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  1. #21

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    Quote Originally Posted by E. von Hoegh View Post
    Technical Note: Independence of Focal Length
    Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).
    Let me (not a physicist) ask this: I can see the above being the case for a simple long focus lens, but what about a true telephoto where the focal length is greater than the physical length of the lens? (I know this has something to do with one of the nodal points but I barely remember my optics class.) The light's "distance traveled" could include some distance before it actually crosses the aperture edge. Or is this simply magnification and not "distance traveled" and a non-issue?

    s-a
    I photograph things to see what things look like photographed.
    - Garry Winogrand

  2. #22
    bsdunek's Avatar
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    I have a Lentar 450mm f8.0 and a 500mm f8.0 mirror lens with some camera shop name on it. Both do a good enough job to get 16 X 20 prints from 35mm. The mirror lens is a little low in contrast, but nothing a no. 4 printing filter won't fix. I say use them and enjoy them. Just hold them still, preferably on a tripod, unless you're panning race cars or something. I've got some pretty good shots panning, and they work fine.
    Bruce

    Moma don't take my Kodachrome away!
    Oops, Kodak just did!


    BruceCSdunekPhotography.zenfolio.com

  3. #23

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    As I remember, the 500mm came out about the same time as the 1000mm. The UK magazine, Amateur Photographer did a test around about 1963 on the 500 and it wasn't at all bad. What struck me was the fact that it was a very simple lens with actually only 3 elements, of which 2 were cemented together and the two groups were at opposite ends of the tube. The 1000 was the same as the 500 but bigger, again the same optical setup.

  4. #24
    ic-racer's Avatar
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    Quote Originally Posted by Diapositivo View Post
    The Cambridge In Colour site quoted above specifies that the f/number is relevant, not the absolute diametre of the diaphragm aperture. It says:

    Technical Note: Independence of Focal Length
    Since the physical size of an aperture is larger for telephoto lenses (f/4 is has a 50 mm diameter at 200 mm, but only 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size).
    Another way to put it is that "Long focus lenses magnify the circles of confusion and Wide lenses make the circles smaller with the same linear aperture size."
    All, in all it works out the same "F number" (not linear aperture size) across lenses gives the same CoC size on the film.

    When the physics of optics gives such an elegant solution, why do some not want to believe it

  5. #25
    Hatchetman's Avatar
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    Quote Originally Posted by desertratt View Post
    A one time back in the 60s and 70s I worked for the world's largest news organization. Their photogs (including me) used the Takumar 500 for sports photography (Dodgers, Yankees, etc.) and it was a fine lens. We mounted 'em on Nikons and had a Mickey Mouse system like a car gear shift to instantly focus on first and third base, second, home, and the pitcher's mound. The photos were used in newspapers and magazines all over the country and it is one of my favorite lenses. Large, but a goodie. I think you bought a gem.
    Great story!

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