Nigel Sadler - Digital Photography

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Lenses focal length v CCD size


35mm focal length lens with a full frame 35mm camera or professional range digital SLR with full-size CCD (36 x 24mm)	On a typical amateur digital SLR (70% of 35mm) a 24mm lens would be needed to achieve the same view	A 17mm lens would be needed for a typical compact digital camera (50% of 35mm) to achieve the same view
The rectangles give a size comparison of the CCD receptor

Most digital cameras have a considerably smaller image recording area than the traditional 35mm film camera. Therefore to achieve the same field of view, shorter focal length lenses are required. However, to help the user understand what the lens on their digital camera can see, 35mm equivalent focal lengths are usually quoted. Therefore with a 35mm camera the following would apply:

Lens focal lengths for the traditional 35mm film camera

20mm and shorter = fish-eye view of the world
24mm = very wide-angle lens - used for dramatic effects
28mm = a regular wide-angle - large groups, big objects or dramatic architecture
35mm = mild wide-angle - as 28mm, but with less distortion
50mm = 'standard' - gives a field of view similar to the naked eye
80mm = portrait lens - good for isolating subjects from the background
105mm = long portrait lens - as the 80mm, only more so.
200mm and upwards = telephoto lenses - typically used for viewing distant objects.

Digital cameras have CCD sensors of varying size - small in a cheap camera, and larger in a top of the range model. Therefore, camera manufacturers tend to quote lens focal lengths as if for a 35mm camera. This allows the consumer to judge the field of view of a particular camera and lens combination, be it fixed or more commonly a variable focal length zoom.

The result is that a fixed focal length 35mm camera would probably sport a 35mm lens, while its digital equivalent with only a 8mm x 6mm CCD may only have an 8mm focal length lens. Yet both would have a similar angle of view. Likewise a 35mm camera with a zoom lens of 35-135mm would be matched with 8-36mm of its digital brother.

This only becomes an issue with cameras that have interchangeable lenses - digital single lens reflexes (SLR's). If you already own a quality SLR photo outfit with interchangeable lenses, then is possible to buy a digital back to which you can attach your lenses. However in most cases the CCD will be smaller that 36mm x 24mm and so you will need to know the conversion factor so you can establish what angle of view your lenses will now offer. Typically, everything would have moved up a scale, so a 35mm lens may now be roughly equivalent to a 50mm and a 50mm an 80mm and so on.

Updated:
24 May, 2006

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