Recalculating MF focal length to 35mm equivalent?

Discussion in 'Medium Format Cameras and Accessories' started by awaken77, Sep 20, 2008.

  1. awaken77

    awaken77 Member

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    How to match MF lens focal length to 35mm equivalent (in terms of the same FOV)?
    For example, 80mm in 6x6 approximately equals to 45mm in 35, so divide factor is 1,76-1,77 ( 80 / 1,76 ~ 46 ).
    What about other MF formats ?
     
  2. Ian Grant

    Ian Grant Subscriber

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    35mm 120
    28mm 45mm
    50mm 80mm
    135mm 200mm

    Equivalents are always approximate as the Formats are quite different

    Ian
     
  3. ic-racer

    ic-racer Member

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    There are a number of ways to normalize the view. Since I use varying format aspect ratios, I normalize on the diagonal. Though some use the horizontal measurement.

    To get the correct numbers for you system, just measure the diagonal of the film image on a negative. Or use the same frame you use to figure out the actual view from the viewfinder (if you want to match the 'viewfinder angle of view'). Once you have the measurement for two formats, just make a ratio of the two and use that for the calculation.

    As an example, my system has the following values:
    35mm = 43mm diag
    "6x6" = 80mm diag
    "6x9" = 100mm diag
    "4x5" = 153mm diag.
    "8x10" = 310mm diag.

    So my 35mm camera with a 50mm lens matches:

    6x6 camera with a 93mm lens
    6x9 camera with a 115mm lens
    4x5 camera with a 178mm lens
    8x10 camera with a 360mm lens
     
  4. Ian Grant

    Ian Grant Subscriber

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    This is where using a calculation breaks down slightly. It's not that it's wrong but using 35mm as the starting point is false.

    The typical standard lenses for Large Format are:

    4x5 camera with a 150mm lens
    8x10 camera with a 300mm lens
    6x6 camera with a 75-80mm lens
    6x9 camera with a 90-110mm lens

    A 35mm camera standard lens should be nearer to 45mm as the OP surmises but for technical reasons it was more practical to design and make high quality 50mm lenses particularly as lenses needed to be fast to allow the cameras to be hand held with the slower film speeds of the 30's.

    It's better to think more loosely as there are a few ways you could mathematically calculate the equivalents.

    Ian
     
  5. dancqu

    dancqu Member

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    Multiply any 35mm focal length by 1.57. That is the
    ratio of the long edges of each format, 6x6 and 6x4.5,
    relative to that of 35mm; 55mm over 35mm. The long
    edge is the most wide angle. IMO the long edge best
    describes the angle of view.

    My RZ67 has a long edge length of 69mm. So 69
    over 35mm then multiply. Dan
     
  6. Windscale

    Windscale Member

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    One can never, or indeed, never need to be too exact. I have had many 6x9 cameras and the 9 sometimes turned out to be 8.4, 8.7, 9 or the longest one I have seen at 9.2 (a Bessa II folder). The 6 is also rarely exactly 6!
     
  7. Ian Grant

    Ian Grant Subscriber

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    Welcome to APUG. That's what I've been saying but in a different way. Likewise a lens marked 150mm might actually be a 153mm, a 50mm standard on a 35mm camera could be a 48mm, others 52mm etc. So it's pointless being exact.

    Ian
     
  8. Christopher Walrath

    Christopher Walrath Subscriber

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    One easy way I've found to convert 645 to 35mm in my head. It's roughly the same as km to miles. 8.0 - 5.0. 80mm to 50mm or so.
     
  9. wiltw

    wiltw Subscriber

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    I found that the best recommendation came from Sinar, about converting FL from one format to another is to NOT express it as a diagonal, with a conversion factor. Instead, look at FL with respect a multiple of the narrow dimension of the frame!
    For example 24mm WA (135) is (1*side) which is same as 15mm (APS-C) same as 42mm (645) same as 90mm (5x4 sheet film)
    Or, 90mm portraiture short tele (135) is (3.75* side) is same 55mm (APS-C) same as 160mm (645) same as 340mm (5x4)
     
  10. wclavey

    wclavey Member

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  11. Thomas Bertilsson

    Thomas Bertilsson Subscriber

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    Think of the image circle you have to capture. On 35mm you have - 35mm (OK, 36mm), with 645 and 6x6 you have 60mm (or 56mm or so), 6x7 is larger again (almost 70mm), 6x9 (almost 90mm), and 4x5 (about 110-115mm). Divide the image circle numbers, and you have your conversion factor. There's some slop built into my method, but it works OK.

    - Thomas
     
  12. Q.G.

    Q.G. Inactive

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    "Some slop"? :wink:

    A lens projecting an image circle of 36 mm would produce severe vignetting on 35 mm format.
    The same for your other circles.

    What you are doing here is compare the size of the sides of the different formats, not that of the image circle.
    And not consistently too: long side for all formats smaller than 4x5", but short side for 4x5"?

    A good way to compare formats is the one that compares horizontal angles of view. We tend to compose using the horizontal angle a lot; the vertical less. But not always. And who knows the horizontal angle of view of his or her lenses?
    But probably the best way - which you are using - is indeed to use the long side of the format. The ratio between those is also the ratio between focal lengths.