend on the shutter speed with slower speeds resulting in more blur.
Taking another situation – you may want to photograph a waterfall. A fast shutter speed would freeze the motion of the water, perhaps showing individual drops of water suspended in mid-air. A longer shutter speed would make the water “flow”.
In both of these situations, there is no “correct” shutter speed – it all depends on how you want your photograph to appear.
Even if the subject matter of the photograph is not moving, the choice of shutter/aperture combination can still affect how it appears. This is because the aperture affects what is called the “depth of field”.
The depth of field is basically how much of the image is in focus. Photographs with a wide depth of field are generally completely in focus everywhere in the image, whereas photos with a shallow depth of field have the subject in focus but anything behind or in front of the subject will be blurred. Wider apertures produce a shallower depth of field and narrow apertures give a wider depth of field. The desired effect obviously depends on the type of photograph – for a landscape shot, you will want everything to be in focus, but for a portrait of a person or a shot of a flower, you may want a blurred background.
Choosing a combination
As I said earlier, there will be several different shutter/aperture combinations that give the correct exposure. In automatic mode, your camera will pick one of these and will generally come up with a correctly exposed photograph. However, the camera doesn’t know how the photographer wants to treat movement or depth of field and if you want to have control over these aspects of creating an image, you will need to move away from letting the camera make all the decisions.
As a general rule, for landscapes, you will get better results with a narrow aperture to maximise depth of field. This means you will need a slower shutter speed to get the exposure right. The slower shutter speed should not affect the image as landscapes don’t usually move when having their photo taken!
Alternatively, for portrait work, where a shallow depth of field may be wanted to give a blurred background, best results can be achieved by using a wide aperture and a faster shutter speed. However, the final decision is down to the photographer as only he or she knows how they want their photograph to appear.
Okay, now I understand why I might want to use my own settings, but how will I know which shutter speed & aperture to use to get the correct exposure if I ignore the camera’s suggested settings?
Don’t worry – if your camera allows you to override the automatic settings, it almost certainly has two handy modes:
Aperture Priority – you set the required aperture and the camera selects the correct shutter speed to give the correct exposure.
Shutter Priority – you set the required shutter speed and the camera selects the correct aperture.
You should find that using these modes when you want to achieve a particular effect will greatly improve your photography. Don’t forget – with a digital camera, it costs nothing to experiment!