At 17:27 1/20/01, you wrote:
John, are the calculated AOV values for a given focal length absolute, or
does the lens design come into play? Some 28's seem to provide a wider view
than others. Is this due to fudging the stated focal length?
Mickey
Possibly:
First, ensure they are giving you the "diagonal" AOV. I have seen a few
specs giving the "horizontal" AOV along the 36mm edge of the film
frame. This will be noticeably narrower than the AOV for the diagonal.
After that, it's probably fudging of the true focal length. Very few (if
any lenses) are exactly the stated focal length. They should be very, very
close to it though. The biggest focal length fudging I've seen is for
after-market super-wides . . . trying to get just one mm shorter than the
competitors' for marketing . . . and stretching the truth slightly. I've
found the Zuiko's to be reasonably accurate. Anyone doing photometrics
requires exacting measurements for specific lenses.
-- John
P.S. Beyond the short answer, there are two things related to this with
practical lens designs:
1. Aside from the angle of view, each lens design has a "coverage." A
lens' coverage is its "image circle" taking in the entire acceptance angle
of the lens. This is almost always bigger than the film frame (which
determines the angle of view), even at infinity focus. With 35mm and MF
this is done to eliminate some of the severity of cos^4 falloff at the
edges of the image circle which would end up in the corners of the frame.
It's a design trade-off between that and having too much extra light
(unused by the film frame) bouncing around inside the "light box" at
closest focus and risking flare (area between back of lens and film
frame). [see #2] If you look at the back of the 200/4 or 300/4.5
telephotos, you will see a somewhat rectangular shaped baffle inside the
lens near the back. It is designed to block some of the image circle that
won't be used before it enters the light box. For view camera lens
designs, the "coverage" is important for other reasons . . . to allow
tilting the standards and still having the image circle cover the film frame.
2. Most prime lenses are focused by the helical moving the entire lens
forward. As you focus from infinity to closer distances the image circle
gets larger at the film frame. Imagine a cone of light getting taller; the
base gets bigger. Non-macro lenses do not focus much closer than 6X to 8X
of the focal length, so this isn't a big deal. The focusing helical of a
normal 50mm only moves the lens 7-8mm forward at closest focus and the
image circle doesn't grow by much. For a macro lens or when using an
extension tube that racks the lens out much farther for very close
focusing, it does. As a result, the AOV at closest focus for a macro lens
will be measurably less than that at infinity. It's why someone who does
much macro work eventually gets entangled in at least understanding
"bellows" or "extension" factors. The same light spread out across a much
wider image circle at the film plane requires exposure compensation. TTL
metering will automagically compensate. For external metering,
compensation must be computed and taken into account at focusing distances
less about 8X the focal length, the point at which the compensation
required exceeds about 1/3 stop. Lenses using internal focusing change the
rules entirely (this is more than a floating aberration correction element).
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