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Thread: dumb question

  1. #1

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    dumb question

    Is there a simple or easy way to determine if a particular focal length lens will cover
    a particular format? I don't even know how to ask the question.
    Thanks, Larry

  2. #2
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    Re: dumb question

    The easiest way is to click on LF Home Page on the blue tool bar above.

    Scroll down to lens comparison charts.

    Click, enter the format you need, and you'll find the answer.

  3. #3
    Lost mike rosenlof's Avatar
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    Re: dumb question

    Not a dumb question. A good one. And there's no easy answer.

    It all depends on the lens design, and to a smaller effect, your working aperture. A smaller aperture gives a little more coverage.

    If you think of the 'normal' focal length of a format as the length of its diagonal, that focal length lens (i.e. 6 inch (150mm) on 4x5) "normally" covers that format with reasonable room for movements.

    Some lenses, often with a 'W' or 'XL' in their names are designed to cover larger formats than their focal length might indicate.

    Many process lenses, cover a narrower than 'normal' range. The 'narrow' aspect doesn't get displayed as loudly as Wide coverage, but it reflects the priorities of the lens designers. Apo (red dot) Artars, are one example of a process lens with a somewhat more narrow than 'standard' coverage.

    In LF lenses, more elements often mean wider coverage. Not because they have to, but that's when LF lens designers usually do with more lens surfaces.

    You normally need to find lens manufacturers' specs to determine what a lens covers. Most general purpose lenses are specified at infinity (or nearly so) focus. Most macro or process lenses have their circle of coverage specified at 1:1 magnification which is usually double their coverage at infinity.

  4. #4

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    Re: dumb question

    Thanks Gem

  5. #5

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    Re: dumb question

    No
    Go to cameraecccentric and peruse some old lens catalogs to get a taste.
    "Image Circle" is related to focal length only in a most tenuous way, such as a 15mm lens almost certainly won't cover 4x5 [ but wait there was that........]
    Then there's "circle of illumination" vs "circle of best definition"- and you gotta know what, and whose, the definition of "best" is.
    The good news is that there are lots of threads here that discuss these topics and specific lenses. The most direct way to begin might be to search for information about specific lense[s]- "Will a 7 inch f6.3 Frazanar cover 7x11", or maybe "Who made Frazanars?" so you can find a catalog on line. A little searching should get you enough so that you will be able to frame good questions, and understand answers.

  6. #6
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: dumb question

    The parameter you're looking for is the diameter of the image circle (IC).

    You'll find this on the data sheet for any modern lens.
    Since the IC gets larger as the lens is stopped down you'll usually find two values, one wide open (f/5.6 or ?) and one at f/22.

    To cover a particular film, the IC diameter must be larger than its diagonal dimension (e.g. 163mm for 4x5).

    Since large format work frequently requires lens movements, we like to have an IC larger than the minimum.

    A good example of the range of values you might encounter can be found with two Nikor 150mm lenses:
    The 150mm f/5.6 Nikor-W has an image circle of 210mm, which covers 4x5 adequately, while
    the 150mm f/8 Nikor-SW has a huge image circle of 400mm, which easily covers 8x10.

    - Leigh

  7. #7

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    Re: dumb question

    Quote Originally Posted by clarryd View Post
    Is there a simple or easy way to determine if a particular focal length lens will cover
    a particular format? I don't even know how to ask the question.
    Thanks, Larry
    The answer to your exact question is what Mike said.

    The relationship between focal length and image circle size is linear for a particular lens design/formula. The lens design can also be expressed in terms of angular coverage. After you know this value, it's just a matter of calculating it for the focal length:

    http://en.wikipedia.org/wiki/Angle_of_view#Example

    GTW

  8. #8
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    Re: dumb question

    Quote Originally Posted by Leigh View Post
    (e.g. 163mm for 4x5)
    Less than that. 4x5 is a nominal dimension. My standard film holders provide a diagonal of 154mm. For some lenses, that's a critical difference (65/8 Super Angulon comes to mind).

    For the OP, one of the challenges of lens design has been to provide a larger image circle for the same focal length. That's a big reason why tessar designs gave way to plasmat designs, and why plasmat designs were unsuitable for lenses shorter than 90 or 100mm, leading to wide-field designs like the Super Angulon and Grandagon.

    Many large-format cameras have ground glass with the corners notched to allow you to look at the lens directly from the corners. If you can see the whole aperture shape through that notch, then the lens is providing coverage. If part of it is blocked off, the corner will be dark. If you can't see the aperture at all, it's outside the lens's coverage. Smaller apertures are less likely to be hidden behind the barrel of the lens, which is why coverage improves at smaller apertures.

    Rick "who likes long tessars and short Super Angulons" Denney

  9. #9

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    Re: dumb question

    Quote Originally Posted by mike rosenlof View Post
    ...

    If you think of the 'normal' focal length of a format as the length of its diagonal, that focal length lens (i.e. 6 inch (150mm) on 4x5) "normally" covers that format with reasonable room for movements.

    ...
    That's like saying - if you can see the dog you can see it...
    First of all, you have to know for what format the lens is made - only then can you judge if it is a "normal" focal length for that format or not. 150mm lens for SLR or 150mm lens for digital backs or 150mm lens for LF... etc.

  10. #10

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    Re: dumb question

    Just FYI for the math-challenged: The diagonal sizes for standard film formats are

    4x5: 6.4 inches = 163mm
    5x7: 8.6 inches= 219mm
    8x10: 12.8 inches = 326mm
    16x20: 25.6 inches= 650mm
    20x24: 31.2 inches= 793mm

    So, if the image circle of your lens is larger than the diagonal film size listed above for your format, then you should be OK.

    Image circle and format diagonal sizes should not be confused with a lens focal length, even though they're all measured in mm.

    The focal length of a lens can GENERALLY *but not always also be an indicator of the lens coverage. The normal lens for 6x7 has a focal length of 80mm, for example. For 4x5 a normal lens is 150mm. But an f/5.6 XL Super Angulon lens has focal length of 90mm will cover 4x5 because it is designed to be a wide angle lens, not a normal lens. It has an image circle of 259 mm which means it can just cover 5x7 formats too, but not with much movements.

    As mentioned in other earlier posts, if you're using a process lens, things are a bit different.

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