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Is my understanding of equivalent focal lengths correct?

Discussion in 'Adapted Lenses' started by colbycheese, Jul 13, 2015.

  1. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    I think i have finally made sense of this stuff. So say we have a 50mm prime lens designed for a full frame dslr. When we use that lens with an adapter on micro four thirds, the sensor is half the size, thus we get double the focal length. a 50 will become an 100mm. And doing it the other way, if we put a 25mm lens design for m43 on a full frame dslr, it would have the focal length of 12.5mm. However when we talk about the equivalent focal length for a 25mm lens designed for m43 it is 50mm. Meaning to get the same field of view on a dslr, we would need a 50mm lens. And when i get into legacy lenses designed for half frame cameras, which is similar to the size of m43. Say i have the industar 69 which is 28mm and designed for half frame. To get that same field of view on full frame, we would need a 56mm lens. Since the size is similar to micro four thirds, does this mean we would have the same equivalent focal length. (i.e the 28mm industar would be 4 mm) on micro four thirds? Or am i still confused? I dont understand why the focal length would be doubled when the lens was designed for half frame. Anyways sorry for the wall of text but can someone tell me if i am on the right track
     
  2. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    NO...
    The FL NEVER changes. A 50mm FF lens on mu43 will give the same FoV as a native 50mm mu43 lens.


    ANY 25mm lens mounted on mu43 will give the same FoV as ANY 50mm mounted on full frame.

    If you don't have any FF cameras, then it's not worth worrying about.

    Barry
     
  3. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    alright, this makes more sense now.
     
  4. tkbslc

    tkbslc Mu-43 Hall of Famer

    Focal length is an actual physical property of the lens. It has nothing to do with sensor size. A 50mm lens is still 50mm even if it is sitting on your shelf.

    Now where it gets confusing is that different sensor sizes will show a different field of view with the same focal length. A 50mm is a short telephoto on m4/3, but a normal prime on FF. So people try to compensate for this by normalizing on the old 35mm standard (FF digital) and giving other systems focal lengths in "equivalent" terms for comparison. This does allow for easier comparison, but it also creates confusing language.

    The thing you have to remember is that the focal length DOES NOT CHANGE. It is always the same focal length. A 25mm on a 2x crop sensor is not a 50mm. It is a 25mm lens that is giving the view of a 50mm lens on FF due to the smaller 4/3 sensor. But the lens is still a 25mm. This means that if you adapt a 25mm for another system, it is still a 25mm lens, too. And it will give the FOV of your native 25mm, or any other 25mm you can fine (as long as the lens covers the sensor).

    There is no such thing as reverse crop factor. A 50mm lens on FF will not act like a 25mm on m4/3. It will be an actual 50mm.

    The reason that you still have the same "crop factor" with lenses designed around smaller sensors is that, again, the focal length is a physical lens property that has nothing to do with the sensor. The sensor size affects the FOV that lens will have, but the focal length is the focal length.
     
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  5. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    So in the case of the industar 69 designed for a half frame camera, would the field of view change that much?
     
  6. tyrphoto

    tyrphoto Mu-43 All-Pro

    May 25, 2014
    Seoul | NYC
    ㅇtㅈyㅅr
    It's a 28mm lens so on m43 it would be a 56mm equivalent.

    No matter what system the lens was designed for,, for m43 just double whatever focal length is stated on the lens.

     
  7. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Here's a quick image I made up that hopefully can help you understand why the same lens on different sensor sizes will give you a different field of view.

    Think of the light cones as the field of view that your sensor sees. Is it clear now why the smaller sensor has a "cropped" or narrower field of view?

    l84Zx5u.
     
    • Like Like x 1
  8. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    No, the FoV is not determined by what camera the lens was made for.

    FoV is only controlled by FL and the size of the sensor you're currently shooting on.

    Barry
     
  9. davidzvi

    davidzvi Mu-43 All-Pro

    Aug 12, 2012
    Outside Boston MA
    David
    Here might be the simplest picture I think I've ever found to explain crop / field of view / angle of view.

