At 01:34 12/16/01, Warren wrote:
Although the extension tubes are less costly, because
magification is basically a function of amount of
extension, the 65-116 wins out because of its ease of
use in the field.
Warren
Whether the 65-116 or a set of extension tubes are used depends on how much
extension is required. That depends on both lens focal length and desired
magnification (on film!). That depends on the lenses on hand and desired
subject distance. The tube "set" has a much shorter extension range than
the shortest length that can be set on the telescoping tube.
Bizarre as it may seem, I've made macros using the 18/3.5 Zuiko. Only the
OM 7mm tube works with this lens. Everything else is too long, including
the 12mm tube from the Vivitar AT-21 tube set. The focus point with lens
focus set at infinity ends up physically inside the lens. Why would
someone *use* the 18mm f/3.5 Zuiko for a macro? Perspective!
Although the OM tube set is designed for continuous magnification coverage
with a 50mm lens with 7mm, 14mm and 25mm tubes, it can be used with any of
the OM Zuiko rectilinear prime lenses, out to the 200/4. Lens weight and
the moment arm forces on the tube flanges make me reluctant to use
longer/heavier lenses with them. I haven't handled the 180/2.8 or 180/2
and would be reluctant to use them with the tubes if their size/weight
approach that of the 300/4.5 Zuiko.
The Vivitar AT-21 set with 12mm, 20mm and 36mm tubes appears to be designed
for continuous magnification coverage with an 80mm to 90mm lens. I have
used the 24mm f/2 Zuiko (12mm tube only) and the 200mm f/4 Zuiko lenses
with them, plus others in between. The same caveat about using lenses
longer than the 200/4 applies to this set also. I don't know if the 12mm
tube is too long for a 21mm Zuiko or not. I don't have either 21mm lens.
Since there have been many macro questions lately, here's a "crash course"
about using lens extensions in making macros . . .
First Principle:
Macrophotography is driven by "magnification." This is the ratio of the
subject's image size _on_film_ to the actual subject size. If the image
(on film!) is 1/2 life-size, the magnification is 1:2, or 1/2, or 0.5 (take
your pick).
Making macros requires thinking about:
- how large the subject is
- desired size of subject image on film
- desired distance from the subject
- focal lengths available
- range of lens extension available
Since I'm accustomed to "English" units, I think of 35mm film as
approximately 1" by 1.5" in size. This helps me get a handle on desired
magnification. I do all the rest of the math in millimeters because tube
lengths and lens focal lengths are in millimeters. The important point is
using the _same_ units of length in the equations!
Basic Lens Equations:
1/f = [1/(x+f)] + (1/y)
f = lens focal length
x = lens extension from infinity focus to focus subject on film plane
[x+f = distance betwen rear lens node and film plane]
y = distance between subject and front lens node
M = (f + x) / y
M = magnification of subject on film
f = lens focal length
x = lens extension from infinity focus to focus subject on film plane
[x+f = distance betwen rear lens node and film plane]
y = distance between subject and front lens node
For use with typical 35mm and medium format cameras (see note about large
format below), the basic equation for finding the length of extension tube
required is:
x = M * f
x = distance lens must be extended _from_infinity_focus_point_
M = magnification desired (on film!)
f = lens focal length
[Note: Large format users often calculate the *total* "bellows extension"
needed for distance from lens board to film back. The lens focal length
must be *added* to "x" in this equation to approximate that
distance. Why? The lens board usually approximates the location of the
rear lens node. To focus the lens at infinity, the lens board (hence,
approx. rear lens node) must be positioned approximately one focal length
in front of the film back. For them, it's easier to measure the distance
from lens board to film back. If you see these equations in a text, ensure
you understand *what* is being calculated: additional extension required
from the infinity focus point or total extension from rear lens node to
film plane.]
Subject Distance Problems:
(a) A shorter lens requires less extension for the same magnification, but
will also have less distance from the subject. The distance from lens
front to subject can be *approximated* using magnification desired and lens
focal length:
y = f + (f/M)
y = front lens node to subject distance
f = lens focal length
M = desired magnification
(b) If you know the desired magnification and the minimum subject distance
you want, the minimum focal length required can be *approximate* using:
f = (M * y) / (M + 1)
f = lens focal length
M = desired magnification
y = desired front lens node to subject distance
Important Note:
These give *approximate* results when *approximating* the lens front
(filter ring) location using the "front lens node" location. The front
lens node can be behind the filter ring, or in front of the filter
ring! These equations do not work as well when the lens is extremely close
to the subject. The closer to the subject, the greater the error will be.
Unfortunately, wide variation among complex compound camera lens designs
makes a universal equation using the filter ring location impossible (in
lieu of the front lens node location). Equally unfortunate: Olympus does
not publish the location of the front and rear lens nodes for its
lenses. A "y" shorter than the _physical_ length of the lens is cause for
concern. Less than half the _physical_ lens length is cause for *great*
concern. Under these conditions I put the best guess for tube length on
and move in on the subject slowly and *carefully* making sure the lens
objective doesn't bang into the subject.
Final Note:
Effective aperture narrows as magnification (extension) increases. This
means if the lens is set to f/4, the effective aperture is something
tighter than f/4 when extension tubes are added. Workaround? None really
needed unless you're using a flash in Manual mode. Using the OM's
magnificent TTL/OTF metering solves this problem.
Post Final Note:
Be CERTAIN to keep units of length/distance the SAME! DON'T use a subject
distance in feet and expect a lens focal lengths in millimeters!!
-- John
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