Crop factor using digital lenses

Discussion in 'Adapted Lenses' started by Crdome, Feb 12, 2012.

  1. Crdome

    Crdome Mu-43 Hall of Famer

    Sep 11, 2011
    West Central Indiana
    Chrome
    The crop factor for 35mm to m4/3 is 2:1. What is the crop factor for digital SLR to m4/3? Thank you. -Chrome
     
  2. Always the same: 2x
     
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  3. Crdome

    Crdome Mu-43 Hall of Famer

    Sep 11, 2011
    West Central Indiana
    Chrome
    Nic, thanks for the clarification. I obviously wasn't thinking. Strangely I thought the crop factor going from 35mm to digital was due to a change of the flange focal distance. But, now I realize the ffd by design must always remain constant to remain focused. The crop factor is relative to the sensors size, specifically the center portion (striking the sensor) of the larger image circle that would extend past the sensor's edges. -Chrome
     
  4. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    OK, first, there's no such thing as "digital crop factor." The crop factor is based on the relationship between two sensor sizes (with 35mm film counting as a sensor). Which I guess you've figured out. So the crop factor of m43, compared to 35mm or full frame digital, is 2x.

    But the truth is that relatively few people actually shoot FF digital cameras. Most people coming to m43 from a DSLR are coming from an APS-C format camera (e.g., Nikon DX). And compared to those cameras the crop factor is less.

    Take a 20mm lens on m43. Compared to a full frame camera, that's equivalent to a 40mm lens FOV. But it's equivalent to about a 26mm lens on an APS-C camera. (A 40mm lens on APS-C would have the same FOV as a 60mm lens on a FF camera).

    Another example: Panasonic is getting ready to release a 35-100mm zoom. People keep comparing this to the 70-200 mm zooms from Canon and Nikon. But if you've used one of those CaNikon lenses on an APS-C camera, you're really used to a 105 to 300mm field of view (compared to full frame). Someone used to one of these lenses on an APS-C format camera is going to be surprised at how much wider the FOV is going to be with the m43 equivalent.

    Looked at another way, the crop factor of m43 compared to APS-C is only about 1.3x, not 2x.
     
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  5. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Oh, and the same thing is true for DOF comparisons. Compared to full frame, at a given FOV, an m43 lens needs to be set 2 stops faster to get the same DOF. Compared to APS-C, though, the difference is a bit more than 1 stop, not 2.
     
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  6. nickthetasmaniac

    nickthetasmaniac Mu-43 All-Pro

    Jan 11, 2011
    Perhaps, but Panasonic is making the zooms 12-35mm and 35-100mm to replicate the classic 24-70mm and 70-200mm full-frame fast zooms. You may have noticed that several companies do the same for APS (Pentax DA* 50-135/f2.8 for instance).
     
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  7. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Yes, I understand that. But I suspect *most* of the people currently using 70-200s are using them on APS-C bodies.
     
  8. That's true, and Sigma did something similar with it's APS-C 50-150mm f2.8 lens. However, the two big SLR brands (Nikon and Canon) have never released APS-C spec fast telephoto zooms, although of course both have fast 17-55mm f2.8 standard zooms. For anyone used to a 70-200mm f2.8 or f4 lens on an APS-C Canon or Nikon body, a 35-100mm m4/3 lens is going to come up short in comparison.
     
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  9. Farcanalman

    Farcanalman  

    144
    Feb 12, 2012
    Hmmm, most strange!

    I have just spoken to the technical advisor for a company that manufactures adapters for all kinds of photographic purposes, including Micro 4/3, and he said that for example my E-PL1 should multiply the focal length of any Legacy or AF lens by 1.6x not 2x, and the 0.4x that I was led to believe is quite a difference!

    So are you all 'rounding up' or does this '2x' figure apply to certain sensor sizes only?

    :confused:
     
  10. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    No, 2x is the "technical" crop, but the advisor is right about the 1.6x being more accurate in terms of getting a similar perspective (which is what most people use "Equivalent Focal Length" for!). The reason for this is that Four-Thirds uses a 4:3 aspect ratio which is a lot squarer than the 3:2 aspect used on 135 film, Full-Frame DSLRs, or APS-C. That's because 135 film was originally a "double frame" system created by putting two 35mm video frames together (Olympus had a "single frame" system in the original Pen cameras of the 50's and 60's). That's why all the formats based off of 135 film is so wide. The 2x crop factor measures only the diagonal angle of the Field of View only, and doesn't account for that change in aspect ratio.

    Here's an example.... Take this as the control image for "Full Frame":
    sensor-size_comparison.

    This is what the image would look like on an APS-C sized sensor which has a true 1.6x crop (APS-C is the same 3:2 aspect as Full Frame, so the crop factor is an accurate representation in this case):
    sensor-size_aps-c.

    Now this is what that image would look like on a Four-Thirds sensor...
    sensor-size_four-thirds.

    You notice the APS-C and Four-Thirds image look nearly identical, except that the Four-Thirds image shows less of the legs, right? So for the purpose of perspective the 1.6x crop is much more accurate... Basically, take an APS-C crop then cut down the long ends.
     
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  11. Crdome

    Crdome Mu-43 Hall of Famer

    Sep 11, 2011
    West Central Indiana
    Chrome
    Crop Factor

    Meyerweb, thank you for taking the time for this lengthy explaination.

    Looked at another way, the crop factor of m43 compared to APS-C is only about 1.3x.

    This is exactly where I was trying to go with my question. Please correct me if I'm wrong. A 100mm APS-C lens on m4/3 will equal the FOV of a 130mm m4/3 lens.

    Also thank you for the insights regarding the DOF.

    -Chrome

     
  12. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Well...

    Not quite. Let's start by noting that 100mm is 100mm, whether used on m43, APS-C, or FF. A "100mm APS-C lens" has the same focal length as a 100mm FF lens. It's angle of view changes, because of the smaller sensor on APS-C compared to full frame (crop factor), but the actual focal length is 100mm, no matter what. So it doesn't matter what system this 100mm lens comes from, it will have the same FOV on an m43 camera whether it was original designed for m43, APS-C, FF, or even a 4x5 view camera.

    So let's look at the actual calculations. If you take FF (35mm) as the baseline, a 100mm lens on a FF camera has a crop factor of 1. A 100mm lens on APS-C has a crop factor of 1.5 or 1.6, depending on brand. So on an APS-C it has an FOV equal to (about) 150mm on FF. On m43, the crop factor is 2, so that same 100mm lens as an FOV of 200mm on FF.

    Note again, it's not because the lens was designed for FF vs. APS-C vs. m43, it's because as you make the sensor smaller, you're cropping just the central part of the image the lens produces, so you get a smaller FOV with the same lens.

    So what's the crop factor between m43 and APS-C? Divide 200 by 150: 1.3333.

    You can do the calculations between other formats as a test: Between FF and APS-C: Divide 150 by 100 = 1.5. Between FF and m43: Divide 200 by 100 = 2.0.

    Now, what does this mean? A 100mm lens mounted on m43 will have approximately the same field of view as a 130mm lens mounted on an APS-C camera. (But note, I'm not saying APS-C lens or m43 lens.) The sensor on an m43 camera is smaller than the sensor on an APS-C camera, so it crops a smaller FOV out of the image produced by the lens.

    Gee, I do ramble on, don't I? HTH.
     
  13. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    More realistically though, the image will look the same but with extra width added. :)