Fitting a 35mm film camera lens to most Dslrs has the effect of increasing their
focal length, at least when it comes to angle of view. The only digital cameras that
don't do this are called "full frame". This refers to their image sensor being the
same size as a 35mm film frame (24mm x 36mm). In all other Dslrs, the sensor is smaller
than this and so the frame is cropped by a certain amount.
This is the crop factor and it is typically around 1.5 - 1.7. You have to multiply
the focal length of a lens by this factor to find it's 35mm equivalent. For example,
if you buy a 50mm lens and your crop factor is 1.5, then your lens will behave like
a 75 mm one. (50x1.5 = 75). If your crop factor was 1.7, then you would have an 85mm
lens (50x1.7 = 85).
There are two reasons for working out the 35mm equivalent of your lens.. First it
allows you to compare like with like and secondly, most photographers still relate
to focal lengths in 35mm terms. If you read a tutorial on photography, any reference
to focal lengths is likely to be in 35mm equivalents. For instance a 28mm lens is
a standard wide angle that photographers have got used to over the years. But, if
you want the same angle of view on a Dslr, you might need a 16, 17 or 18mm lens,
depending on your camera's crop factor.
If you are not familiar with 35mm cameras and their lenses, here is a brief guide
to focal lengths in 35mm terms.
At the top end, 1000mm was around the longest lens available, but anything much above
500mm would be called a "super telephoto". These lenses are highly specialised and
would be most useful if you were forced to be a very long distance from your subject.
That's why you tend to see them mostly at sports events and rocket launches.
150mm - 500mm would be called a telephoto lens. These lenses allow you to creatively
isolate your subject or part of your subject at medium distances.
75mm - 150mm is a medium telephoto. Lenses in this range are much favoured by portrait
photographers. This is also the range in which you find "macro" lenses. These are
lenses that allow you to focus much closer than normal. A "true" macro lens is one
that allows you to get close enough for a life-size image on a 35mm frame. That definition
isn't much use for digital cameras, but if a lens has macro written on it, it should
let you focus much closer than normal. The medium telephoto is preferred for macro
work because it puts you at a reasonable distance from the subject for an extreme
50mm was known as a standard lens but not because it was very popular or useful.
It was just easier to make good quality cheap lenses at this focal length, so it
came fitted to most 35mm cameras as standard, hence the name.
35mm - 28mm, medium wide angle. These are probably the most popular and useful lenses
because you can use them in just about every branch of photography.
28mm - 20mm or less. Extreme wide angle. These can appear to alter perspective and
produce very dramatic shots, which is why people buy them.
Below 20mm, fisheye lenses. These are defined by the fact that the image is a complete
circle inside a 35mm frame. Of course, when you take into account the crop factor,
it doesn't quite work the same way with digital cameras. Unless the Dslr is full
frame, you will not get the full circle.
All the above statements refer to the 35mm equivalent, that is after you have multiplied
the focal length by your camera's crop factor. Only you can tell what type of photography
you want to do and therefore what focal length lens you will need. The list above
should help you to decide what you need for your camera, as long as you remember
to use the crop factor.