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  1. #1
    Dr Croubie's Avatar
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    Calibrating Apertures (on LF lenses)

    So I can't be the first person to ask this, but i can't find anything in the search (maybe I'm just searching for the wrong thing).

    Anyway, I have a few LF lenses, all bought second-hand, and I'm not sure if their real apertures are the same as the ones written on their scales.
    Specifically, I've got a 65/8 'Technika' Super Angulon in #00, a 90/8 'Technika' Super Angulon in 'Compur Electric 1' (#1), a Xenar 105/3.5 in #0, and a 180/5.6 Symmar-S in #1.
    The 180/5.6 is the most dodgy-looking, for a start the aperture scale on the shutter only goes down to 6.8, when the aperture is fully open it's off the scale, and there's also a sticker on it that says 'Rodenstock Quality Seal'. So it's definitely not the shutter it came in. All the shutters work (even the Electric one that I had to hack together a battery holder for), and the times all sound right, it's just the apertures need calibrating.

    So what's the best way to calibrate them? I'm just going to put a white sticker around the shutter and marking my own aperture-values on that, but it's how to take the light-readings to know where to put the marks that I'm wondering.

    What I've got available is a Gossen Digisix (it does both reflected and incident), I've got my Toyo-View, I've got any number of slrs with TTL metering (probably the most accurate would be the digital on live-view), and I've got a nice plain evenly-lit wall in my house.

    I've previously calibrated my Soviet MF TTL-Prisms by mounting a long-lens on my tripod, attaching the dslr via adapters and taking light readings, then attaching the MF-body and setting the prism accordingly. (sometimes I've used the digisix instead of dslr).

    But how can i do that with LF? I can get the reflected/incident reading on the wall easily enough, say I get something like iso100, 1/30s, f/8.
    But then how do I get the TTL reading through the LF lens to calibrate? Obviously I can get a film-holder-board with an EF mount on it, set camera to iso100 Av, and move the aperture until I get a 1/30s reading, ergo that must be where f/8 is (although it won't be focussed to infinity, and I'd prefer not to have to buy anything I wouldn't use after this).
    I was thinking the best way is to use the digisix somehow through the LF lens. Should I use it on incident or reflected (ie dome or not?). And then how do I know when I'm at the correct spot to get 1/30s at f/8? Won't the light being read be 'different' because it's already gone through an f/8 lens? Maybe I hold a grey-card at the film-plane and reflected-meter off that (whilst pointing at the wall that gave me the first reading of iso100, 1/30s, f/8)?
    Any other methods that can use the stuff i've got lying around? (I'd rather not take a bunch of test shots, I doubt my developing is anywhere near calibrated)

    (ps, please don't say Sinar Six, I can't afford one of them)
    An awful lot of electrons were terribly inconvenienced in the making of this post.

  2. #2

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    It's really not that complicated. fl/d. Focal length divided by aperture=f stop.
    The measurement of the aperture has to be done from and through the front objective.
    Heavily sedated for your protection.

  3. #3
    Dr Croubie's Avatar
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    Ah yeah, I should have said that I've also considered that.
    Except for the one problem of the apertures not being properly circular, especially the 65/8 SA having only 5-blades. I can calculate 'roughly' how big the aperture size needs to be, but mostly I'm going to be shooting Velvia with these so I can't afford to be 'roughly' correct to within a stop, I'd prefer to be within 1/6-stop or better, hence trying a TTL method...
    An awful lot of electrons were terribly inconvenienced in the making of this post.

  4. #4
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    This depends on what you want to have in the result: F-stops or T-stops. And how precise it should be. For F-stops you need to determine actual focal length and measure the diameter of the aperture as it is seen through the front element from infinite. A device called collimator might be useful, or a laser pointer mounted on a rail perpendicular to optical axis. Non ideal roundness of the aperture would be in most cases negligible, or you can measure for several times rotating your lens around its axis and calculate average.

    For T-stops you can use an incident light meter, or better a 'booster' for on-ground-glass metering (you have to calibrate it with your ground-glass first).

    For practical application it should be enough to:
    1) believe that maximum aperture stated by manufacturer is true
    2) assume F-stops are equal T-stops
    3) meter relative, so calibration with the ground glass is not needed, 1 EV (or whatever else) T-stops from maximum aperture
    || Cezary Żemis <cezary.zemis@pronet.pl> | www.cezaryzemis.name
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  5. #5

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    Or you could contact Bob Salomon over at LFPF ans see if he stll has any aperture scales for that lens in that shutter, I know had some. This would be the simplest way.



 

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