Alignment of lens axis with CCD on XL1s at DVinfo.net

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Old May 1st, 2002, 09:40 AM   #1
 
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Alignment of lens axis with CCD on XL1s

I've reposted this message here, since, it is buried under another quality related topic for the XL1s.

Here's another curious situation that I've encountered and wonder if anyone else has experienced this. I routinely leave a UV filter mounted on the front of the standard 16x IIs lens. I mounted a polarizing filter on top of the UV lens so that both filters were stacked onto the lens. In the widest angle setting, I could see some vingetting of the image. This is normal and I expected it. What was weird was that the vignette wasn't symmetrical around the image frame. It vignetted the top right corner, but, no other corners!! This tells me the CCD is not concentric (centered) with the optic!!! What's up with that? Is that because of the stabilization system, perhaps?

If, in fact, the lens is decentered, say from poor asssembly QA, the image will suffer from coma and astigmatism. Basically, this will degrade the image resolution, read: sharpness.

Last edited by Bill Ravens; May 1st, 2002 at 12:10 PM.
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Old June 8th, 2002, 10:49 AM   #2
Andreveroli
 
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In my XL1S Pal is the same cuestion !!
The lens is not concentric to the CCD
put a cardboard with one circulate in front of the lens and perfectly centered . Trim in the image will not be seen centered !!
It is a serious problem .


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Old June 8th, 2002, 08:40 PM   #3
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In the land of large format photo, the fact that the lens could be moved up,or down, etc. was an important consideration. My guess is whether the lens is parallel to the chips is the more important thing.
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Old June 9th, 2002, 07:52 AM   #4
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Don't judge quality just by whether or not symetrical vignetting. Judge by the video image produced by the lens/CC combination.

Stacking filters can lead to interesting issues with flare, stray reflections, dust, etc, beyond vignetting.
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Old June 9th, 2002, 10:38 AM   #5
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Dear Sirs.:

In my opinion, a lens that not is aligned with its optical axis, has astigmatism.
And the quality of the image can lower much in the edges.
The lens of the XL1, is not centered with respect to the CCD, this does not have discussion, because it is only necessary see to front of the camera with the removed lens, to see that the mount of the lens is not aligned with the CCD.
If we make a small hole ( of 2 o 3 mm. ) on a black cardboard and place it to about 3 meters of distance, illuminating it from back, we will see that the light that happens through the hole, has comet form, instead of a light point.
This is astigmatism.
The direction where points the comma (or tail of the comet) marks the direction in which the optical axis separates of the center axis.
Whatever of this is due to the own defects of the lens and whatever is due to the lack of centered, I do not know it.



Thanks and sory for my english


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Old June 9th, 2002, 10:53 AM   #6
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xl-1

I have experienced the same vignetting when using a UV filter. It is so slight that it flew under my radar (a tv or the view finder) but one day i was watching footage on the final cut viewer and there it was. when i discovered it i had the uv filter and a polarizer on at the same time.

my xl-1 is from the beginning, spring of 1998, and i chalked it up to the age of the lens. but i did notice, and i am not totally sure this could cause it (maybe someone could give me an idea) but the mounting ring for the lens seems to sit crooked, at an angle on the xl-1.

anyway i am kinda relieved to hear other people have noticed this. Someone should tell Canon.
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Old June 9th, 2002, 11:22 AM   #7
 
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As I originally posted.....vignetting is normal when stacking filters, so this is not a flaw. However, as Veronica points out, a decentered lens will cause degradation of image resolution due to coma. A Canon optics expert may, however, explain the phenomenon by understanding how the image stabilization system works on this lens. The IS works by a moveable prism as one of the last elements in the lens. This could, potentially, cause decentering and would be normal. But, I don't know for sure. One possible test of this theory would be to stack filters on a 16x manual lens( the manual lens has no IS system) and see if the asymmetrical vignetting still exists. Anyone with a manual lens care to check it out and report back would be greatly appreciated.
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Old June 9th, 2002, 11:34 PM   #8
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Speaking of the steadyshot, what happens when you turn it off? Does it recenter the prism, etc? Or is it possible that it just shuts off whichever direction it was last in? That could explain why the axis is off. complex toys, hey?
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Old June 10th, 2002, 08:15 AM   #9
 
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I forgot that the Canon wide angle lens has no built in Image Stabilization. After remembering, I ran the test I described above with it and still see an asymetrical vignetting pattern. Guess it's not the IS system causing this.
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Old June 10th, 2002, 08:26 AM   #10
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vignetting with UV filter alone

No, I experience vignetting with ONLY the UV filter on but it is slight and i had never noticed it UNTIL I stacked a polarizer on top of the uv filter. Now I notice the vignetting all the time, since i leave the uv on all the time. And it only occurs in one corner.

I mentioned this because I use an older XL-1 with the 16x, auto lens and, if this is some sort of design issue it has been there since the beginning.
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Old June 14th, 2002, 10:28 PM   #11
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Hi,

I have the manual 14x lens and several EOS lenses that I will try the double filter arrangement on tomorrow. However, since everyone that looks for this problem finds it, my best guess is that it has to do with the pixel shifting technology.

Jeff
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