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Old 17-05-2015, 09:21 PM
cfranks (Charles)
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QSI683ws-8 with Canon EOS

Hi,

I'm trying my Canon EF 70 - 200 f2.8L IS USM lens with my QSI683ws-8, using the QSI-EOS adapter. Wide open at 200mm produces a field curvature of 46%, a bit more than I need! Using an aperture mask of 28mm dia, the curvature drops to 22%. Has anyone used this combo and got better results? I'm not too knowledgeable here but could an incorrect back focus distance give this kind of problem?
Thanks
Charles
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Old 17-05-2015, 09:46 PM
cfranks (Charles)
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Forgot to include these.
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Old 17-05-2015, 10:09 PM
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DavidTrap (David)
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Hi Charles,

I've got a QSI583 that I've used with various camera lenses.

I've had similar disappointing results with zoom lenses (Nikon 14-24 and 70-200). Had better results with an 85mm prime and some distortion in the corner with a 50mm prime.

My hunch is that while the physical back focus distance is correct for camera lenses, when you put a filter in the way the refractive index of the filter shortens the effective focal length sufficient to really disturb the optics of zoom lenses. The lenses came to focus a significant distance from the infinity marker on the lens. The prime lenses were less affected. I had some improvement using a 1mm spacer behind the QSI-Nikon adapter.

You've got a fairly tilted image - how are you supporting the weight of the camera / lens?

I also wonder if the image stabilization mechanism needs to be "powered up" to maintain the optics in perfect alignment? The prime lenses I used were 25 years old, so definitely manual.

Just my thoughts,

DT

Last edited by DavidTrap; 17-05-2015 at 10:20 PM. Reason: Clarity
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Old 17-05-2015, 10:11 PM
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Joshua Bunn (Joshua)
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Hi Charles, I would try to sort out your ccd tilt first. Are you able to go past infinity focus like you can on a DSLR? Personally, I would get out my calipers and measure all the flange distances and see if it's correct.
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Old 18-05-2015, 10:54 AM
cfranks (Charles)
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Thanks David & Joshua,
Since the change from 46% to 22% tilt was due to the aperture mask, I'm fairly sure it is an optical problem. CCDInspector has (for me) given variable results depending on the amount of stars in the image.
The camera is bolted to the dovetail plate, square, both across the plate and vertically. The lens was attached and the camera placed on it's back with the lens vertical and the front clamping ring fitted with the screws just touching the lens, ie. no strain on the camera adapter. The lens cannot move anywhere near enough to cause that amount of tilt.
David, I had thought about adding a spacer, taking into account the filter but I think your comment about the IS mechanism might be a better clue. How does one 'power up' said mechanism? That could also possibly explain my T-Point calibration run last night. Last time I used the MX mount was with my camera and a Pentax 300mm Prime and the Sky RMS was 4.3 arc sec. Last night with the Canon, I couldn't get it better than 43.6 arc sec! The scatter diagram was all over the place suggesting something may be loose.
I have a small Surface Plate on order to do as Joshua suggested and accurately check the flange and adapter thickness, spacing etc. but that won't arrive for a week or more, probably at the same time as the next clear sky!
Thanks again,
Charles
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Old 18-05-2015, 03:26 PM
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DavidTrap (David)
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Your bracket looks sound.

Guess you can only power up the IS mechanism using a Canon body... I gave up on using my 70-200 for that reason.

DT
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