At 8:50 PM -0700 3/11/07, Peter K. wrote:
Introducing the 2 element lens you are talking introduces the glass than light must pass through in which case it reduces the sharpness and would deteriorate the actual sharpness. The extension tube (not tele converter) has no elements so the lens would remain as sharp as normal except you have a closer focusing point and no infinity range on the lens. So I do not understand how this is possible, I mean even introducing a filter will reduce light transmission, albeit negligible. Peter K
Lenses can only be optimally corrected for one reproduction ratio. For standard (non-IF) lenses this is generally about 1:30 to 1:50, depending on focal length and anticipated use.
Most fast lenses today, or ones that are pushing the envelope, use internal focussing not only to keep the lens from moving away from the body to keep the balance of the lens, but to optimize the performance of the lens through a wider focussing range. If the 135/2 didn't have internal focussing, it's performance at distances such as 5ft would be dramatically lower. Wheras a lens with standard focussing has to be reasonably optimized over a wide range of reproductions, the designer of an IF lens doesn't have to care about what happens when the whole lens assembly is moved forward, as specific correction for a given distance is part of his design, achieved by differential movement of element groups.
If you now move that whole lens assembly further away from the camera as you would when using extension tubes, you completely upset the corrections introduced by the IF system. Using high quality achromatic close up lenses is similar to the IF mechanism itself, which shortens the focal length to achieve close focus. Close-up lenses shorten the focal length further, but keep the main lens assembly at the same distance, so the result is closer focus.
The close-up lenses do their job ahead of the main lens, which maintains the proper relationships between lens groups and the virtual image produced by the close-up lens, therefore maintaining the lens performance.
This also applies to zooms, which all now use internal focussing, even if the front group moves during focussing.
In any case, it's pretty easy to check for yourself if you have extension tubes and a 500D available. The 135/2 would be a superb candidate for this, as a drop from its stunning performance would be easy to see.
Actually, not that good. IF lenses like the 135/2 do better with good 2 element achromat close up lenses like the 500D than extension tubes. --
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