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How to interpret focal length of m4/3 lenses and adapted lenses

Discussion in 'Native Lenses' started by noodlehaus, Feb 22, 2010.

  1. noodlehaus

    noodlehaus Mu-43 Veteran

    Feb 14, 2010
    I'm a bit confused with interpreting the focal length of m4/3 lenses. For example, does the 2x crop rule still apply to the 20mm f1.7 lens? Is it effectively a 40mm lens on m4/3 cameras?

    I'm quite certain that this applies to legacy lenses or adapted 4/3 lenses. But please correct me if I have it all mixed up. Thanks!
  2. Alan Wolf

    Alan Wolf Mu-43 Regular

    Jan 20, 2010
    Berkeley, CA
    angle of view

    The lens focal length stays constant, no matter what the image size (sensor or film) is. What changes is the angle of view. When people speak of converting lens focal lengths, what they mean is "using a standard image size (typically 35mm film - 24 x 36mm rectangle) as a reference point, what field of view will this (given lens) give me with this image size." So, since 35mm is our current standard, we can take the crop factor (how much of that 24 x 36 mm rectangle are we going to use to make our image?) and multiply the lens focal length by that to get a similar field of view. This sounds far more confusing that it is (probably my inability to simply explain anything!).

    So, as an example: the 4/3 sensor is about 12 x 18 mm, which gives us a diagonal of about 21.6 mm. The 35 mm diagonal is 43.3mm, close to 2x as large. So, a 20mm lens on a 4/3 camera has the same field of view as a 40mm lens would on a 35 mm camera. (Just as a side note, this diagonal is generally considered the "normal" focal length for a given format, so the 20mm Panasonic was a traditional choice, not random.) Anyway, with 4/3 you just multiply a lens's focal length by 2 and get what the equivalent field of view that lens's focal length would be if you were using a 35mm camera, or a FF sensor.

    If this is still confusing, picture that 35 mm camera. Say you've got a fixed, 50mm lens on it, and you're looking at something far away. You think—I wish I could get closer, or had a longer lens with me, but you don't, and take the picture. Later, when you go to print it, you just print the center part of it—what you've just done is "cropped" the image. This is exactly what happens with a smaller sensor. So, in a way, the 20mm lens on your m4/3 camera IS now a 40mm lens (at least in 35mm terms...) even though its focal length is, and always will be, 20mm.
    • Like Like x 3
  3. hodad66

    hodad66 Mu-43 Top Veteran

    Jan 27, 2010
    Indialantic, Florida
    Just double the focal length..... not in reality but in
    effective usage. The "20" is a 40 for all purposes as
    it gives just under a "normal" cropped view. An old
    OLY 50mm will seem to be a 100 mild tele.
    • Like Like x 1
  4. noodlehaus

    noodlehaus Mu-43 Veteran

    Feb 14, 2010
    Thank you very much for the explanation guys. This clears things up greatly for me =)
  5. noodlehaus

    noodlehaus Mu-43 Veteran

    Feb 14, 2010
    Wait, sorry but I forgot to ask. So do the MFT lenses also go under this 2x view angle rule? For example the Panasonic 20mm f1.7 lens, is it also effectively 40mm on MFTs? Or does the conversion/rule only apply to 4/3 lenses?
  6. Amin Sabet

    Amin Sabet Administrator

    Apr 10, 2009
    Boston, MA (USA)
    As Alan said, the focal length does not change depending on the camera. So, a 20mm MFT lens put on a MFT camera will give you the same angle of view / field of view as any other 20mm lens properly adapted for use on a MFT camera. A 50mm MFT lens gives the same view as an adapted 50mm 35mm film SLR lens or an adapted 50mm medium format lens for that matter.

    If you want to think of everything in 35mm frame field of view equivalents, then double the focal length regardless of whether it is a MFT lens or an adapted lens.

    A Lumix 20 is therefore a normal (or wide normal depending on how you look at it). A Nikon or Canon 20mm lens adapted for use on a MFT camera is also a normal. A 35mm lens used on a MFT camera is a short tele, regardless of whether it is a native MFT lens or adapted for that purpose.
    • Like Like x 2
  7. noodlehaus

    noodlehaus Mu-43 Veteran

    Feb 14, 2010
    Thank you very much Amin :) 
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