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