Lens focal lengths...

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  • 08-06-2010, 08:50 AM
    Lens focal lengths...
    Hi All,

    Ok, so here's my question: I know that in the 4/3 systems, you double the focal length to get the 35mm equivalent. Does that mean the f-stop doubles as well.

    Does a 150 f2 = a 300 f4 in 35mm? Or does is it a 300 f2?


  • 08-06-2010, 10:52 AM
    Re: Lens focal lengths...
    The crop factor effects focal length not the aperture. Aperture has to do with exposure so in those terms f2 is f2.

    That being said, a smaller sensor will affect the depth of field. Wide open will still give you the narrowest DOF and vice versa, but it will be different scale for a full format camera vs. a 4/3 sensor.

    I'm a rank amateur and it may be more complicated than this but I think my basic premise is correct.
  • 08-06-2010, 11:42 AM
    Re: Lens focal lengths...
    That's kind of what I thought, but wasn't sure.

  • 08-06-2010, 12:10 PM
    Re: Lens focal lengths...
    As I understand it, the focal length of the lens has not changed.
    All that happens in 4/3 and micro 4/3 is the sensor is smaller than 35mm.


    So if the lens focal length is 300mm, then the field of view with that sensor is equivalent to a 35mm sensor on double that focal length.

    But the focal length does not change.
    That's important because the f/stop is related to the lens aperture size and the focal length.
    N = f/D
    where f is the focal length, and D is the diameter of the entrance pupil.

    A 100 mm lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 135 mm lens with a setting of f/4 will have a pupil diameter of about 33.8 mm. The 135 mm lens' f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

    "It is important to note that neither the light gathering ability of the lens, nor any of its traits are actually affected: a 50mm f2 lens on a 4/3rds body has the same f-stop as a 50mm lens on a 35mm camera."
  • 08-06-2010, 02:32 PM
    Re: Lens focal lengths...
    As it's been explained to me, the focal length doesn't change, but since the sensor is smaller than the 35mm film, the effect of that length changes. I could be wrong. I've read the the 14-42mm that came with my camera, would be equivalent to a 28-84mm in 35mm.

    I guess the way I see it, the relative focal length doesn't really matter, my question was if the f-stop has a similar change as the focal length.

    This is hard to explain, hopefully you guys can understand what I'm getting at.

  • 08-06-2010, 03:03 PM
    Re: Lens focal lengths...
    F-stop measures intensity, not total volume and does not change based on the area of the sensor.

    IMHO, nothing changes except the size of the image on the sensor. If you took some tape and covered up another 3/4 of the sensor would that 150mm then become a 600?

  • 08-06-2010, 11:57 PM
    Re: Lens focal lengths...
    Simple answer: no

    Also the depth of field is too unchanged, a 150mm F2 lens is going to have the same depth of field at this setting whether the sensor is a full frame or APS-C or 4/3, just the field of view changes. So the lens will provide you the SPEED and equivalent field of view of a 300mm F2, but not the depth of field, the depth of field will still be the same as a 150mm F2 whichever camera it may be mounted it.
  • 08-07-2010, 04:24 AM
    Re: Lens focal lengths...
    Thanks for the answers, everyone. This is why I joined this forum, to learn about my camera from people who have had them for a while.