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Re: [OM] shift lenses; plastic lenses; bellows factor

Subject: Re: [OM] shift lenses; plastic lenses; bellows factor
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Tue, 01 Oct 2002 01:08:04 -0500
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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