iris problem fixed in XL2? at DVinfo.net

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Canon XL and GL Series DV Camcorders
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Old January 14th, 2005, 11:44 AM   #1
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iris problem fixed in XL2?

Hello.
In the XL1 there was a problem when closing the iris too much (11, 16). The image turned a little bit blur.

Does this occur in the new XL2?

thanks
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Old January 14th, 2005, 12:01 PM   #2
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The issue is called "diffraction", there's an article on it here on the DVInfo site, and I think there's also an article on bealecorner. On the original XL1 it happened mostly at f/32, on the XL2 with its smaller pixels it can happen at smaller-numbered apertures.

The FX1 warns about it in its manual, I don't remember if the XL2 warns about it in the manual. The FX1 has the smallest pixels of the bunch, and they implemented a feature in the menu system to limit how small the camera can make the aperture, specifically to deal with the issue.

It's not a "fault" of the cameras, it's a property of optics, and any camera with CCD pixels of a certain size will encounter it when using a small-enough aperture.
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Old January 14th, 2005, 12:04 PM   #3
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It's my understanding that this is a physical issue with any lens - see the excellent article on http://www.dvinfo.net/canon/articles/article19.php. It has to do with the way that optics function in general. It is best to open the iris a bit for better focus. This is not actually a problem - it happens to be the way that video cameras work. Adjusting exposure through small variances in gain, ND and shutter speed would be the preferred way to go.
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Old January 14th, 2005, 07:32 PM   #4
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Miguel,

Use the ND filters built into the lens to cut down the amount of light hitting the CCD's. This should help you stay at a medium to large-ish aperture.

As mentioned before, the softness is due to the diffraction caused by stopping the lens down to a small aperture.
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Old January 14th, 2005, 07:53 PM   #5
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Yes. This problem is so bad that most consumer cams with the smaller sensors and even some professional cams, like mine, have undocumented ND filters built-in to keep from having to close the iris too much.
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Old January 15th, 2005, 09:27 AM   #6
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The XL2 manual does warn that in bright light the lens may close down to the point where the image becomes blurry, that this is more likely to happen at short focal lengths, and that the fix is to use the neutral density filters. This may very well be a diffraction effect. A lens doesn't focus parallel light rays to a point but rather to a blurred point described by a "point spread function". The width of the PSF is proportional to the f/ number i.e. it is twice as wide at f/11 as it is at f/5.6. The image is degraded by this when the PSF is wider than the resolution of the sensor. As the sensor is smaller in the XL2 the phenomenon is likely to be more noticeable at a given f/ number with an XL2 than with an XL1.

At http://www.pbase.com/agamid/image/38660757 I've put two images (the second is at http://www.pbase.com/agamid/image/38660761 but it's easiest to go to the first one and then use the "next" button to flip over to the second and the "previous" button to go back) showing the performance of the stock 20X lens at the minimum focal length at f/11 (the smallest aperture) and f/4.4. The differences are there for you to see. The smaller aperture will definitely cost you some resolution. Notice also the barrel distortion (in both - from the short focal length).
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Old January 20th, 2005, 07:45 PM   #7
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I assume this is the same problem/issue that I've experienced with every 1/3" camera when not using a ND filter outdoors? I can't imagine experiencing it when shooting indoors.
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