Tuesday, April 24, 2007

Using Your New Camera -Technical skills

Technical skills are an essential basis for photography, and until recently made it a relatively difficult medium to master. Both film and digital cameras now offer so much in the way of automation that most things are easier. Technical decisions produce differences in your pictures and cannot be separated entirely from composition and aesthetics.

Learn to Use Focus

Most modern cameras now have auto-focus systems, although there are still occasions when it is better to focus manually if your camera allows it. Older cameras always had a focus scale on the lens against which the subject distance was set. Usually the scale was also marked to give an indication of 'depth of field.'

'Depth of field' is one of the more difficult ideas for new photographers to understand and put to use. Although although only one particular distance is precisely in focus when we take a photograph, objects at a range of distances from the camera will look acceptably sharp in any print. If the closest object in a particular print that seems sharp is at 5 feet, and the most distant is at 15 feet, then the depth of field extends from 5 to 15 feet.

Depth of field is not an absolute measure, as it depends on the size of the print, viewing distance and the visual acuity of the observer. Suitable assumptions are made to come up with actual figures, and the idea remains a useful one. For any particular format and focal length, the depth of field gets larger as the lens aperture gets smaller.

The smaller format of most digital cameras results in them having greater depth of field for any given angle of view and aperture. This means that there are less focus problems, but also that it is harder to make backgrounds unsharp.

A few cameras have a built in system for setting the focus and aperture to give the required depth of field, but most auto focus cameras lack any such facility. If you want to be precise you will need a camera that allows manual focus and lens with a depth of field scale.

Even with an auto-focus camera I spend much of the time working with auto-focus switched off. Working rapidly using auto-focus on too often gives a sharp background and the closer subject out of focus.

When using wide angle lenses at moderate distances in good light, faster and more reliable results are obtained by rough manual focussing. With a 28mm lens set to f8 and a little under 1.5 metres, everything from 1 metre to about 2.5 metres will be sharp. I know because the scale on my 28mm tells me this, with depth of field markings for f8. In practice I don't set the distance at the centre mark for focus, but set the nearest distance against the aperture in use, then read off the far distance.

Using focus means using both sharpness and unsharpness. The far distance - in this example around 2.5 metres - tells me that if the background is further than this from the camera, then it will appear less sharp than my subject. This helps to make the subject stand out. At times I'll deliberately shift the focus of the lens closer than necessary for my subject to make sure that the background is unsharp

With cameras that don't have this facility, it is worth remembering that except for close-up pictures, the range of sharpness extends roughly twice as far from the point of focus away from the camera as it does towards it. To get the maximum sharpness across an area you should focus not at its centre point, but roughly one third into it.

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