At 14:26 9/29/02, William Sommerwick wrote:
>> The magnification is the lens extension (from infinity) divided by the
lens's focal length.
Your basic math is correct, however, it needs clarification:
This works as a practical method with the normal lenses (lens helical only)
and normal lenses when used with tubes or bellows (lens helical plus
telescoping tube or bellows length) on 35mm and MF SLR's. Unfortunately,
when using the OM's 135/4.5 or the 80/4 bellows lenses, it gets more
complicated. Using pure additional extension provided by the tube or
bellows won't work.
These lenses *must* be mounted on the telescoping tube or bellows to be
used. The 135/4.5 requires 65mm extension from its mounting flange to the
camera body lens flange to get its rear principal point far enough from the
film plane for infinity focus (total of 135mm, its focal length). The 80/4
would focus at infinity with 29mm extension of its mounting flange from the
camera body lens flange, but neither the telescoping tube nor bellows can
go that short. It cannot focus at infinity, but will always focus at some
closer distance, even when used on the bellows with it fully
compressed. IIRC, the 80/4 minimum magnification when used with the
bellows is 0.09X which gives a minimum focus distance as measured from its
front principal point (not the filter ring) as 96.67 cm (approx. 38
inches). Distance from lens front to subject at maximum focus distance
will be different than this as the front prinicpal point is *not* the same
as the front surface of the lens objective, or the front of the filter
ring. It should be within a few cm though.
The bellows further confounds things with its indexing marks which read
33mm more the true additional extension it is providing (e.g. if its index
markings show 69mm, then true additional extension is 36mm).
"Lens extension" is defined as the entire distance between film plane and
lens' rear principal point (what you call its "optical center"). This
comes from extension being originally measured on view cameras wherein the
film plane and lens board are two convenient points between which the
distance can easily be measured. With most view camera lenses, the lens
board closely approximates the location of the lens' rear principal point;
it's close enough for most purposes. At infinity focus, lens extension is
lens focal length.
"Extension" as you've used it is the additional extension beyond focal
length. Works for someone dealing with 35mm and MF SLR's that can measure
tubes and helical, and use that. However, it doesn't match the traditional
definitions and equations found in most texts (e.g. Ansel
Adams: _The_Camera_).
>> By definition, a lens's focal length is the distance from the lens's
optical center to the film plane. As you focus closer, the lens's focal
length increases. The f-number -- which is the focal length divided by the
aperture -- also increases, which requires an exposure increase.
No! Focal length *never* changes unless somehow one or more elements
"morph" themselves into a different shape, or the lens focusing helical
somehow shifts internal elements to deliberately change the focal
length. Focal length is defined as the distance between the lens' rear
principal point and the point where ray paths entering the lens parallel to
the lens axis converge. The only time ray paths from a point will enter
parallel to each other and the lens axis is when that point is at an
infinite distance from the lens. Thus, the focal length is also the
extension (distance from rear principal point to film plane) for infinity
focus.
What does occur that makes the subject grow in size:
When a lens is extended farther from the film plane to focus closer than
infinity the diameter of the image cone where it intersects the film plane
grows. All the light gathered by the lens that makes it through its back
end is contained in the image cone. Since the film gate does not grow in
size (for 35mm format, it's always 24mm x 36mm), and the diameter of it at
the film plane grows, less light passes the film gate to the film. IOW,
the same amount of light is spread over a larger area, the image circle, a
piece of which is the fixed, unchanging area of the film gate, and the
image recorded uses less light from a larger image circle at closer focus
distance than from a smaller image circle at farther focus distance. For
practical purposes in setting exposures, the difference between the amount
of light at infinity focus and closer distances does not become appreciable
enough to adjust exposure until critical focus distance is closer than
about 8X lens focal length. Total lens extension for this focus distance
is focal length plus focal length divided by seven (f +
f/7). Magnification is 1/7th life-size on film, and technically the
correction for this would be about 1/5th f-stop.
In addition, if the camera is mounted on a tripod and the focusing helical
is used to fine focus, as the focusing is performed from the lens infinity
position to some shorter distance marking, the lens cell is moved closer to
the subject. Not much of an effect at "normal" non-macro distances, but at
extremely close macro distances it increases magnification slightly by
shortening the focus distance between subject and lens. I've experienced
this effect when focusing on 1:1 macros using the 85/2 and
auto-tubes. This is why it's often easier to maintain desired
magnification at high magnification by attaching the lens to the tripod and
allowing the camera body and film plane to move back rather than the lens
moving forward. Difficult with "normal" lenses and auto-tubes; easier with
the telescoping auto-tube or bellows.
-- John
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