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Old Apr 12, 2010, 2:52 AM   #1
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Default Lens HELP!!

Hi guys! I am looking into lenses that can provide adequate focus in close proximity (2-5 cm). I want to mount the lens on a cctv security camera (the small ones approximately the same size of the board mounted cameras).

The lens should provide focusing in the said rage without adjusting the focus (rotating the lens clockwise or anti clockwise). Currently all the lens I have tried need to be focused.

I really do not no much about optics and any help on lens type and brand will be greatly appreciated.
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Old Apr 12, 2010, 5:39 AM   #2
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The closer the focus, the shallower the depth of field. If you don't want to adjust the focus, you need to position the subject at the correct distance. Since you're using a video camera, I presume you're talking about multiple subjects as they pass into and out of the field of view. You can position the camera such that the image will always be at the correct distance as it passes in front of the camera. If the lens has filter mounting threads, you can use close-up lenses, but the degree of sucess will depend on the lens on the camera. The only way to increase the depth of field is to reduce the aperture, and that's something that isn't usually possible on video cameras.
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Old Apr 12, 2010, 1:20 PM   #3
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Quote:
Originally Posted by TCav View Post
The closer the focus, the shallower the depth of field. If you don't want to adjust the focus, you need to position the subject at the correct distance. Since you're using a video camera, I presume you're talking about multiple subjects as they pass into and out of the field of view. You can position the camera such that the image will always be at the correct distance as it passes in front of the camera. If the lens has filter mounting threads, you can use close-up lenses, but the degree of sucess will depend on the lens on the camera. The only way to increase the depth of field is to reduce the aperture, and that's something that isn't usually possible on video cameras.
First of all thanks for the quick reply I am completely going into an unknown domain (optics).

By profession I am an Electronics Design engineer and am currently working on a visual aid device which provides a video feed of the page to the TV. The problem is when I try add a zoom feature to the device. When the camera (a small board type) is moved from 5 cm to 2 cm to provide zoom, the camera looses focus. I would like to know the type of lens which can provide this type of focusing capability (I know adding the feature is possible with the right type of lens).

PS
Any help with the camera will also appreciated. Please remember it has to be small, preferably a board type camera.
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Old Apr 12, 2010, 1:57 PM   #4
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There is an optical property called Depth of Field. Basically, it's when you focus on an object, how much infront of the object and how much behind it, will be in focus. It seems to me that what you're trying to do is use a fixed focus camera to have something in focus, and then move the camera closer, and still have it in focus. That depends a lot on the lens and the aperture. Can you provide some more details about the camera you're using?

By the way, moving the camera isn't actually "zooming". A zoom lens (a lens that can change it's angle of view) might be better for what you want to do. It would be able to zoom in to get a higher magnification image without actually moving the lens, and since the distance won't change, the subject will be in focus whatever the zoom.
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Old Apr 12, 2010, 2:42 PM   #5
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2- 5cm means a macro lens. May I ask why do you need such short distance focusing?
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Old Apr 12, 2010, 9:22 PM   #6
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The CCTV lenses I am familiar with use C-mount, and some have power zoom capability, as well as electrically controlled focus.
I used to use MCM Electronics for parts and supplies - they should have a pretty good selection.

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