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Thread: RZ and focal length calculations

  1. #1
    Shelby Lewis
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    RZ and focal length calculations

    Hopefully you guys can help me here.

    When calculating the 35mm equivalent focal length for lenses on the RZ67, but using a 645-format back, do you treat the lenses as you would on a 645 camera?

    I have the CI Focal Length Calculator, but am not sure how to figure focal lengths for a 6x7 camera using a 645 back (p25, to be exact). If I want to use the 210 on the rz, with a p25... do I enter the "210" under p25? That gives a 312mm equivalent on the rz, which is about 150mm on 35mm-format. This sounds right to me, but just trying to be careful here before I set up my lens lineup. I have a great kit available at a ridiculously good price that also includes a 110mm, 140mm, and 65mm. I want to add one more longer lens, and think the 210 will be the one.

    For headshots, I like a long lens... somewhere between 135mm and 200mm on 35mm.

    Thanks!
    Shelby

  2. #2
    Sr. Administrator Jack's Avatar
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    Re: RZ and focal length calculations

    Yes -- you use the factor for the format you're actually shooting, not the camera. In the case of the P25, you can simply multiply whatever lens you mount an whatever by 0.7 to derive the effective 35mm equivalent focal length. So your RZ 210 would indeed be a 147 effective focal on the P25.
    Jack
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  3. #3
    Shelby Lewis
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    Re: RZ and focal length calculations

    Thanks!

  4. #4
    Senior Member bensonga's Avatar
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    Re: RZ and focal length calculations

    I've been wondering about this myself recently as I mount Pentax 67 lenses on my Pentax 645. Trying to figure out what a 200mm P67 focal length lens is equivalent to in terms of a Pentax 645 lens. I came up with this spreadsheet based on some focal length calculator for 35mm equivalents. I'm not sure if it is correct however.

    What is the CI focal calculator?

    Gary

  5. #5
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    Re: RZ and focal length calculations

    Hi,

    35mm, 6x7, 645 have different proportions; 3:2, 5:4 and 4:3 respectively. Thus there is no exact conversion factor of lenses, except if one consider horizontal or vertical sides of the image separately (albeit calc by diagonals give approximation).

    Actually, it is very simple to calculate exact the equals.

    Exact dimension of formats mentioned are:
    35mm = 36mm x 24mm
    645 = 56mm x 41.5mm
    6x7 = 70 x 56mm

    If I recall correct the P25 is a 48mm x 36mm sensor.


    Thus for width, the equal of 35mm is for example 210mm lens;
    6x7 ; 210mm x 36mm / 70mm = 108mm
    P25 ; 210mm x 36 / 48mm = 158mm
    ... or for P25 the factor by width is 36mm/48mm = 0.75

    Or for height, the equal of 35mm for same lens;
    6x7 ; 210mm x 24mm / 56mm = 90mm
    P25 ; 210mm x 24mm / 36mm = 140mm
    ... or for P25 the factor by height is 24mm / 36mm = 0.66

    Regards
    Anders

  6. #6
    Sr. Administrator Jack's Avatar
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    Re: RZ and focal length calculations

    Anders, the whole vertical horizontal thing is why folks use the diagonal -- but you're correct, the aspect ratio difference makes it tough to get an exact comp.

    Here's an Excel doc I made up a few years ago that covers most formats and focals -- it's not refined, just use it for my own reference, also has a diagonal calculator at the bottom:
    Jack
    home: www.getdpi.com

    "Perfection is not attainable. But if we chase perfection, we can catch excellence."

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