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Aperture vs sensor size relationship question

Discussion in 'Adapted Lenses' started by Phoque, Mar 4, 2012.

  1. Phoque

    Phoque Mu-43 Veteran

    Sep 18, 2011
    Ontario, Canada
    I think I have read somewhere a native lens aperture on a full frame sensor is equivalent to a one stop slower aperture native lens on a m43 sensor.

    If that is right, does that mean that using an f-4 full frame lens with an adapter on a m43 body would be equivalent to using a native f-2 lens? Would the narrower depth of field apply too?
  2. foto2021

    foto2021 Mu-43 Veteran

    Nov 5, 2011
    SE England

    That's wrong, unfortunately. In terms of light gathering, there is no difference between an f/2 native lens and an f/2 legacy lens with an adapter. They both deliver the same illumination of the sensor at f/2, or indeed at any other aperture.

    In terms of depth of field, if you want the same depth of field on m4/3 as on a full frame sensor, you need a lens aperture that is two stops wider for a lens of the same angle of view. If comparing full frame and m4/3, the same angle of view would demand a lens of half the focal length. So to get the same angle of view and the same depth of field of a 50mm lens at f/4 on full frame, you would need a 25mm lens at f/2 on m4/3.

    Complicated, isn't it? But you'll get used to it.

    May I point out that the difference between f/4 and f/2 is two stops, not one? A one stop difference would be f/2.8.
    • Like Like x 1
  3. Promit

    Promit Mu-43 All-Pro

    Jun 6, 2011
    Baltimore, MD
    Promit Roy
    Aperture equivalence is tricky. Aperture controls two different things: depth of field, and amount of light.
    * Amount of light: f/2 is f/2, no matter the format. There's no question of equivalence.
    * Depth of field: The DOF is a function of aperture and physical focal length. Not format. The DOF of the 45/1.8 is exactly the same as a 45/1.8 would be on a full frame.

    So usually what people are talking about with aperture equivalence is the equivalent depth of field at an equivalent angle of view. The 45/1.8 has an equivalent full frame angle of view to a 90mm, and the depth of field in that case is equivalent to a 90mm/3.6. Some idiots try to complain that all m4/3 lenses are horribly slow because of this, but that's only if you want insanely shallow depth of field -- you often don't. In fact, one of the cool things about a smaller format is you can get fast lenses without such shallow DOF.
    • Like Like x 2
  4. Phoque

    Phoque Mu-43 Veteran

    Sep 18, 2011
    Ontario, Canada
    Hmmm, I just checked the theory on Wikipedia and DOF calculations are not cheap math!
  5. usayit

    usayit Mu-43 Hall of Famer

    Are you talking about a thread that went on and on and on on the TPF?

    Its a bit confusing and its no surprise that your conclusion is incorrect (the thread is confusing because much of the miscommunication all mixed up within the thread). Basically, it is a comparison of two different sized sensors where the entrance pupil of the lenses and the field of view are kept constant. Also, note.... that your post can lead to a confusion between Exposure value (EV) and Light Value (LV). If this is the source you read, you can pretty much ignore it.... there is some validity but much of it doesn't translate to anything practical in the real world sense.

    If not the source... move on...
  6. Phoque

    Phoque Mu-43 Veteran

    Sep 18, 2011
    Ontario, Canada
    Actually, all I remember was that I read something about depth of field on m4/3 vs other camera systems from a reliable source. So what I've written to start this thread comes from me badly expressing my thoughts on something which I obviously don't understand well.

    I guess my thread title should have read "Depth of field vs F-stop vs camera system question".
  7. Hikari

    Hikari Mu-43 All-Pro

    Nov 26, 2010
    Given the same focal length, DoF will be less at a given aperture with m4/3 compared with 35mm

    Given the same angle of view, DoF will be greater at a given aperture compared with 35mm

    The difference in DoF is proportional to the crop factor.
    • Like Like x 2
  8. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    What he said ^^^^^^

    Understanding the relationships between aperture, focal length, viewing distance, viewing size and sensor size are only complicated because of the false information that permeates the web. It's actually reasonably simple to understand the basic concepts. Large and medium format shooters have been using these principles for several decades. The principles haven't changed.

    • Like Like x 1
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