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Thread: Newbie question #99999999999.....

  1. #1

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    Newbie question #99999999999.....

    How do I calculate the longest focal length possible w/ a given bellows? I know I need to know the measurement of the length....is there some multiplication factor?

  2. #2

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    Re: Newbie question #99999999999.....

    I am not sure I understand the question. If the bellows is 300mm/12" you can use a 300mm/12" lens but you will be limited to focussing on a subject at infinity - usually 100 times the focal length of the lens.

    I usually recommend that the bellows be at least 25% longer than the longest lens you want to use.

    steve simmons

  3. #3

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    Re: Newbie question #99999999999.....

    The shortest bellows one can get away with for a particular focal length is about the same as the focal length. This will allow you to focus at infinity with that focal length lens. This will give you a tiny bit of wiggle room as the film plane is usually a little behind the rear attachment point of the bellows and the lens sits a little forward of the front attachment point of the bellows.

    How much longer the bellows should be relative to the focal length will depend on how close you want to focus. For landscapes you won't need much more...just a guess, but perhaps 50% more than your focal length would be nice to have. For table-top/still-life work you might want 2x to 4x the focal length. For portraits perhaps up to 2x the focal length.

    Now wait for someone to answer who really knows what they are talking about!

    Vaughn

  4. #4

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    Re: Newbie question #99999999999.....

    Sounds like I was being generous with 50% longer than the focal length.

    I have used a 28" lens with my Zone VI 8x10. I could focus at infinity, but I was at the very end of my focusing racks. I was wishing for another inch or two of extension, but the image came out. A 24" lens is quite usable on the camera...as long as I don't have to focus too close.

    Vaughn

  5. #5

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    Re: Newbie question #99999999999.....

    As the others have indicated, to determine the longest focal length that will work with a given bellows extension, you have to also say how close you want to focus. In words, the formula amounts to the following. Multiply the maximal bellows extension by the minimum closeup distance and also add them. Then divide the first number by the second, and that is the longest focal length you can use. (Of course all lengths should be measured in the same units.) For example, suppose your maximum bellows extension is 300 mm, and you want to be able to focus down to 3 meters of 3,000 mm. Then the maximum focal length will be

    f = 300 x 3000/(300 + 3000) = 90000/3300 ~273 mm

    But there is one warning about this. The focal length is defined to be the distance between the rear principal point and the rear standard when you are focused at infinity. For normal lenses, the principal point is so close to the back of the shutter, you can assume that when you focus at infinity, the bellows extension equals the focal length. But for lenses of telephoto design, the rear principal point is actually well in front of the lens board. For such lenses, when you are focused at infinity, the bellows extension would be well less than the focal length, it would equal the so-called rear flange focal length. So a lens will work if its rear flange focal length is less than the number given by the formula. Thus, in the above example, a 300 mm lens of normal design wouldn't work but a 300 mm lens with rear flange focal length equal to 270 mm would.

    Note that "telephoto design" refers to the design of the lens and may not bear any special relation to the focal length. Some lenses, long for the format, have normal design, and some are of telephoto design; you can't tell by just looking at the focal length. There is also "inverted telephoto design", where the principal point is behind the lensboard. In such a lens the rear flange focal length is greater than the focal length, so you can focus at infinity without getting so close that the back of the lens hits the film or the bellows is compressed too much. Most wide angle lenses are designed this way, but not to the extent that telephotos are.

  6. #6

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    Re: Newbie question #99999999999.....

    Thank you gentlemen. BTW,,,Steve...just ordered your book. Looking forward to it.

  7. #7
    Dave Karp
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    Re: Newbie question #99999999999.....

    The focal length of a lens is a good shorthand way to arrive at the minimum bellows you need to focus at infinity. Ebony includes a chart on its website that covers the flange focal distance for lenses from the big four manufacturers: http://www.ebonycamera.com/articles/lenses.html

    From the chart you will see that the actual extension needed may vary. For example, the Fujinon 450 C requires 425.3mm to focus at infinity, less than the 450mm you would expect. This can be handy information if you are pushing the limit of your equipment and are choosing from more than one lens.

  8. #8

    Re: Newbie question #99999999999.....

    To use it at infinity you need to know the flange focal length of the lens. Basically this is the distance from the rear of the shutter to the lens board. If the flange focal length is longer then your bellows then you will not be able to focus the lens without an extension of some sort. Lens manufacturers publish the flange focal lengths for all of their current lenses. Some might publish it for their older lenses.

  9. #9

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    Re: Newbie question #99999999999.....

    Quote Originally Posted by Bob Salomon - HP Marketing View Post
    To use it at infinity you need to know the flange focal length of the lens. Basically this is the distance from the rear of the shutter to the lens board. snip....

    That distance is usually zero...you probably meant...

    "The flange focal distance of a lens is the distance from the rear surface of the lens shutter (i.e. the front surface of the lensboard) to the focal plane (i.e. the film plane) when the lens is focused at infinity."

    Also: "With standard lenses, the flange focal distance is usually slightly less than its focal length. However this relationship does not hold for wide-angle or telephoto lenses - the flange focal distance is somewhat greater than the focal length in the case of wide-angle lenses, but very much less in the case of telephoto lenses."

    Both quotes are from the link David gave us.



    Vaughn

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