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Old July 1st 04, 04:22 PM
Don Stauffer
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Default what does the focal distance actually mean?

We must be careful to differentiate between zoom and focal length.

A little history of photographic lenses may help here. But first of
all, consider that the longer the focal length is, the larger will be
the image (in physical size) for a given distance for a given object.

Now, I will ignore the original meaning of the term 'telephoto' in
camera lenses as being unimportant for now. But it came to mean a lens
with a longer focal length than a 'standard' lens. So it magnified the
image. Or, in other words, made it look like you were closer to the
object when you took the picture. Sometimes people talked about the
power of a telephoto lens, as the ratio of that lens to a 'normal' or
standard lens (50-55mm for a 35 mm camera).

Photographers used to carry around a whole bag of lenses with different
focal lengths.

Then, they perfected the zoom lens. A zoom lens is one that by moving
various lens elements inside the lens can have different focal lengths.
People talked about the zoom range as a 3X or 4X zoom range. Sometimes
they called this 'power', but that was not really a good idea. It is the
same as the older meaning of the term 'power' if the minimum focal
length for that zoom is the standard lens size (say 50mm for a 35mm

Today, we may have a 33 to 100 mm zoom lens as having a 3X power zoom,
but that is not the same as saying that in the 100 mm position it is 3X
power (magnification), since the ratio of 100 to 50 is 2X, not 3X.

So today a zoom lens of such and such 'power' means the ratio of longest
to shortest focal lengths, NOT the magnification of the image at max
zoom compared to normal lens.

BTW, for any format, the old meaning of standard or normal lens was a
focal length approximately equal to the diagonal distance of the format
(film frame size or image chip active area size).

scott wrote:

When you see a 200mm lens, what is the 200mm actually a measure of? I
assume it's something to do with how much "zoom" there is, but surely
measuring the angle that is shown by the image would be better? Eg a
wide-angle lens would be a 90 degree field of view but a telephoto one would
be maybe 5 or 10 degrees.

Is it possible to work out the field of view in degrees from the focal



Don Stauffer in Minnesota