New Copal Shutter for Schneider-Kreuznach 58 XL Lens.
26/02/11 18:00
I had a problem with the Prontor shutter on my Schneider-Kreuznach 58 XL lens. The fault was with the flash sync. socket, which worked perfectly with flash heads, but would not trigger a Leaf Aptus digital back reliably. The back produced blurred magenta images below 1/8 second, which is symptomatic of high or wavering resistance between the two contacts. The contacts should be a dead short to earth (zero ohms) when the shutter is open, and Leaf backs are notoriously fussy.
I checked the obvious things, like my cables, tried a new cable, and cleaned the sync socket as much as I could. I was not hopeful, or there would be failures at all speeds. I suspect that the shutter needs a service to improve internal contacts, and possibly the replacement of a weak spring, which is what happened when one of my Hasselblad lenses had an identical fault.
However, this Prontor was an odd-man out in my line-up. All my other lenses are in manual Copal shutters. The Prontor is marvellous, with a self-cocking action, but it works a different way to all my other lenses (eg two cable releases) which can be irritating.
Fig. 1: Schnieder-Kreuznach Super Angulon 58 XL f/5.6 in Prontor Professional shutter.
Fig. 2: Lens disassembled (unscrews) with (from left to right) the front group, Prontor shutter, and rear group.
I tried to buy a Copal #0 shutter in the UK, but the dealer was insistent on sending the lens to Schneider-Kreuznach to be mounted and calibrated. It’s an excellent dealer, and their advice was good, because the spacing (shims) between the groups is essential for optimum performance. However, the likely cost was £540 as opposed to £350 for the bare shutter. With my technical background, I decided to see if I could manage it myself.
The procedure is hardly difficult, as anyone knows who regularly fits lenses to new lens boards: the front and rear lens groups unscrew by hand. The shutter requires a special spanner, or you’ll butcher the lock ring, but it’s an essential thing to have in one’s bag anyway, and easy to find.
Fig. 3: Lens groups with new (much more compact) Copal #0 shutter.
The specifications show that the thickness of the Prontor and Copal shutters should be precisely 20 MM. In other words, the spacing ought to be the same except for manufacturing tolerances. Using a micrometer, I checked the Prontor shutter and found it measured 20.02 MM. The Copal shutter measured 19.98 MM. I measured the brass shims that were part of the original assembly, and the thinnest measured 0.05 MM, so I put it safely to one side.
It is conventional for all the shims to be on the rear lens group. However, when I fitted the front group (I had opened the shutter and aperture irises) the shutter iris would not close, fouling on the protruding metal cylinder containing the rear-most lenses of the front group. Therefore I fitted the thickest shim to the front group, and all the rest (except for the 0.05MM shim) to the rear.
Fig. 4: The narrower assembly fits well in a recessed lens board, which makes life easier for me.
The smaller shutter made it possible to fit to a recessed lens board. The old flat board had been fine with the Sinar P2, but the Norma can not tilt or swing with the standards so close.
Fig. 5: Feeling the squeeze in recession, the recessed lens board is not a genuine Sinar part. I was careful to check its alignment using Zig-Align mirrors with a mirror attached to the panel, and also with a mirror on the back of the lens.
The most serious effect of poor spacing is that the plane of focus, which should ideally be flat, becomes curved, and of chromatic aberration at the edges of the picture. Optimum spacing balances one effect against the other. Whether 0.01 MM is important I don’t know, but I suspect there is as much variation by screwing the lens up too loose or too tight. I shall soon see ...
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GMM-P (26/02/2011)
I checked the obvious things, like my cables, tried a new cable, and cleaned the sync socket as much as I could. I was not hopeful, or there would be failures at all speeds. I suspect that the shutter needs a service to improve internal contacts, and possibly the replacement of a weak spring, which is what happened when one of my Hasselblad lenses had an identical fault.
However, this Prontor was an odd-man out in my line-up. All my other lenses are in manual Copal shutters. The Prontor is marvellous, with a self-cocking action, but it works a different way to all my other lenses (eg two cable releases) which can be irritating.
Fig. 1: Schnieder-Kreuznach Super Angulon 58 XL f/5.6 in Prontor Professional shutter.
Fig. 2: Lens disassembled (unscrews) with (from left to right) the front group, Prontor shutter, and rear group.
I tried to buy a Copal #0 shutter in the UK, but the dealer was insistent on sending the lens to Schneider-Kreuznach to be mounted and calibrated. It’s an excellent dealer, and their advice was good, because the spacing (shims) between the groups is essential for optimum performance. However, the likely cost was £540 as opposed to £350 for the bare shutter. With my technical background, I decided to see if I could manage it myself.
The procedure is hardly difficult, as anyone knows who regularly fits lenses to new lens boards: the front and rear lens groups unscrew by hand. The shutter requires a special spanner, or you’ll butcher the lock ring, but it’s an essential thing to have in one’s bag anyway, and easy to find.
Fig. 3: Lens groups with new (much more compact) Copal #0 shutter.
The specifications show that the thickness of the Prontor and Copal shutters should be precisely 20 MM. In other words, the spacing ought to be the same except for manufacturing tolerances. Using a micrometer, I checked the Prontor shutter and found it measured 20.02 MM. The Copal shutter measured 19.98 MM. I measured the brass shims that were part of the original assembly, and the thinnest measured 0.05 MM, so I put it safely to one side.
It is conventional for all the shims to be on the rear lens group. However, when I fitted the front group (I had opened the shutter and aperture irises) the shutter iris would not close, fouling on the protruding metal cylinder containing the rear-most lenses of the front group. Therefore I fitted the thickest shim to the front group, and all the rest (except for the 0.05MM shim) to the rear.
Fig. 4: The narrower assembly fits well in a recessed lens board, which makes life easier for me.
The smaller shutter made it possible to fit to a recessed lens board. The old flat board had been fine with the Sinar P2, but the Norma can not tilt or swing with the standards so close.
Fig. 5: Feeling the squeeze in recession, the recessed lens board is not a genuine Sinar part. I was careful to check its alignment using Zig-Align mirrors with a mirror attached to the panel, and also with a mirror on the back of the lens.
The most serious effect of poor spacing is that the plane of focus, which should ideally be flat, becomes curved, and of chromatic aberration at the edges of the picture. Optimum spacing balances one effect against the other. Whether 0.01 MM is important I don’t know, but I suspect there is as much variation by screwing the lens up too loose or too tight. I shall soon see ...
--
GMM-P (26/02/2011)