f5.6 is the same no matter what lens, right?

Discussion in 'General Technical Discussion' started by Steve S, Jul 17, 2005.

  1. Steve S

    Steve S

    Feb 1, 2005
    SE Florida
    I'm getting ready to do a portrait shoot in a bit here, and want to use 2 D2X cams, 1 with a 28-70 f2.8, and 1 with an 85 f1.4. Will these produce the exact same exposure values? Can you tell I have no film background or even a light meter? :roll:
  2. Hi Steve,

    They may or they may not produce the same exposures. Helpful, huh? Theoretically speaking, f/5.6 should produce identical results with both lenses....but in the real world lots of variables can intervene to contradict theory. For one thing, the 85/1.4 is known for slight overexposure (its speed being conservatively rated, many say).

    Good news is that you can easily "calibrate" your two lenses to one another by shooting an identical test shot with each, then checking the histograms on the two bodies, and adjusting EV compensation as required.

    Hope that helps.

    Let us know how the shoot works out. And post some samples!

    Best wishes,

  3. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  4. Steve S

    Steve S

    Feb 1, 2005
    SE Florida
    ok, then how bout this scenerio

    the 85 1.4 on one cam, the 50 1.4 on the other. Will they be the same exp then?
  5. Re: ok, then how bout this scenerio

    Hi Steve

    Theoretically yes. The same arguments apply as mentioned already. Each lens may be slightly different speeds and each camera sensor (if digital) may have different sensitivity. For example, I compared my D2H and D2X (before I sold the D2H), and there was a 1/6 EV difference, using the same lens in controlled lighting.
  6. nfoto

    nfoto Guest

    Light metering would be very difficult if a f-number differed between lenses. Also, the definition of the f-number ensures it is independent of focal length.

    That being said, there are slight differences in the transmission of many lenses so some tend to give a little darker images (typically, zooms) or lighter (primes), but the difference should be well within the tolerance of the aperture and aperture mechanism itself, which normally is less than 1/3 of a stop. With film this difference was inconsequential, with today's DSLRs with ultraprecise exposure, you can see it in some cases. My 85/1.4 lenses (both MF and AF) are spot on though.
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