Just last night I was testing couple of lenses I have for some time with my new 400D...
So far the best behaved is Russian-made Tair 11A, 135mm, F2.8, I used a lot with my Praktica... (I just found one on ebay for US$45 + postage). Some tests claim over over 74 lps/mm ( line pairs/mm), wide open (
http://forum.manualfocus.org/viewtopic.php?pid=64397).
Even with iris fully opened, there is very little SA distortion at the frame edges. At f4 the images are perfect throughout the image area (this lens was designed for full frame 35mm format, so there are less problems with smaller CCD that 400D uses).
It is rather heavy though (it is made of brass... ideal material for lenses, but forgotten recently because of this cheap but light and adequate (for most purposes) plastic materials..). So I am thinking of mounting the lens on the equatorial platform and then attaching the camera to it, instead of other way around, as it is at the moment.
As far as focusing is concerned, this is another story. Since it is not a Canon lens, and since Canon disabled the focus confirmation for non-canon lenses, I have built right angle viewer with 10x magnification (see the thread in ATM section,
http://www.iceinspace.com.au/forum/s...ad.php?t=23756 ). It is a great help (still there is some ambiguity due to the fresnel lens used as a matte glass in the camera), but all really depends on how precisely the camera diagonal mirror is adjusted. So there are still some tests to be done to find the best focus by taking pictures of a bright star and to record the position of the lens focuser ring. Once this is done, I will fix the position of infinity focus on the lens and never I will have the focus problem again. Of course, that means this lens will be used with that particular camera only.