View Full Version : Too much time on my hands?

paul owen
27-Oct-2005, 04:43
Had a bit of time to kill the other day! So ... as I was swapping lens panels I noticed that a 90mm Rodenstock lens I have has a brass(?) shim on the rear element - nothing new here! I always thought that they were placed here to adjust for correct placement of elements. So what happens if the shim is removed and a photo taken?? Told you I had too much time on my hands! So, without further ado, I took a photo with the shim in place and removed and used Polaroid 55 film. Result - (unscientific of course!) ... I couldn't tell any difference between shots! Both were as sharp as each other across the whole frame! I then started thinking about this shim! The thickness is "microscopic" and therefore surely any effect of having it in place will be negligible? I (unscientifically!) reckon that the degree to which you tighten the elements into the shutter would have a more significant effect on image quality then whether or not this shim is in situ? Okay, granted when the lens is in the factory and shutter and glass are married, a technician may feel the need to use a shim to create a perfect combination - but what happens once the lens is sold and the buyer removes the rear element to install it on a panel. Surely the degree of tightness to which the lens is installed is a very personal thing? How tight is tight? How tight is too tight? Too loose? What effect will this have on image quality? In other words - are shims necessary? I need to get a life :)

Roger Hein
27-Oct-2005, 05:04

My 240 G-Claron has a brass shim on the front cell. I too have tried it with and without the shim in place and could not see the difference. Some have suggested that the shim is there to optimize the lens for 'infinity' while at a close reproduction ratio it should be taken out.

27-Oct-2005, 11:04
You're just demonstrating that casual home lens tests aren't nearly as precise as the ones the manufacturers do. Those shims are indeed there to improve the performance of the lens (fractions of a milimeter difference in front/rear element spacing lead to increase in field curvature, I believe). The lens manufacturer needs to compensate for variances in the shutter, and also in the cutting of the threads themselves, which can never be done to the required precision. So they all get tested and shimmed appropriately. I don't think the different tightnesses you're talking about will make much difference ... you're not really forcing those elements closer together than the shim and shutter body allow. More likely your just stretching the metal with the threads on it a bit. Unless you use a pipe wrench, like I do.

And don't you think "too much time on my hands" is implied in any entry on a large format photography message board?

28-Oct-2005, 10:29
"Some have suggested that the shim is there to optimize the lens for 'infinity' while at a close reproduction ratio it should be taken out."

that sounds bizarre to me. why don't you call Schneider USA? they actually have technicians there who can answer these questions.