50mm 1.4 / 1.8 negligeable exposure difference?

Discussion in 'Adapted Lenses' started by panglos, Feb 22, 2015.

  1. panglos

    panglos New to Mu-43

    Feb 22, 2015
    Hello all,

    I don't post often and I'm new on this forum and I need a little help. I got a good deal on a Lumix G5 for Christmas and I'm playing around with a few old MF lenses and adapters. Yesterday I figured out I'd compare two silver noses: Canon FD 50mm 1.4 against another oldie, the Olympus OM F.Zuiko 50mm 1.8.

    1. I attached the camera to a tripod with the Canon lens at F1.4, iso 800, focused on an indoor scene, and got a reading of 1/50th of a second.

    2. I then proceeded likewise, same scene, same iso, except I switched the Canon for the Olympus and set it to its minimum aperture of F1.8. Lighting conditions were exactly the same (indoor, no light from outside, ceiling lamp), and I got a reading of 1/40th.

    3. By curiosity I switched lenses again and slid the aperture ring on the Canon down to the next click (between 1.4 and 2, most certainly 1.8). To my surprise I got the same value as when set to 1.4: 1/50th!

    Conclusions / Questions:

    a - Wide open, the difference in light transmission between the Canon at 1.4 and Oly at 1.8 is only 20% (1/50 to 1/40), shouldn't I be expecting a full stop (eg a difference more like 1/80th to 1/40th)?

    b - 1.4 and 1.8 give me the same shutter speed on the Canon: why?! Is the M4/3 adapter acting like a baffle that's limiting light transmission at the widest aperture?

    c - Since the Canon at 1.8 gives me 1/50th and the Oly at 1.8 gives me 1/40th, could it be that my Oly is underperforming at 1.8 (it does have very light fungus on several elements)? This would explain conclusion a.

    By the way I compared test pictures and they all seem evenly exposed so I'm not questioning the light meter on the camera.

    All your observations are warmly welcome!

    Many thanks.
  2. HarryS

    HarryS Mu-43 Top Veteran

    Jun 23, 2012
    Midwest, USA
    First, I would make sure the Lock-Open ring on the FD adapter is turned to the right position. It can be positioned such that the blades barely close. I'd also look at the blades on the FD lens and verify they are wide open at f1.4 and close smoothly as you click down.
  3. tkbslc

    tkbslc Mu-43 Hall of Famer

    Metering methods on different cameras can cause minor differences.

    Also, lenses transmit light differently. Thats why video camera lenses use t-stop instead of f stop. F-stop measures size of aperture. T-stop measures actual light transmission. They are rarely the same
  4. panglos

    panglos New to Mu-43

    Feb 22, 2015
    Hello Harry and Tk, thanks for your quick answers. The FD lens adapter was turned to the right position during testing, clicking down from 1.4 to 1.8 I could see the blades slightly closing.

    At this point I understand that the difference in T-stops between the Canon and the Oly, both wide open, may be smaller than what the difference in F-stops indicates, hence my exposure results.

    But it still is a mystery to me why the Canon is giving me the same exposure values at 1.8 as it is at 1.4...
  5. tkbslc

    tkbslc Mu-43 Hall of Famer

    Have you compared photos to make sure it is not just a metering error?

    Fwiw, 1.4 to 1.7 is only half a stop. So that's 1/50 vs 1/37. I'm sure the camera rounded that to 1/40.
  6. panglos

    panglos New to Mu-43

    Feb 22, 2015
    I've compared photos and to the human eye they are strictly identical in terms of exposure. Today I played around with my Canon 50mm 1.4 under all sorts of situations and I can confirm that the lack of any exposure difference between 1.4 and the next click on the ring (got to be 1.8 or 1.7) is absolutely constant.

    ...Meaning that on m4/3 the Canon FD 50mm 1.4 is probably not worth getting over the cheaper FD 1.8 in terms of light gathering.

    PS: my Canon FD 50mm 1.4 is the kind without the "SSC" lettering and it's in mint condition.