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Thread: Is it .27 or .30 when figuring out a focal length - Which one do you use?

  1. #1

    Join Date
    May 2011
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    Is it .27 or .30 when figuring out a focal length - Which one do you use?

    Hi,

    When I see a LF focal length, I usually multiply
    that focal length by .30 in order to get its
    equivalent in 35mm.

    For example, a LF lens with a 150mm focal length
    will be multiplied by the .30 factor to get the 35mm.
    equivalent, that is 45mm.

    However, I was reading a conversation on the
    internet and a guy mentioned that it is not .30
    the right factor to multiply by, but .27.

    That would give a LF lens of 150mm. a focal length
    of 40.5mm.

    Which of the two factors is right? Which one have
    you be using?

    Thank you, kind regards!

  2. #2
    Gary Beasley's Avatar
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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    That would depend on whether you are using the vertical or horizontal measure to compare by. 35mm is closer in aspect ratio to 5x7, 4x5 ratio would be clipping the sides. However I think the .27 may be close as a 150 is just slightly wide.

  3. #3

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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    Nacho, whatever you do will be wrong. Welcome to the club.

  4. #4
    Foamer
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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    Not wanting to have a headache I just multiply the 35mm x3, or divide by 3. Since I shoot 4x5, 5x7, 8x10 I've quite trying to think in terms of numbers anyway. I have a wide lens, a normal lens, and a portrait lens.


    Kent in SD
    In contento ed allegria
    Notte e di vogliam passar!

  5. #5
    Mark Sawyer's Avatar
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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    A six-inch lens on a 4x5 is equivalent to a 150mm lens on a 4x5.

    Best not to think in terms of equivalents and just let large format be its own thing. Asking "what lens would I use on a 35mm?" to figure out what lens to use on a 4x5 is like deciding where to take your girlfriend for Valentine's Day dinner based on your ex-girlfriend's favorite restaurant...
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  6. #6
    ic-racer's Avatar
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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    ...a guy mentioned...
    He would need to know how much of your 35mm negative you print and the diagonal of your 35mm camera's film gate to give correct advice.

  7. #7

    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    I shoot multiple formats, including 4x5, 5x7, Micro 4/3 (gasp ! ) and full-frame. The varying aspect ratios of these several formats make a single angle of view ( usually the diagonal) multiplier not very useful, at least for me. Trying to compare the normal diagonal field of view across several formats with different aspect ratios, such as 4x5 to 35mm equivalent, is apples and oranges.

    So, I personally calculate the equivalence for my own use by comparing the long axis of different formats so that there's an equivalent horizontal angle of view that's actually equivalent and comparable. The same can be done using the short axis if that's your taste.

    35mm full-frame has a 36mm long axis while the image area of the long axis of 4x5 is roughly 120mm and 5x7's long axis image area is roughly 172 mm. I know that the image area of sheet film is slightly different but a percent or two difference is basically irrelevant to a focal length equivalence calculation - if those few percent are important in a particular circumstance, then just use the practical approach of checking the ground glass edges for certainty.

    Dividing 36mm/120mm = .3 long-axis multiplier going from 35mm to 4x5, or a long-axis divisor going from 4x5 to 35mm of roughly 3.3

    For 5x7, it's very nearly the 35mm aspect ratio, so it's a bit more intuitive. 36mm divided by 172mm is just about a .21 multiplier or a focal length divisor of roughly 4.8

    A 150mm lens on 4x5 thus gives a 35mm long axis equivalent of roughly a 45mm lens, whether you use a multiplier or a divisor.

    A 43mm lens on 35mm is its "normal" lens as classically defined using the diagonal angle of view calculation.

    So, 150mm is pretty close to classic normal on 4x5 whether calculating by the long axis method or the diagonal angle of view approach.

  8. #8

    Join Date
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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    When you first learn a foriegn language, you are translating what you hear in your head to your native language, after you get better and do it long enough you learn to think in that language (to an extent.) Focal lengths is kind of the same thing. When you start in a new format you have to translate to a format you know, but after a while, you don't translate 135mm, you just know what it will look like (approximately) in the format you're shooting.

    For some reason the 35mm focal lengths have become so ingrained that it is even a standard entry in digital camera EXIF data--"FocalLengthIn35mmFormat". So my reccomendation is not to get too hung up on the specifics--when you're just moving to the format, use the multiply/divide by 3 rule. Its not really accurate but it give you a ballpark number. Then learn to think in the new focal lengths.

  9. #9

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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    I'm using 3.5 as the devisor which is approximately equal to 0.29.
    But since the ratios are different there is no right or wrong.
    Don't waste to much time...

    And then try to equivalent 4x10 to 35mm
    Lasse Thomasson | Instagram

  10. #10

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    Re: Is it .27 or .30 when figuring out a focal length - Which one do you use?

    multiply or divide by 3.5 for 4x5 and 35mm. that's 4x5 with the rails cropped out and based on equivalent normal lenses (152mm and 43mm).

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