Measuring and marking aperture

Discussion in 'Camera Building, Repairs & Modification' started by amuderick, Apr 7, 2008.

  1. amuderick

    amuderick Member

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    I am looking for a reliable way to measure the aperture marks on my lens with the eventual goal of adding f/32 to an aperture whose markings end at f/22.

    I tried placing the camera at a fixed distance from a bright 500W light. I put my Sekonic light meter at the film plane and opened the shutter on bulb to take a reading.

    The results weren't quite consistent given the apertures as marked so I'm a little hesitant to use the readings to mark the f/32 point. Is there a better way to do this?

    Perhaps the light fall-off from vignetting is affecting the reading? The light meter sensor isn't quite in the center of the film plane.

    Thanks!
     
  2. David William White

    David William White Member

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    I would first use a light meter or a micrometer for approx. 0.7 diameter of f/22 and mark the barrel. Then confirm with a contact strip test: Set f/22 and do frames for 1/30, 1/60, 1/125, etc. against out-of-focus grey card. Then repeat for your proposed f/32 starting with 1/15, etc. Compare and adjust/repeat. Proof is in the pudding.
     
  3. Maris

    Maris Member

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    Measuring the aperture on shutters or lens barrels gets darn difficult for the smaller stops. The problem is even worse for aperture mechanisms that have a non linear scale; the ones where f8 and f11 are a long way apart but f32, f45, and f64 are all crowded together. But there is a way.

    I found that I can open the negative stage part of my Omega D2 enlarger enough to get most shutters and irises in. Then I crank up the enlarger head to 10x magnification and measure the projected image of the iris. The big magnification factor makes for big increase in precision. Even a 1 millimetre aperture can be measured to +/-5%.
     
  4. David William White

    David William White Member

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    Damn clever. Follow with a strip test and you're done.
     
  5. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    If you can mount the lens in your enlarger then the Darkroom Automation enlarging meter can read out the f-stops to 0.01 stop. URL below.

    You can also try mounting the lens in an enlarger and use a modern digital light meter that reads out in 1/10ths of a stop at the easel - that should allow you to get the aperture to 1/5th of a stop which is good enough for film work. Meters that read in 1/3's of a stop don't have enough resolution.

    This method produces 'T-Stops', but since you are only interested in where is one stop down from f-22 the difference between f and T at small stops will be negligible except in the case of uncoated lenses. In any case the error will be in your favor.
     
  6. Jim Noel

    Jim Noel Member

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    David,
    Your method assumes that the factory marks f-stops are accurate, and that is not always the case. I have tried it with several lenses in the past and found some to be off as much as 1/6 stop.
    Maris plan is likely the easiest,fastest and most accurate for those of us who do not have specialized equipment.
     
  7. David William White

    David William White Member

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    Did you mean Nicholas?
     
  8. Curt

    Curt Subscriber

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    This sounds like a great way to do it, can you tell us what you use to measure with?

    Curt
     
  9. amuderick

    amuderick Member

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    What I ended up doing was positioning my Canon SLR at the film plane with the door open and the shutter on bulb. I shone a bright light into the defocused lens from some distance away. I checked each setting and each metered as 1/2 of the previous one. Since I can easily measure the fully open aperture (which is thus correct), I can show they are all pretty close to accurate.

    Is anything wrong with this quick and dirty measurement method?
     
  10. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    This is a good way to measure pinholes and also to check the hole is round and doesn't have any burrs.
     
  11. edz

    edz Member

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    This is EXACTLY the way to go. For exposure you want T-stops and the advice given here is useful for ANY lens. I would suggest that one performs this with all objectives for exposure consistency. F-Stops is a geometric measurement useful for depth-of-field but not for exposure (other than as a rough guide).

    Two objectives of the same true focal length (we need to keep in mind that the focal lengths reported on the barrels of most objectives is not their real but only their nominal lengths) and set to the same f-Stop won't probably have identical transmission of light. The number of lens elements, coating etc. all have a significant impact on transmission. F-Stops can even be quite misleading. A typical f/0.95 superspeed, for instance, might have a geometric aperature greater than the focal length and whence an F-Stop less than 1 but can't, as we all know, violate the basic laws of physics and pass more light out then going in.. In still photography given the relatively low dynamic range of photographic papers we safely ignore this but the effect is not insignificant w.r.t. the dynamic range of film so in cinema and diapositive processes we tend to talk about T-Stops (Transmission Stops) for lighting and F-Stops for Depth-of-Field. Good cinema objectives are for this reason calibrated to T-Stops.

    One does not, of course, need the meter above but nearly any baseboard enlarging meter is sufficient.

    With the enlarger you can cheaply (as one would say "Good enough for government work") calibrate both the F-stops and T-stops!