I believe there are some definitions needed to make distinctions between
effects and some myths to be exposed:
1. "Magnification" and "perspective" are two _different_ things. I
believe this distinction was made in my original posting.
Magnification:
Typically considered and calculated at the "subject distance," a very
specific and exact distance, not in the entire image depth. Often it's the
critical focus distance, but not necessarily if that is moved to envelop a
subject with depth by the DOF.
Perspective:
How quickly (or slowly) horizontal lines not parallel to the film plane
converge to a "vanishing point." Shorter lenses will converge these lines
more quickly giving a greater sense of depth, and will take in a larger
area of distant background. Longer lenses will converge these lines more
slowly giving a lesser sense of depth, and will take in a smaller area of
distant background.
2. Myth: Increasing focal length increases blur of background.
This is much too general a statement. Yes you can get a "more out of
focus" background (as perceived by the image viewer), but _only_ if the
background is sufficiently distant from the rear of the DOF. If posting
images were not considered poor form on a mail list, I would attach some
graphs of what happens to the "Circle of Confusion" as distance increases
behind the critical focus points having the same magnification for
different focal lengths. If the background is very close to the rear of
the DOF, changing focal length won't make much difference. If the
background is relatively far from the rear of the DOF it can make a
difference. The farther it's behind the rear of the DOF, the more
difference it makes.
-- John
At 14:24 5/24/01, Scott Nelson wrote:
It's not that significant a factor. What is significant is the concept
that you "get the same magnification" at different standoffs using a 50
and a 90 or 135, while technically correct, this does NOT produce the same
image on film. Have you ever seen those tests where a subject is shot at
the same image size using short, medium tele, and long teles? The
subject's relation to the background is dramatically different. Shooting
macro with a long lens not only gives you working distance, it throws the
background more out of focus, an effect you may want given the tendency
for distracting elements to sneak in and detract from macro field work.
Also, consider working distance's effect not only on disturbing and
shadowing the subject, but also on flash. I once again suggest consulting
John Shaw's *Close-ups In Nature* as a seminal work which demonstrates the
use of lenses up to 300mm for macro work.
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