Re: How to Calculate Zoom
- From: "Marc Sabatella" <marc@xxxxxxxxxxxxxxxx>
- Date: Tue, 13 Jun 2006 19:52:17 -0600
The baseline for comparison is equipment non specific, ZERO
magnification, what the naked eye sees.
That might be the baseline *you'd* like to use for comparison, but it is
*not* the baseline actually used when you see cameras with zoom lenses
quote magnification ratios. A point & shoot camera that advertises a
"7X" zoom is not saying that it magifies things seven times more than
"what the naked eye sees", but rather, that its maximum magnification is
seven times its minimum magnification. Most likely, the minimum
magnification is a little more than half "what the naked eye sees" (to
the extent it makes sense to define this at all - see below), and the
maximum magnification three to four times "what the naked eye sees".
Whereas another zoom lens might be only "4X", but that might mean
minimum is twice "what the naked eye sees" and the maximum is eight
times "what the naked eye sees". Meaning the 4X zoom would actually
magnify more than the 7X. That's why you don't see SLR lenses using
ratios like these. You see real, hard numbers to give focal length.
The 7X zoom I mentioned would actually be sold as 28-200, the 4X as
100-400. Then we could compare the focal length ranges directly to see
that the former did a lot more at the wide angle end of the range but
the latter did a lot more at the telephoto end. This is all relevant
information, and that's why camera lenses are labelled with all of it.
Why is it that binocular, rifle scope, spotting scope and video camera
makers all seem to easily come up with a baseline for comparison, and
yet that seems to be near impossible for SLR camera people?
Video camera zoom ratios are exactly like point & shoot zoom ratios.
That is, they aren't telling you how much a camera magifies comared to
what the naked eye sees, but simply telling how its maximum
magnification compares to its minimum magnification. As for binoculars
and scopes, it is possible to come up with a definitive number for these
because the images are being projected on the same screen: your retina.
Now, if you want to take the lens off your camera and hold it up to your
eye like a telescope, you could similar come up with an absolute number
to measure the magnification. But that number would only be meaningful
when projecting the image onto your retina. It would cease to have
meaning when projecting the image onto a piece of film or a camera
sensor. Because in order for that image to eventually get on to your
retina, it's going to have to be displayed or printed somehow, and you
have no way of knowing how much it is going to be magnified in the
process of displaying or printing it.
Can't the baseline for comparison be equipment non specific?
Not really, at least, not in the way you want. See below. The best
convention we have is that of a "normal" lens, which as has been
mentioned here, is considered around 50mm on a 35mm film camera, or
around 30-35mm on most digital SLR's. What is relevant in making this
"normal" isn't so much magnification, since any picture can be blown up
bigger, but rather, angle of view - how much you can actually take in at
once without moving your eyes. A normal lens is one that approximates
the angle of view that we typically see. Of course, that differs
between people, and since our vision isn't constant across our field of
view but rather drops off toward the edges (and at different rates for
things close up than things far away, most likely), there is some leeway
in defining even for a given person what his angle of view is. But
there is general agreement that somewhere around 50mm is a "normal"
focal length for most people when projecting an image onto a piece of
35mm film. And since most digital sensors are smaller than that, a
correspondingly smaller focal length is "normal". Of course, it order
for that image to actually *look* normal, we'd have to project them onto
a wall that just barely fills out field of vision. When looking at the
image printed 4x6, we sort of have an inutive sense of what looks
"normal" and what looks like it was taken with a longer or shorter focal
length length, but this can be deceiving. It can be hard to tell the
difference between an object taken with a 50mm from 10 feet away versus
the same object taken with a 200mm from 40 feet away (actually, I'm not
sure I have the math right there - it might be 20 feet, or some other
number involving the square root of 2, to get the object to appear the
same size on a same sized print, but whatever). Still, the notion of a
"normal" lens is the closest you're going to get to "what the naked eye
sees".
So if you want to compute a "magification" factor in this way, divide
the focal length by, say, 33 for most digital SLR's or 50 for a 35mm
film SLR or "full frame" digital. Thus, the aforementioned 28-200 zoom
would not be called a "7X" zoom, but rather, a ".6X - 4X" zoom on a 35mm
camera, or a ".8X - 6X" on most digital SLR's. That is, you'd list the
magnifications at both ends of the zoom range, not just the ratio
between those magnifications. And indeed, one *could* do this. But I'm
not sure I have any intuitve sense of what ".6X" or "4X" looks like.
The only way to know would be get a feel for it through experience. And
one can develop a feel for what "200mm" looks like just as surely as one
can what "4X" looks like. So I don't see what is gined by this
approach.
Say a 1 inch bumble bee (or white square) at 30 feet is 1/32 of an
inch at 100% with no magnification (what the naked eye sees).
1/32 of an inch where? On your retina? If you hold out a ruler at
arm's length? This is problem #1 with computing what sort of
magnification we are talking about in absolute terms.
With X lens/camera that same 1 inch bumble bee at 30 feet is 1/2 of
an inch at 100% 15X zoom?
1/2 an inch where? On the camera sensor? When you display it on your
comuter screen? When you make a 4x6 print? When you project it on a
wall? That's problem #2 with computing what sort of magification we are
talking about in absolute terms.
Does anyone have any way to calculate a equipment non specific, "Zoom"
factor in this manor rather than comparing equipment specifics?
No, because such a thing is quite impossible to do for the reasons
described. Howeve,r it is trivially simple to divde focal length by 50
or 33 to get something resembling what you are talking about if calling
a 200mm lens a "4X" zoom has more meaning to you than simply calling it
a 200mm lens.
---------------
Marc Sabatella
marc@xxxxxxxxxxxxxxxx
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