None constant aperture zoom lenses

Discussion in '35mm Cameras and Accessories' started by benjiboy, Apr 30, 2010.

  1. benjiboy

    benjiboy Subscriber

    Messages:
    8,688
    Joined:
    Apr 18, 2005
    Location:
    U.K.
    Shooter:
    Multi Format
    I was just wondering if your using for example a f3.5-f4.5 none constant aperture zoom at what point does it change aperture during the zoom travel ? because this could be very significant if it is used with a hand held light meter or none TTL strobe, what I'm asking is, is the transition gradual throughout the zoom range ?.
     
  2. markbarendt

    markbarendt Subscriber

    Messages:
    7,718
    Joined:
    May 18, 2008
    Location:
    Beaverton, OR
    Shooter:
    Multi Format
    It is gradual on the lenses I have.

    On lenses of this type I tend to avoid using the wider/variable apertures. With the example you gave I'd treat it as a 4.5 and use A or M to keep it out of the variable range.
     
  3. Q.G.

    Q.G. Inactive

    Messages:
    5,682
    Joined:
    Jul 23, 2007
    Location:
    Netherlands
    Shooter:
    Medium Format
    It's gradual on all variable aperture zooms.

    Basically (though not exactly), the focal length changes while the pupil size remains the same. And given that the f-number is f(ocal length)/pupil size, any change in focal length, no matter how minute, will change the f-number.

    And that provides the clue to how to use such a lens with non-TTL metering: first calculate the pupil size by doing the sum focal length/f-number at that focal length (always given for both extremes, so doable), and jot that down somewhere handy.
    Then after changing focal length do the sum backwards again: f-number = focal length set/number calculated in first step.

    What you also could do is do the sums and calculate at what focal length setting you would need to add 1/3 or 1/5 stop, and mark those points on the focal length scale of the lens.

    Mind you: while close enough, it's not necessarily correct, nor exact. It depends on what happens inside that lens, and that varies with design.
    But it works.

    And another mind you: this changes with focus setting too. The closer you get, the more light you lose.
    And that can be a significant amount not just when doing macro. Especially not when using wildly asymmeric lenses, which zooms are.
     
    Last edited by a moderator: Apr 30, 2010
  4. benjiboy

    benjiboy Subscriber

    Messages:
    8,688
    Joined:
    Apr 18, 2005
    Location:
    U.K.
    Shooter:
    Multi Format
    Thanks guys , I suspected that was the case, I just have one variable aperture zoom a 35-70 my others are constant aperture.