At 10:32 12/2/02, Ag Schnozz wrote:
>If this was the LUG, there would be no stopping after a
>comment like that.
I'm still not satisfied.
50mm = 1x
100mm = 2x
???mm = 3x
200mm = 4x
If these are the "standard" multiplication factors regarding
general "magnification" of 35mm cameras, then why is 3x 135mm
and not 150mm?
AG-Schnozz
Not true! Magnification is the ratio of the size of the object being
photographed on film to its actual size. This is dependent on object
distance from the front lens node and focal length, but **independent** of
film format. Whether or not it fits in the film frame is determined by
film format. Thus, the same object at the same distance will be very
nearly the same size **on**film** with an 85mm Zuiko on an OM and an 80mm
Sekor on an M645. The slight difference will be the 5mm difference in
focal length. If you were to make a photograph of the same object with
each one, then take the developed film out of the M645 and chop it down to
24mm x 36mm, the size of a 35mm format full frame, you would end up with
very, very nearly the same, exact photograph as was created using the OM.
Here's the math for magnification as related to focal length and distance
from object (subject) to front lens node:
Start with two of the basic lens equations:
1/f = 1/u + 1/v, where:
f = focal length
u = subject distance from front lens node, and
v = critical focus distance of rear lens node to image on film plane
M = v/u, M = I/O, and therefore I/O = v/u, where:
M = magnification
I = image size
O = object size
u and v are defined as above
Solve for "v" in the first equation and get:
v = (f * u)/(u - f)
Substitute into the second equation and reduce:
M = f/(u - f)
Now see what happens to magnification for an object at 5 meters (or 5000mm)
when we use the lens focal lengths you listed:
M = 50/4950 for a 50mm lens
M = 100/4900 for a 100mm lens
M = 150/4850 for a 150mm lens
M = 200/4800 for a 200mm lens
You should be able to see it's not 1X, 2X, 3X and 4X magnification! Going
back to the original problem of what focal length is effectively halfway
"between" 100mm and 200mm, as regards magnification (versus FOV for a
specific film format), it depends on object distance from the front lens node!
f = [(150 * u) - 20000]/(u - 150), where:
f = focal length with magnification halfway between
u = distance from object to front lens node
To derive this, set the difference between magnification of a "halfway"
focal length "f" and that of a 100mm lens equal to the difference between
the magnification for a 200mm lens and the same halfway focal length, then
solve for the halfway focal length. At an object distance of infinity, the
focal length is 150mm. As object distance is reduced, the magnification
midpoint focal length increases and approaches 200mm.
-- 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 >
|