At 21:23 6/11/00 , Giles wrote:
>The lens filter size, is to a great extent, dictated by physics. The F
>number of a lens is roughly the ratio of the focal length to the diameter
>of the front element , for lenses other than wide angles.
If the largest apparent aperture diameter as viewed from the lens front
(wide open) is approximately the same as the front element diameter, then
your statement is a "ballpark" approximation. Not one I would have thought
of. Yes, the front element diameter _must_ be bigger for a faster lens
because it _must_ capture more light being reflected from the subject. To
do so it _must_ have a larger cross-sectional area (on a plane) to do it.
Note this is _not_ the same as front element surface area, but something
less (I won't do Stokes' Theorem here).
More precisely, and a better explanation of optical physics relating to
exposure (approximation assuming a very, very large subject distance and
_not_ applicable to macros):
The f-stop number is the focal length divided by the diameter of the
apparent aperture, as seen through the front element (not counting slight
adjustments for absorption). This is also called the "entrance aperture"
or "entrance pupil."
Why a lens speed is often written f/#.# which gives maximum entrance
aperture diameter:
f=focal length
D=aperture diameter (as seen from the lens front)
N=f-stop number
Some simple algebra:
N=f/D
ND=f
D=f/N
Also often written as a ratio, 1:#.# on lenses is the ratio of maximum
entrance aperture diameter to focal length.
BTW, entrance aperture diameter can be *different* from the apparent
aperture from the rear, called the "exit aperture" or "exit pupil." "Pupil
Magnification" (p) is the ratio of the exit aperture diameter to the
entrance aperture diameter. Look at the aperture diameter through a
telephoto, standard, and a wide angle some time, stopped down with DOF
preview button depressed, from front and back. You will see the differences:
telephoto: large front and tiny rear; p < 1
standard: about the same front and rear; p ~ 1
wide angle: tiny front and large rear; p > 1
[OK guys, let's not^H^H^Hget crude here.]
This is a demonstration of why 40-55mm focal length (for 35mm small format)
produces natural looking perspectives, a telephoto image appears flattened,
and wide angle perspective lines appear exaggerated. It is why the fast,
long telephoto lenses must have a very large front element. It also
explains why Olafo used his 300/4.5 for his TOPE 1 self-portrait. He was
trying for a huge eyeball image using magnification from lens rear to lens
front.
-- John
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|