Crop factor and perspectives question

Discussion in 'Native Lenses' started by agentlossing, Aug 18, 2013.

  1. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    I've wondered about this for awhile: say you are using a 100mm lens for a portrait on a full-frame sensor. You have a nice perspective flattening effect that compliments a person's face and makes the portrait look especially pleasing.

    Now suppose you take a 50mm lens and stock it on a micro four thirds camera. The focal length is the same due to crop factor; 100mm. But is the perspective flattening roughly equivalent to that of the 100mm full-frame lens, or is it the same as it would be on a 50mm lens on full-frame? In other words, does sensor crop factor contribute at all to perspective flattening, or do the lens characteristics stay the same based on optical focal length regardless of the camera's crop factor or lack thereof?
     
  2. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    They stay the same as you take the photo from the same position. It's the positioning of the camera that alters the perspective.
     
  3. Mattr

    Mattr Mu-43 Regular

    62
    Aug 5, 2013
    Melbourne, Australia
    Matt Robinson
    Correct, the FL only changes the angle of view or how close you need to be in order to fill the frame with the subject. Perspective can only change with a change in relative distance between observer, subject and background. So, keep all those factors the same, the perspective will stay the same. At least, that's how I was taught it back in the good old City & Guilds days!

    Sent from my GT-I9100 using Mu-43 mobile app
     
  4. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    Yeah, but if you're taking a portrait of a person and it's say a headshot or something... then obviously you will back up further to fill the frame the same way when using a camera with a crop factor... thus, the perspective is more flattering than using that same lens on a full frame, or the same as using a longer lens on the full frame. I think that's what the OP is getting at, and he is absolutely correct about it. The whole idea of using a longer focal length is to increase your distance from the subject. If that were not the case, then a 100mm lens wouldn't look flattering on Full Frame either. Switching from a 50mm lens to a 100mm on the same camera without changing your distance would result in nothing more than a big distorted nose, lol. You need to back up to make it a flattering perspective - that is an assumed part of the process.

    However, I wouldn't use a 50mm lens on m4/3 to get the same perspective as a 100mm lens on full frame, I would use a 70mm lens. Remember that the aspect ratio (4:3) of a 4/3 or m4/3 camera is very different from the aspect ratio (3:2) of a full frame camera, and thus the "crop factor" idea is NOT accurate, like it is on APS-C which shares the same 3:2 aspect. The perspective of a 70mm lens on a 4/3 sensor is much closer to 100mm on full frame, but squarer. That's difficult to explain without pictures, but let me know if you want and I'll give you a visual demonstration.
     
  5. dwig

    dwig Mu-43 Top Veteran

    624
    Jun 26, 2010
    Key West FL
    It's true that the crop factor isn't "very accurate" but your statement about substituting a 70mm lens is poor since you don't specify which image is being cropped to match the other's aspect ratio. The "error" in the crop factor is one way when the 35mmFF image is cropped to match the m43 shape and the other way when its the m43 image that cropped to match the 35mmFF image. The normally stated crop factor is based on matching the diagonal angle of view between m43 and 35mmFF.

    @OP: You should note two things:
    1. The focal length NEVER CHANGES, period. A 50mm lens used on an m43 camera is still a 50mm lens. It's the field of view (FOV) than changes.
    2. Focal length DOES NOT affect perspective, at least directly. Shooting distance is the only factor. FL only has an influence in that it causes you to shoot from a particular distance to get a particular framing.
     
  6. dwig

    dwig Mu-43 Top Veteran

    624
    Jun 26, 2010
    Key West FL
    This would happen only if you also moved closer to match the framing seen with the 100mm. If you simply change lenses, without changing distance, there would be absolutely no change in perspective.
     
  7. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    Thanks guys! That answered my initial question to a tee. I don't know how I missed the fact that distance from subject is the only thing that alters perspective, but now it all clicks, since it also explains why, say, an environmental portrait from my 14mm doesn't do anything uglifying to a person's face (so long as they're close enough to the center of the frame to avoid distortion), while a close portrait framing the face exclusively would, as has been noted, give them a nose enlargement.

    I knew this happened thus, but it's nice to know the mechanics behind it.
     
  8. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    No, that's wrong. A 100mm lens on FF and a 50mm lens on m43 will give you exactly the same FOV when shot from the same position. No need to back up at all. Assuming that the subject image will fit within the frame of both formats.

    This depends more on the aspect ratio of the final output rather than the sensor. If both are going to be used to make an 8x10 print, for example, the sensor format doesn't really matter.
     
  9. agentlossing

    agentlossing Mu-43 Hall of Famer

    Jun 26, 2013
    Andrew Lossing
    I think the difference in sensor aspect ratio is simple enough to grasp; you just frame differently based on whether you have extra real estate, either horizontally or vertically. Obviously that makes a slight difference in effective focal lengths, but that depends heavily on framing, you might just as well not need that extra horizontal real estate, resulting in not stepping further back, point moot. Uh, right? Am i on the same wavelength?
     
  10. pdk42

    pdk42 One of the "Eh?" team

    Jan 11, 2013
    Leamington Spa, UK
    You know, it's amazing how many people just don't get this whole issue of perspective and its relationship to distance to subject. I was idly looking at some photography books in the library the other day and just had to take a look at "Digital Photography for the Over 50s in Simple Steps" - if only for the amusement factor. I wouldn't recommend that anyone hanging around these forums looks at it (over 50 or not) - it's a whole bunch of condescending twaddle. I don't know the author's credentials (a certain Marc Campbell), but I found this howler:

    [​IMG]

    (If you must, you'll find the preview at: http://www.amazon.co.uk/Digital-Photography-Over-Simple-Steps/dp/0273761064)

    So, getting closer achieves "the very same result" as staying put and increasing the focal length???? Try taking a shot from 6 inches with an UWA and then another from 20 ft with a long tele and see if the result is "the very same". Incredible...
     
  11. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Well, not exactly the same, but you'd have a hard time telling the difference. :tongue: :biggrin: