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In techincal terms. Tell me about crop factor.

Discussion in 'Adapted Lenses' started by ke7dbx, Oct 7, 2011.

  1. ke7dbx

    ke7dbx Mu-43 Regular

    76
    May 11, 2011
    An someone ether explain to me or point me to a good website. In technical terms, how crop factor works. Not the simplified version. Thanks!
     
  2. blue

    blue Mu-43 Veteran

    280
    Jun 1, 2010
    UK
    I'm not really sure what you're asking - because the "technical" explanation is very simple. There isn't some magical rocket science brain surgery version.

    Take a picture on a 35mm sensor then cut a piece out of it. This is your smaller sensor. Take the image on the smaller sensor and blow it up to the same area as 35mm - the subjects in your "blown up" image will look bigger. The end.
     
  3. alessandro

    alessandro Mu-43 Regular

    153
    Jan 29, 2010
    From a "non-technical" guy:
    "Crop factor" is not really technical stuff, it's a relative concept.
    To define a crop factor, you need to define a 1:1 model.
    People have chosen to say 24*36mm (the old 35mm film) is the 1:1 model.
    This has only to do with the fact that the diffusion of that format has given the habit to recognize the field of view (FOV) based on that format, where 35-50mm was a "normal" lens.
    Keep in mind, for example, that a 35-50mm lens on a 6*6cm (rollei) is a wide angle lens.
    Speaking of "crop factor" has the only meaning of giving the idea of the kind of lens/FOV you are using on a different sensor format.
    The "crop factor" of a micro 4/3, "2", means that using a 25mm on it will give a similar FOV to what you get with a 50mm on a 24*36 sensor (or film).

    The only thing that really changes is the depth of field, that is longer in shorter lenses.
    A "normal" lens on micro 4/3 (17-25mm) will have the same FOV of a normal lens (35-50mm) on 35mm, or (more or less) 60-80mm on 6*6.
    But the DOF will be deeper for the shorter.

    In conclusion, speaking of crop factor is the same as saying "equivalent to", where the chosen standard is 35mm film/sensor. Just because we are (were?) accustomed to that format.
     
  4. FetterSteve

    FetterSteve New to Mu-43

    7
    Oct 10, 2011
    Another newbie question here following on from this explanation.

    Why does the Crop factor of 2 on my camera mean that my 14mm lens is a 28mm equivalent?
     
  5. RobWatson

    RobWatson Mu-43 Hall of Famer

    Would'nt need crop factor if lenses were marked for angular field of view instead of focal length - except for those silly people putting lenses designed with image circles that don't particularly match their cameras.
     
  6. Christopher

    Christopher Mu-43 Regular

    72
    Oct 8, 2011
    Toronto
    what's 14x2=?

    it's referring to the crop factor compared to a full frame camera aka a 35mm camera. the smaller sensor magnifies
     
  7. WT21

    WT21 Mu-43 Hall of Famer

    Feb 19, 2010
    Boston
    Do you have a protractor? Put it in front of your face with the middle pointing forwards, and think in terms of "angle of view"

    A 50mm lens on a 35mm full frame camera will cover a 39.6 degree arc of view in front of you. So, about 20 degrees to the left of straight ahead, and 20 degrees to the right. Angle of view - Wikipedia, the free encyclopediaIf you don't have a protractor, and you failed geometery, then think of it this way Straight ahead of you is zero degrees. To your right (off your right shoulder) is 90 degrees. 20 degrees is 1/4 between straight ahead and to your right.

    That same lens on an m43 body with a "2X crop factor" that has a smaller sensor (twice as small, which is where the 2X crop factor comes from), only uses 1/2 the angle of view, or about 10 degrees to the left of center and 10 degrees to the right. Very similar to if you had mounted a 100mm lens on a full frame camera. This is all assuming 3:2 aspect ratio. Differing aspect ratios impact these calculations a little differently, but it's close enough for horseshoes and hand grenades.

    So, some (myself included) often say "a 50mm lens on a 2X crop body is the equivalent angle of view as a 100mm on a full frame." That's it. The portion of the view is narrowed, because some of the image circle is cut away because the sensor is smaller. In relating m43 to Full Frame 35mm, it's quite easy, because it's a 2X factor. APS-C relations are harder to calculate, because they are 1.6X (Canon) or 1.5 (Sony and Nikon).

    You do yourself a great service if you learn the angles of view for each of your lenses (assuming you are good at estimate degrees of arc in front of you).

    See also Crop factor - Wikipedia, the free encyclopedia.

    Don't forget to ask about APERTURE equivalence. Now there's a fun one!
     
    • Like Like x 1
  8. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    The "crop factor" is an idea that has out lived its time. Especially since most people don't even shoot 35mm anymore. I would just learn what focal length means on your camera rather than converting it to an equivalent angle of view to a diffferent format.

    Basically, crop factor gives you the equivalent angle of view of your lens to the equivalent focal legth for a 35mm camera.
     
  9. Hyubie

    Hyubie Unique like everyone else

    Oct 15, 2010
    Massachusetts
    Herbert
    I would not disagree on the concept being somewhat obsolete only because 35mm is not very common anymore, but I think it has its uses. At the top of my head, one use I can think of is: I have a DSLR and an m43 camera. Sometimes it is useful to know that a 50mm lens on my 1.6-crop-factor DSLR has the same FOV as a 40mm on my m43. Just my 2 cents'.

    As to what WT21 mentioned about Aperture equivalence, I tried but I think that is honestly beyond my brain's reach, and I have no plans of learning about it at all. I prefer to use that time to reading a good book. Or taking pictures. :smile:
     
  10. oldracer

    oldracer Mu-43 All-Pro

    Oct 1, 2010
    USA
    Smaller sensors subtend smaller angles, hence the resulting image has a narrower field of view than one from a larger sensor.
    Consequently, what all these other guys have said.
     
  11. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
     
  12. I think that 35mm equivalence is still very useful given that including compacts, m4/3, APS-C (Canon vs the rest), Sigma Foveon, APS-H, full-frame, etc there are probably at least 10 different sensor mediums. Compacts are particulay prolific with slightly different sizes. I have no idea what a 6.3mm lens on a 1/1.7" sensor gives me for example, but if the camera says it has a 28-300mm equiv. lens then I know what to expect.

    The Ricoh GXR is a good example of where equivalent focal length is important, since you can mount 3 different size sensors (lensors) on the one camera.
     
  13. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    On the contrary, APS-C is 3:2 aspect like 35mm, so I think it's a LOT easier to calculate. A 1.5x or 1.6x crop factor with APS-C will tell you exactly what the difference in crop of the image will look like. It's literally 50% smaller or 60% smaller, in the same dimensions. It's no wider or taller, just smaller. The 2x crop factor of Four-Thirds is nothing but misleading if you ask me...
     
  14. I know, the sooner m4/3 dumps the 4:3 ratio and switches to 3:2 the better :biggrin:
     
  15. yottavirus

    yottavirus Mu-43 Regular

    115
    Aug 13, 2011
    The FOV of u43 is closer to 1.9 than 2 because of the extra height.
     
  16. GaryAyala

    GaryAyala Mu-43 Legend

    Jan 2, 2011
    SoCal
    I agree that Hikari fella, (sadly I must admit). It is a concept which is generally useful only to those who grew up with 35mm camera in their hands. For me, by using crop values, it is very easy to calculate and visualize what a lens delivers in focal length regardless of sensor size.

    Gary
     
  17. WT21

    WT21 Mu-43 Hall of Famer

    Feb 19, 2010
    Boston
    I think I ALSO said

    "That same lens on an m43 body with a "2X crop factor" that has a smaller sensor (twice as small, which is where the 2X crop factor comes from), only uses 1/2 the angle of view...This is all assuming 3:2 aspect ratio. Differing aspect ratios impact these calculations a little differently, but it's close enough for horseshoes and hand grenades. "

    It is really close enough for rough field work, at least for me. 39 degree field of view vs. a 43 degree field of view -- I wouldn't be able to really tell the difference until I look through the viewfinder (or at the LCD).
     
  18. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    Actually, I think a more accurate way to describe it would be to say a 1.7x crop, which is then cropped shorter on the sides. The perspective of the image looks almost exactly the same as shot on a Canon APS-C body, but wth the sides trimmed off.

    Visual example:
    Canon APS-C (1.6x crop):
    sensor-size_aps-c.
    Four-Thirds (2x crop):
    sensor-size_four-thirds.
     
  19. Sammyboy

    Sammyboy m43 Pro

    Oct 26, 2010
    Steeler Country
    :biggrin: Ned, you should know better than to use that bee as an example. How long do think it's going to take before someone questions it's size; if it's life size (1:1) on an APS-C format, then what size would it be on a m43 using the crop factor of 2. :biggrin: :biggrin: :biggrin:
     
  20. Ned

    Ned Mu-43 Legend

    Jul 18, 2010
    Alberta, Canada
    Heh, you're absolutely right! That cold open a can of worms...