 |
brainiac
Offline
[X]
|
I don't believe that Contax tolerances were as bad as a tenth of a millimetre.
I do believe that there is an optimal adaptor thickness that is correct for all correctly calibrated CY lenses, whether it be 1.38mm or 1.42mm. However, that assumes that lens and adaptor will be mounted on a zero tolerance Canon body. In real life it seems Canon mount-to-sensor lengths can vary too, so the right adaptor thickness will probably depend as much on the Canon body as on the wear/calibration of the lens mount. Adaptors loosen up with use, and lens and body mounts also wear down. Contax lenses can also become miscalibrated with age/use, which is why Zeiss recommends they are sent in for a regular service. With all respect to Son, I don't see how he can have insulated his results from the vagaries of these factors, unless he used a large sample of EOS bodies in each model, and had all his lenses serviced by Zeiss before testing.
Lenses which have neither floating elements (CRC) nor internal focussing (IF) need only focus past infinity to deliver correct performance. Most CY lenses do not have CRC or IF. However, even CRC/IF lenses will produce very little or no disappointment when adaptor thickness errors are small.
A more serious issue is the adaptor's ability to hold the lens perfectly normal to the sensor plane. Poor corner performance is often just the result of a slanting field of focus. Many Zeiss lenses have a lug which protrudes far enough to touch the inside of the camera body. Apparently these helped to avoid lens droop with longer or heavier lenses. They often scrape the inside of EOS bodies and can be responsible for the lens sitting skew on the mount. All materials are elastic, and a heavy lens can deform the mount system by small but significant quanitites. Adaptors accentuate this problem as they double the number of physical interfaces. Results can vary depending on how you hold the lens/camera, or how the camera is oriented. It is almost impossible to tighten an adaptor evenly using the three tightening flanges, and most adaptors will focus on slightly different distances from one corner to another. Equally, like most Canon lenses, many CY lenses suffer from increasing CA as misfocus increases, as can be seen in green and pink fringes around blurred highlights.
What all this boils down to is that, with great respect to Son, I don't believe the situation is nearly as simple as his theory suggests, and I don't think one should aim for a non-standard adaptor thickness based on focal length. I think a better policy is to get your lenses serviced by Zeiss, get your bodies calibrated perfectly by Canon, start with a normal 1.42mm adaptor, or whatever your favourite supplier provides, test all corners using Liveview instead of trusting the manual focus system, and if you find results you don't expect, try your spare thinner and thicker adaptors to see if they help. If they do, don't assume it's because that lens model always needs that adaptor thickness.
Edited on Jun 21, 2008 at 07:15 AM
|
| Jun 21, 2008 at 07:08 AM |
| |
|
 |
