Viewfinder coverage

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At short distances the M-viewfinder covers approximatly 100 % of the image but not so at longer distances, can anybody elaborate?

-- Jonas Vilslev (jonasvilslev@groenjord.dk), December 19, 2001

Answers

A lens that is focused closer by moving the optical cell farther from the film plane gains a bit of focal length and therefore loses some angle of coverage the farther it is extended. So at closer distances the field of view becomes smaller. The Leica framelines are sized at this smallest dimension, less "the amount masked by a typical slide mount", so that one will always be sure at least everything in the frames will be on film. Since the field of view in reality gets larger as the lens is focused toward infinity but the framelines retain the same dimension, at longer distances they show less than what will be on film. Since longer lenses require greater extension when focusing, they have more discrepancy between fields of view at near and far distances. (The 135mm is an exception with the Leica because it only focuses to 1.5m and the frame is tiny to begin with, so the effect isn't as great as with the 90mm lens that focuses to 1m and has a larger frame making the difference more noticeable). In reality, at infinity with a 50mm lens you will get a view on film equal to an area amout midway between the 50 and 35mm framelines. With the 90mm lens at infinity the on-film coverage is almost exactly equal to the view seen through the 75mm framelines. With the 135mm you can just compose just outside the framelines at infinity and be ok. With 35 and 28mm framelines, you can all but forget the issue altogether. A camera like the Contax G2 adjusts frame size as it is focused, the Leica mechanism does not allow for this. Do not confuse the movement of the frames down and right: that is parallax correction, to compensate for the fact the viewfinder is offset from the lens' axis and this becomes more critical at closer distances. FYI, with internal-focusing lenses (not found on the Leica M), the focal length remains constant as the lens is focused (since there is no extension); however the maximum aperture changes slightly, eg. with the 280/2.8 at its 2.5m closest range, the effective aperture is actually about f/2.2.

-- Jay (infinitydt@aol.com), December 19, 2001.

Jay's explanation is perfect. If you want to really observe this in real life, just grab an SLR. Go out in good light and focus at infinity while holding the depth of field preview lever with the aperture set to its minimum setting. Now focus slowly to the closest distance and watch the background area reduce in coverage. The use of the depth of field preview has nothing to do with this phenomenon, it just keeps the focus deep enough so you can see deep into the image which will be out of focus as you focus closer.

The same thing happens with your M camera, but you don't have the benefit of seeing the image from the actual lens, so the designers chose to make the frame lines correspond to the closer distances.

-- Al Smith (smith58@msn.com), December 19, 2001.


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