Lens confusion--calculating aperture

[BetterSense]

I have a Symmar 210/370 convertible lens in a Linhof shutter. The front element says "1:5,6/210 1:12/370". The shutter works, but the black cover that has the aperture markings on it is broken and rotates, so the aperture scale is not correct. So I was trying to figure out the actual apertures of the lens so that I can glue the cover back on in the correct orientation.

The maximum diameter of the aperture when wide open is about 29mm. I measured this using calipers with the front cell removed. However 210mm/29mm is f/7.3 or so. That's a lot different that f/5.6 that is marked on the front element. Even with the rear element unscrewed I can measure the maximum aperture of the rear element as about 30mm. But f/5.6 should be 210/5.6=37.5mm. What's going on here? How could the max aperture be that far off?

Incidentally 30mm jives well with 310mm/12. But definitely not with 210mm/5.6. What am I to do?

[ic-racer]

With the front element off it is a 370mm f12, your measurement of 29mm is close enough to the calculated 30mm so that is OK.
For the 210mm lens configuration, measuring the aperture size is more difficult, because you need to don't want to scratch the lens elements as you eyeball the aperture through the glass. I have cast the shadow of the calipers onto the aperture diaphragm to get the size.

[aduncanson]

Incidentally 30mm jives well with 310mm/12. But definitely not with 210mm/5.6. What am I to do?

Imagine that! As often repeated here, the aperture calculated by FL/f-no. is not the physical size of the aperture, but the entrance pupil of the lens. Think of this as the physical aperture as magnified by the front cell of the lens (although I maintain that this is not an unambiguous definition either since the apparent size of the aperture when viewed through the front cell will vary with the viewing distance.) When converted, the Symmar has no front cell so the physical aperture IS the entrance pupil. Hence, your observation proves the rule.

M. Bigler recently gave an excellent discourse on measuring the entrance pupil:
http://www.largeformatphotography.in...63&postcount=2

[BetterSense]

So! That's enlightening.

I suppose what I will do is linearly correct the diameter of the actual diaphram for the purposes of aligning the aperture scale. I will assume that when the diaphram is all the way opened, that corresponds to f/5.6.

In other words, if the lens has an exit pupil of 37.5mm and a real diaphragm size of 29mm, then I can assume all the other diaphragm openings to be scaled by a factor of 37.5/29mm. Then the 2.2mm minimum diaphragm opening will give 2.2*37.5/29=2.8mm which gives f/74 for the smallest opening.

Sound reasonable?

[aduncanson]

Assuming that the two scales are not moving independently, why not just set the physical aperture to 23.1mm (=370/16) and fix the scale so that the f/16 mark on the 370mm scale is opposite the pointer?

[BetterSense]

Ah. A more elegant solution. I will just do that.

[rdenney]

Both aperture scales are on the same ring. Using f/16 for just the rear element will work fine. But don't glue it down--tape it down. Then, do test exposures to be sure.

Sort of a bummer that your new Symmar had a loose aperture ring.

Rick "who is going to look at his 180/315 to see how that ring is supposed to be attached" Denney

[BetterSense]

The mystery continues. Measuring the diaphragm opening with the front cell off, and using the result to calculate the f/ratio for the 370mm length results in the 370mm aperture scale not lining up at more than one value. I can set it so that it lines up at f/45 or at f/12 or any other value, but then the rest of the values are wrong. Apparently the "exit pupil modifier" is itself a function of the diaphram opening, even for the 370mm configuration.

I think I will reattach it at the orientation that it was originally on, and stop trying to reverse engineer the thing. I have managed to deduce the original position by looking at how it mates with the rest of the shutter.

There is no real way to tape it on. The cover actually is needed to hold the whole shutter together. Without it the shutter comes apart. From looking at the hole in the cover, I can see that the inside bore of the cover has three 'bumps' left in it. And the outer surface of the round journal 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 see no alternative but to glue the cover on using something like Loctite #680. It will be a permanent act, but the shutter is broken as it is. If the shutter later fails completely, can I put these lens cells in another one? What size do I need and how expensive are they?

[rdenney]

I see no alternative but to glue the cover on using something like Loctite #680. It will be a permanent act, but the shutter is broken as it is. If the shutter later fails completely, can I put these lens cells in another one? What size do I need and how expensive are they?

I'll look at my old Symmar Convertible tonight and try to understand the situation.

My 180/5.6 is in a Compur No. 1, and a 210/5.6 should also be. It will fit in any Compur or Copal No. 1, but then you'll still have to deal with the aperture scale issue.

I'm also thinking that anyone with an old Symmar 210 can measure the aperture at f/5.6 and tell you that measurement. I would be surprised is the aperture location ever changed, unless they were mounted in a different type of shutter such as a Compound.

There is a possibility, which could be confirmed if you can post a picture of the shutter. Some of the Linhof-select Schneider lenses were mounted in Compur #2 shutters of one sort or another. My old 121/8 Super Angulon is that way.

Rick "pause overnight for further study" Denney

[Dan Fromm]

#1

Info about older Schneider lenses, including yours, can be found here. http://www.schneiderkreuznach.com/archiv/archiv.htm

Inform yourself.