At 11:03 12/8/02, you wrote:
Wow. This is way more complex that my memory "stored" from physics class.
I guess I distilled it too far and stripped out complexity. Thanks, John
for setting me straight. Now I'll have some reading material for the week.
Is this phenomenon of magnification varying with the distance from the lens'
front node related to the focal length being different for a macro lens used
at 1:1 as opposed to infinity, then?
Lama
Lama,
It's related to image circle diameter increase. I don't know about
specific macro lens designs which may have internal elements or other
aspects of the design that compensate for this.
With non-macro prime lenses that focus by moving the entire lens cell
forward to get from infinity focus to something closer that, the image
circle diameter increases. Think of making a cone taller by adding to its
base. If the angle between side and axis is maintained, the base diameter
must increase. At minimum focus distance the film gate is capturing less
of the image circle. Another way of thinking about it is image circle
"magnification" because its light is being spread over a larger area. This
is a second order magnification in addition to the most noticeable first
order magnification achieved by moving closer to the subject of interest.
This is the reason exposure compensation is required at distances (from
front lens node to subject) that require extension of the lens greater than
about 1/7th the focal length from infinity focus. Look at the standard
50/1.8, 50/1.4 and 50/1.2 lenses and take a rough measurement of how much
they are extended at minimum focus distance. It's about 7mm or so. I got
curious about this one time, measured a 50/1.4 and then did the math on
light loss for a 50mm lens at 7mm extension from infinity focus. IIRC, it
was just under 1/3rd f-stop and the error is [usually] negligible.
If you're using TTL metering for primes with extension tubes, the metering
compensates. It's not a big deal except to be aware of it and that you
might need more light to get a tighter aperture. However, if you're using
a hand held meter, ambient or flash, it does matter and it requires
compensaion. Been there, done that, using monolights, flash meter and
chrome film which can dramatically demonstrate an exposure error.
-- John
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