Quote:
Originally Posted by Shiraz
just a thought, but some optics designs use the aperture placement to help control aberrations in addition to limiting the amount of light (eg a MN uses the aperture placement to control coma). I have no idea if it makes sense with your lens, but it might be just be possible that it would work better at f2.8 if a full open aperture stop was placed where the original one was.
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I was thinking about it.. and I will try it soon.
Especially because it seems this lens should behave a tad better at full aperture, according to another owner.
However, based on experience with 200m and 300mm Canon lens (similar optical design),
(
http://www.iceinspace.com.au/forum/s...ht=canon+200mm), (and some russian lenses like Tair 11A (135mm) and Tair 33 in particular), the external aperture is way better.. Canons 200mm f2.8 and 300mm F4 have a lot of coma in corners (APS sensor.. and lenses were designed for full frame!) with internal iris, which is not there when external aperture is applied.
I think the reason why iris is where it is in Canon large telephoto lenses is simply because it would have been mechanically very complicated (for manual design.. with motors it is easier of course) to place it near front group.. so the current position is a compromise IMO.
However, smaller lenses are different (symetrical tessar-like designs). The external aperture doesn't work with them, it actually worsens the coma and/or CA.
I converted this lens to EOS by removing everything FD from it (including iris), so I can't really tell.. and I don't want to bother the only other person I know to own this lens to do the test for me (it is not really hard to cut the aperture.. but still.. )
In order to put iris back, I need to redesign the whole iris mechanism.. and this is not easy to do.