Are a LF shutter markings more accurate wide open or stopped down?

Discussion in 'Camera Building, Repairs & Modification' started by BetterSense, May 4, 2010.

  1. BetterSense

    BetterSense Member

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    Odd question. I have a 210/360 Symmar convertible in a Linhof shutter. The shutter works and the lens is pristine, but the black, ring-shaped cover that goes on the front of the shutter, and that has the aperture scale engravings, has been ripped off at some point. I'm contemplating gluing it back on with rubber cement or something.

    The trick is figuring out how to rotate it so that the aperture scales line up accurately with the aperture lever. The trouble is that the aperture lever swings farther than the full scale of the aperture engravings. I can position the ring so that it reads f/5.6 with the aperture blades fully open, or I can set it so that it reads f/45 with the aperture blades fully closed. But not both.

    Which do you suppose it is, or could it be somewhere in the middle?

    Would this lens fit in a different shutter?
     
  2. 2F/2F

    2F/2F Member

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    I would open up the diaphragm all the way, and push the aperture control lever as far as it will go past that. Then, slowly move the lever back the other way, as if you were stopping down. Stop when the diaphragm actually begins to close. Back up a teeny bit, until it is just wide open again, and then align the wide-open f stop mark on the aperture scale (Is it f/5.6?) with the needle on the aperture control lever. (Then test some film to see if you got it right! :wink:)

    You want the diaphragm to begin closing down if the lever is moved past the wide open mark any amount. In other words, you do not want to calibrate the scale to your minimum aperture (f/45), but to your maximum aperture (f/wide open). This is because most diaphragms close down to a little bit smaller an aperture than you get when the lever is set to the last marked f stop of the scale.
     
    Last edited by a moderator: May 4, 2010
  3. BetterSense

    BetterSense Member

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    I was leaning toward doing that, since I can't figure the lens would be spec'd as a f/5.6 lens unless it was exactly f/5.6 when exactly wide open.

    Now I have to figure out how I'm gonna glue it on, since it holds the rest of the shutter together actually and I can't glue it to what's underneath, I have to glue it to where it contacts the barrel of the shutter around the circumference of its hole; there's not much area there.
     
  4. epatsellis

    epatsellis Member

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    there's supposed to be a retainer to hold it on, if you look carefully, there will be two holes that when the cover is pushed down, index the position of the cover. You need to find a retainer to hold it in place.
     
  5. richard ide

    richard ide Subscriber

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    Shellac is sometimes used for cementing parts together. You just heat it and apply a small blob or three with a toothpick. It will fracture easily if removal is desired. A jeweller might give you a small bit rather than trying to buy it. If you match the cement residue on the shutter and ring, that will give you the exact positioning otherwise the suggestion from 2F/2F will work.
     
  6. BetterSense

    BetterSense Member

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    Well, from looking at it, I can see that the inside bore of the cover has three 'bumps' left in it. And the outer surface of the round bit it slides onto has three matching notches in it. It won't fit on unless it's rotated so the bumps match the notches. So you are supposed to line the bumps up with the notches, push the cover on, and turn it a few degrees clockwise, and the bumps slide into milled notches, locking the cover on. A very common scenario. The trouble is the notches that the bumps normally slide into have been brutally torn out so that after you put the cover on and turn it, the bumps have nothing to slide into and the cover just falls off.

    I have determined the angle the cover is supposed to be put on...there is only one angle here the shutter speed tick-mark lines up with the correct shutter speed...and oddly, it's a middle angle such that the diaphrams are neither totally open at f/5.6 nor fully closed at f/45. Go figure.

    I tested all the speeds and they all work consistently, although some are off from the marked speed. I will probably glue the cover on with green Loctite.
     
  7. fschifano

    fschifano Member

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    Too easy. The f number is nothing more that a ratio. Measure the diameter of the aperture and divide that into the focal length of the lens to get the f number. Or take the f number and calculate the diameter of the aperture.

    So, 210 / 5.6 = 37.5. When the aperture is set to have a diameter of 37.5 mm, the f stop is 5.6. Set it there, line up the pointer with 5.6 on the scale and you're done. When you have the lens set up to be a 360 mm optic, that same aperture will get you an f number of 9.6.

    Oh yeah, measure the aperture from the front, not the back. If you can remove the front cells to measure it, even better. A tolerance of +/- .5 mm is good enough for a lens with a focal length of 210 mm or greater.
     
  8. epatsellis

    epatsellis Member

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    Minimum aperture is when the aperture blades just eclipses the lens. The aperture is the ratio of the entrance pupil to focal length, there are numerous posts outlining this here and on LFPF.
     
  9. BetterSense

    BetterSense Member

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    Removing the front cells is no problem considering it's a convertible lens and the front cell is designed to come off for the conversion. When I get my loctite I will measure the aperture diameters with calipers for curiosity's sake, but I will probably still put the cover on in the position that lines it up with the shutter speeds.
     
  10. BetterSense

    BetterSense Member

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    Now I'm thoroughly confused. Ignoring the aperture lever and markings, I just measured the size of the aperture hole with the front element off. Using a dial caliper to measure the diameter of the aperture, I get 29mm fully open--that's 210mm/29mmm =7.25! That's quite a bit smaller than f/5.6 eh? And at this fully open position, the aperture lever indicates a larger aperture than f/5.6 as it's off the scale there.

    The aperture closes down to about 2.2mm or about f/95.

    I can make a custom aperture scale with tape and pencil, but am I calculating this wrong? The max diameter of the rear cell even taken out of the shutter is 29mm. Yet the front element says "1:5,6/210 1:12/370".

    370/12 equals about 30mm but 210/5.6 almost 38mm. The rear element says "12/370". How could it be off by that much?
     
  11. fschifano

    fschifano Member

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  12. jnanian

    jnanian Advertiser

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    frank is spot on ...

    btw its a 210/370 ...

    ( have fun )
    john
     
    Last edited by a moderator: May 5, 2010
  13. epatsellis

    epatsellis Member

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    You need to measure the entrance pupil, the apparent size of the aperture through the front element, I hold the lens at arms length and measure with plastic calipers.

    The f.l. is easy to measure, focus at infinity, mark the bed/rail. Focus at 1:1 (use a ruler and measure the image on the g.g.), mark the rail/bed. Subtract the second number from the first and you have your f.l.

    Aperture is simply f.l. divided by entrance pupil diameter (expressed in the same units, of course)
     
  14. fschifano

    fschifano Member

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    Not that much really. The difference is approximately 2/3 stop. Don't use a dial caliper. A steel ruler marked of in millimeters is good enough and easier to get right. Measure the apparent diameter of the aperture through the entrance pupil. If the hole isn't circular measure from corner to corner and side to side and average the two. It's close enough.
     
  15. John Koehrer

    John Koehrer Subscriber

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    So measure apparent rather than actual diameter?
     
  16. fschifano

    fschifano Member

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    That's what you do.
     
  17. Q.G.

    Q.G. Inactive

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    means that you shouldn't