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Old February 23rd, 2005, 09:07 AM   #1
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question about 35mm lens and video camera

Im sure this issue has been addressed but i can seem to find it when i search.

I have a canon l2 with a single 1/2in ccd. Why can't i mount a 35mm lens right to the camera and get shallow DOF? i know i would loose some of the image or most of it because of the difference in the image plane(right word?)

Can someone please explain this to me? im getting all confused with all the different terms flying around.

Thanks in advance.
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Old February 23rd, 2005, 09:49 AM   #2
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so i found this page
http://www.dvinfo.net/conf/showthrea...threadid=39858

if there was some added space between the 35mm lens and the ccd would that help in making the ccd pick up more of the image.
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Old February 23rd, 2005, 10:01 AM   #3
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The problem is the size of the CCD. The imaging surface is too small to give good DOF.

You can get good DOF by moving WAY back and zooming in.

Try these links for more info.

http://www.mediachance.com/dvdlab/dof/index.htm
http://www.dvinfo.net/articles/optics/dofskinny.php
http://www.photo.net/learn/optics/dofdigital/

Hope this helps.

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Old February 23rd, 2005, 10:04 AM   #4
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Justin.

A long course in film optics is not practical for the forum space, but the short answer given here;-

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Yes, but since the XL1s CCD is only 1/3", it causes the focal length of the lens to be cropped (thus magnified) 7 or more times. A ultra wide fisheye lens suddenly turns into a telephoto. A normal lens turns into a super telephoto. This makes it not much use.

This is why expensive 35mm adapters like the mini35 exist. They project a 35mm lens image onto a ground glass, and then that projects onto the small 1/3 CCD, so a 50mm lens (say, a cheapie Canon 50mm f/1.8) is still a 50mm lens when it reaches the CCD, and depth of field is kept.

With the EF adapter and no ground glass in between to transfer the image, the same lens would give you the equivalent of a 350mm lens.

____

That was in the thread you cited, effectively answers your question. Lenses have a focal point and a focal plane. Y0u must mount the lens at a specific distance in order for the image to become "Focused" on the ccd. Simply moving it farther away is not going to work. The purpose of the special expensive adapters, is to allow those 35mm lenses, to focus their images ON TO a focal plane (the spinning ground glass) which creates the same sized image you would get if they focused on the FILM they would normally focus on. (An area much larger than your ccd in your camera) Your camera then focuses on that IMAGE and sends it to your video cassette.

The adapters, in essence, allow your video camera to video an image that is being 'projected' onto a tiny screen within the adapter.
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