Tuesday, April 24, 2007

Using Your New Camera -Technical skills

Learn to use Aperture

The aperture is the hole through which light enters the camera, also called the iris. Apertures are measured using a relative scale, and designated by 'f numbers' such as f4, f5.6 or f8. The smaller numbers refer to larger holes to let in light. The larger the aperture (smaller f number) the less time the camera needs to take a picture. For any particular light level and film speed (or digital equivalent) there are a range of 'equivalent exposures' which will produce the same exposure on film, for example

1/500 f4 : 1/250 f5.6 : 1/125 f11 : 1/60 f16

All these settings will produce the same exposure, but the results will often look rather different. If you are photographing a moving car, 1/500 f4 will probably give a fairly sharp result, but the slower shutter speed of 1/60 at f16 will produce a blurred result.

For any particular focal length, 1/500 at f4 will give a relatively shallow depth of field, perhaps useful to isolate a single figure from a background, but probably inadequate for a landscape subject with much depth. For this the much greater depth of field given by 1/60, f16 might be more suitable.

Many simpler cameras do not allow you to control the aperture, but use a 'programmed exposure' that chooses a combination of shutter speed and aperture. Sometimes there may be more than one program available, with a 'sports' program give faster shutter speeds and a 'landscape' program giving smaller apertures for any given light level.

With such cameras - and many digital cameras - the only way to control the aperture at all is to use a faster film speed, which will result in a smaller aperture being set.

Learn to use Shutter Speed

The shutter controls the length of time that light is allowed to enter the camera, and is usually expressed in a standard series of fractions of a second, such as:

1/8 1/15 1/30 1/60 1/125 1/500 1/1000 etc

Conventionally cameras used this set list of speeds. Some modern electronically controlled shutters using LCD panels for setting, allow speeds between these. It is one of those "added features" that make operating a camera slower and less intuitive, and is seldom if ever of use.

Correct exposure depends on the light level and film speed (or sensor sensitivity.) As mentioned above, there is always a choice of suitable combinations of shutter speed and aperture, such as:
1/500 f4 : 1/250 f5.6 : 1/125 f11 : 1/60 f16
all of which give the same amount of light.

Shutter Speed and Subject Movement

Shutter speeds become important with moving subjects, especially those close to the camera. Few photographers realise the importance of shutter speeds when photographing people walking. At a normal walking pace you might cover a kilometre in around 10 minutes, giving a speed of roughly 1.5 metres per second. This is 1,500 mm per second, so in 1/500s, you will have moved on average 3 mm. Not a great distance, but certainly enough to cause some blur if you are walking across close to the camera.

In fact the situation is both worse and better. A person walking towards or away from the camera will seem considerably less blurred. However, as walking is a complex motion, although the average movement will be 3mm, both arms and legs may well be moving at something like twice this speed in part of the movement cycle.

As you move further away from the subject, the significance of subject movement decreases, as the subject gets smaller in the image.

Shutter speed and focal length

Shutter speed is also important in cutting down camera shake. For 35mm cameras a good rule of thumb is always to use a shutter speed that is equal to or faster than 1/focal length. So with a standard focal length lens of 50 mm, you should use shutter speeds of 1/50 or faster, while a 500 mm lens needs 1/500.

When working with a digital camera, you should use the '35mm equivalent' focal lengths for similar calculations. As you zoom out towards greater magnification you need to remember you are also zooming your camera shake. If you can set a faster shutter speed you should consider doing so.

The faster speeds are needed because the longer the focal length the more any slight movements are magnified in the image. Some recent lenses incorporate image stabilisation using motion sensors; in some digital cameras the same function is performed digitally. Image stabilisation enables you to use significantly slower shutter speeds without getting camera shake, but does not of course reduce any problems caused by subject movement.

In the traditional SLR camera design, one cause of camera shake was the mirror which had to flip up immediately before exposure to allow the light to reach the film. Cameras without this - including most digitals except some of the interchangeable lens SLR digitals - have an advantage in avoiding shake. Many photographers have used rangefinder cameras such as the Leica series handheld with standard 50mm lenses with success at 1/15 or even 1/8 second.

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