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Thread: Aperture scale conversion

  1. #1

    Aperture scale conversion

    Quick question:
    I have a Copal 0 shutter that was used with a 150/5.6 lens. I want to replace the lens with a 120/5.6. What will my actual aperture be at any given stop on the older aperture scale?
    Follow up: where can I buy a scale to fit the 120?

    Thanks,
    Guy

  2. #2

    Join Date
    Apr 2000
    Posts
    711

    Aperture scale conversion

    1: it will be a quarter stop faster than indicated 2: skgrimes.com

  3. #3

    Aperture scale conversion

    Unless the lenses are of the same design, the correction to apply to the aperture scale is not exactly known because the lenses might have different pupil magnifications. The f-stop is defined by the apparent diameter of the aperture, as viewed through the lens, and not be the physical diameter of the aperture.

    If the lens is or was recently sold by the manufacturer mounted in a shutter, you could try contacting the manufacturer, or their distributer or authorized repair service, to see if they can supply an aperture scale as a replacement part.

  4. #4

    Join Date
    Oct 2003
    Location
    Middletown, NJ - Land of the Living Dead
    Posts
    191

    Aperture scale conversion

    If the lens is or was recently sold by the manufacturer mounted in a shutter, you could try contacting the manufacturer, or their distributer or authorized repair service, to see if they can supply an aperture scale as a replacement part.

    Unless it's a current lens, you may have to calibrate a new f/stop scale yourself. Don't worry, it's not an impossible task.

    Here's how I do it...

    You will need the following: Camera (duh!), lens with calibrated f/stop scale - mounted on board for camera, subject lens - also mounted on lens board, focusing cloth, and an incident light meter.

    Procedure:

    1) With the "known" lens mounted on the camera, focus the lens on infinity.2) Stop lens down to the largest full stop smaller than wide open. Examples: if the lens is f/5.6, stop down to f/8, if the lens is f/6.3, stop down to f/8, if the lens is f/4.5, stop down to f/5.6. Regardless, make sure that you stop down at least as far as the max speed of the uncalibrated lens.3) Aim the lens at a light colored, evenly lit surface - do not change the focus setting. (I use an interior wall where I can control the lighting)4) Remove the ground glass focus panel (or camera back) and place your incident light meter at the film plane position. Use your focusing cloth as a shield so no light besides what is coming through the lens enters the back of the camera. Make a note of the meter reading. (HINT: A nulling meter such as the Luna-Pro SBC or Luna-Pro F makes this whole procedure easy.)5) Put the uncalibrated lens on the camera, open the iris all the way, focus it on infinity, then without changing the focus setting, aim it at the same evenly lit surface that you used before.6) With the light meter placed at the film plane as in step 4, and with the meter protected from stray light, slowly close the iris until the meter reading matches the reading from the known lens. Make a mark on your f/stop scale (you've just found your first f/number).7) Close down the iris until the meter indicates 1 stop less exposure, then make another mark on the scale.8) Continue until you run out of iris!9) From the max whole f/stop marking, repeat this process opening the iris until the max speed is marked.

    I've done many lenses this way with excellent results. You can also use this method to find exact 1/2 stop or 1/3rd stop marks on your iris scales.

  5. #5

    Join Date
    Sep 2003
    Location
    God's Country
    Posts
    2,080

    Aperture scale conversion

    Hi Guy,

    I've just gone through this very exercise with my 360 Symmar F6.8. I looked for the appropriate scale for my Copal 3 shutter but couldn't locate one. A brand new scale from Schneider was going to cost CDN$128.00 plus 14% in tax... and it has to be ordered from abroad!

    S.K. Grimes engraved the scale directly onto the shutter for a total of US$79 which included the cost for shipping. I'm expecting it back within the next 4 days and have been told by others that they do a terrific job of it. I'll post a commentary on it when I get it back into my mitts!

    I'm sure it'll be even less expensive for you down in the US.

    Cheers
    Life in the fast lane!

  6. #6

    Join Date
    Jan 2004
    Location
    Halifax, Nova Scotia
    Posts
    444

    Aperture scale conversion

    I am about to try this for a 150mm G Claron going into a 105mm Tominon's shutter and this is the idea I had:

    With a pair of calipers measure the diameter of the openning in the barrel at a known aperture.

    Set the shutter to the same diameter and mark where the pointer is.

    Repeat for other aperture settings.

    That will only work for you if you have the original shutter the 120mm lens came in or if it came out of a barrel. Can anyone see any flaws in my logic?

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