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Old Mar 15, 2004, 4:36 PM   #1
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Default Less chromatic aberation?

Rather than waste Steves storage space on an explination, I'll sumerize and post a link. Do some of our cameras and there mounts provide poor alignment of add on lenses that cause significant, additional chromatic aberation not native to the lens?
Here is my original post on it on dpreview.
http://forums.dpreview.com/forums/re...essage=7978461
Basically I have noticed that at least one lens I have does not have the severe chromatic aberation that I thought it did, but rather since it is a huge heavy lens that droops on my sloppy loose canon adapter, the adapter etc. may be causing the cromatic aberation.
Any thought would be very much appreciated!!
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Old Mar 16, 2004, 9:54 AM   #2
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Seems to me that the camera threads may be worn or defective. One thing I am beginning to look around for is a clamp-on tripod mount for large lenses. So far I have not located a generic source.
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Old Mar 16, 2004, 12:17 PM   #3
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The canons use a bayonet mount (on the camera side anyway) and it was lose fitting the day I got it. There is an after market metal adapter (the canon one is plastic) that is suposed to fit tighter. I'm almost thinking the loose canon one will be beter if I manage to come up with a lens suport that provides adjustment though. The slop will allow for some movement when adjusting.
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Old Mar 19, 2004, 5:28 PM   #4
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From a pure optics standpoint, chromatic aberation shouldn't be affected by alignment to your camera. It is a characteristic of the lens(or system of lenses).

Im a designer of optical systems, but not really a camera guy. My systems are all very monochromatic, so chromatic aberation isn't anything i deal with regularly. I would think that very dramatic lens misalignment would only really create focal plane problems, not chromatic issues.

Any camera guys agree?
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Old Mar 19, 2004, 5:37 PM   #5
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AHH just read your original post.

2 teleconverters that are mis-aligned with eachother would certainly create some chromatic error.

I think you are really asking for trouble trying to do this, but if you must, make sure that the converters are perfectly aligned with eachother. Once they are aligned with eachother, the alignment to the cam shouldn't be that sensitive.

You are creating a lens system by screwing together these converters. optically, you want them to approximate a single (perfect) lens. To do this, you MUST align the optical axes pretty darn perfectly. both tilt and translation are VERY important. Using an elastomeric adapter would not likely provide the mechanical stability needed for such an adapter. It really should be fashioned from aluminum.
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Old Mar 20, 2004, 1:05 AM   #6
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Actually I am wondering if it depends on the teleconverter. Ignoring stacking, some of the lenses I have do it and some do not. I can just hand hold the lens (just a single teleconverter) and tilt it slightlly and see chromatic aberation very quicklly show up. If the lens is straight there is not chromatic aberation (that I can see with my eyes anyway). I am pointing them at a small desk lamp with a white lamp shade (as it seems to bring out the chromatic aberation) from about 10 feet away.
The following lenses do show a lot of CA with even a slight mis-alignment.
Tiffen 2x (the old version, not the megaplus)
Pheonix 2x (the big 160$ one designed for still cameras, not the cheaper one designed for video)
kenko sgt-20
Unknown 1.5x quantaray that reverses for a 0.6 wide angle.
The following lenses do not show substantial increase in CA when canted slightlly.
Olympus centurion lens D-1.45X
sony vcl-1452H 1.4x
I have no idea if it is because these two are lower magnification or if they are beter lenses or what.
I figure if I can see chromatic aberation with my eyes when the lens is slightlly tilted, so can the camera. If I am not mistaken, all of these teleconverters are 2 or 3 element.
Any thoughts?
Fyi, trying to stack the teleconverters is just what made me realise that misalignment was causing CA which got me looking at it.
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Old Mar 20, 2004, 3:30 PM   #7
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What you are seeing is the chromatic error in the lens. It has nothing to do with the tilting. The chromatic error displays itself away from the center(optical axis), which is why you see it when you tilt. Try projecting your white light source through your lenses onto a white card.
As for some having it and others not--you bet! It is one parameter by which lens quality is measured.
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