    I found it on Wikipedia here:
    https://en.wikipedia.org/wiki/Crop_factor

    Note the focal length doesn't change, just the height (or crop) of the sensor forming a narrower angle of view and resulting in a smaller field of view.

    LensCropFactor.

    Say the above is using the 50mm example. Now if you change the focal length to 25mm so the lens is closer to the focal plane the angle is sharper so you get the same field of view on the crop sensor image.
     
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  10. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    meaning the equivalent focal length on full frame is 56mm or the opposite, or am i still confused?
     
  11. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    does this mean when using a full frame lens on the m43 crop sensor, the edges won't be as unsharp as they would be on full frame?
     
  12. colbycheese

    colbycheese Mu-43 Veteran

    378
    May 1, 2012
    Way up there.
    okay. I guess it would be pretty close. I will figure out for sure once i get the lens.
     
  13. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    28mm shooting on mu43 gives same FoV as shooting 56mm on FF.

    Barry
     
  14. davidzvi

    davidzvi Mu-43 All-Pro

    Aug 12, 2012
    Outside Boston MA
    David
    Focal length never changes. It's the Angle of View generated by the sensor size that changes.
     
  15. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    Usually, but there are other factors such as the fact that mu43 is effectively magnifying the image, so apparent sharpness may decrease. Also, sensor stack size differences can cause distortion, esp. with older wide angle lenses.

    Barry
     
  16. davidzvi

    davidzvi Mu-43 All-Pro

    Aug 12, 2012
    Outside Boston MA
    David
    Yes. And it was one of the biggest things that needed to be overcome when the FX digitals came out. Using a full frame lens on a crop sensor results in an image that mainly cuts out the edges of the lens that are normally softer anyway.

    It's often referred to as using the sweet spot.
     
  17. dwig

    dwig Mu-43 Top Veteran

    622
    Jun 26, 2010
    Key West FL
    The phrase "equivalent focal length" really means "equivalent field of view as that of as nn focal length on 35mm full frame". It doesn't refer to any change in focal length. Focal lengths DO NOT CHANGE. It refers to a change in field of view. Stop thinking about "equivalent focal length" and start thinking about "equivalent field of view".
     
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  18. tkbslc

    tkbslc Mu-43 Hall of Famer

    I think this is a bit misleading in that a lens may or may not project an image larger than the "cropped" sensor.

    Why would he double the focal length if he is comparing to other lenses to use on his m4/3 body? Statements like this are what create the confusion that the OP is expressing. It's a 28mm lens, period. Just like any other 28mm lens. It will have a FOV like any other 28mm lens when used on his m4/3 body. No need to convert or double anything.
     
  19. tkbslc

    tkbslc Mu-43 Hall of Famer

    Relatively, yes.
    Absolutely, maybe not.

    FF lenses often have edges that are noticeably softer than the center of the frame. However a softer corner on FF may still be sharper than the "sharp" edge on m4/3, in absolute terms. Hypothetically, lets say a FF sensor captures twice the detail of a m4/3 sensor. Maybe an OK lens on FF captures 80% of that detail potential at the center and 60% at the edges. The edges would appear softer than the center and some might call it soft. Now take that m4/3 sensor that is starting with 50% of the detail of FF. Even if the lens captures 100% of the detail across the whole frame, it is still behind the FF shot in absolute terms. However, since the lens is perfect across the frame and capturing 100% of the detail possible with that sensor, we would call it insanely good, even perfect. Think of it like the fastest human racing a mediocre horse, maybe. Slow for a horse is even faster than a human can run. (again this was hypothetical, it's probably not those exact numbers. I made them up to illustrate a point)
     
  20. tyrphoto

    tyrphoto Mu-43 All-Pro

    May 25, 2014
    Seoul | NYC
    ㅇtㅈyㅅr
    For the same reason when I use my O25/1.8, it behaves like a 50mm lens in terms of FoV or 50mm equivalent focal length compared to full frame.

    In order to understand m43, or whatever other sensor size system one may use, one should understand the basic crop factor and how it affects the lenses attached to that camera.

    It's not a confusing concept to grasp, yet it's an important one to consider when you've chosen a crop sensor system.