iris closing when zooming at DVinfo.net

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Canon XL and GL Series DV Camcorders
Canon XL2 / XL1S / XL1 and GL2 / XM2 / GL1 / XM1.


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Old March 11th, 2005, 04:15 PM   #1
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iris closing when zooming

Does anyone know why the iris closes when you zoom in?
That's not only on the XL2, the SONY PD 170 does the same thing.
It goes from f1.6 to f2.4 or even more.

THX
Tim Commeijne is offline   Reply With Quote
Old March 11th, 2005, 04:50 PM   #2
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The iris does not physically close down, but the f/stop does change. Here's why. The f/stop is a simple ratio of the focal length divided by the size of the iris. For example, if your iris is 50mm at its widest opening then you have an f/stop of f/2.0 when the lens is at 100mm (100/50=2.0). When you zoom the same lens to 200mm then you have and f/stop of f/4.0 (200/50). So as you zoom to a higher magnification the ratio changes, which makes a change in the f/stop.
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Old March 11th, 2005, 06:22 PM   #3
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Hi Tim

Small F-stop means more light and this is technically very difficult to achieve on a lens at higher zooms. Have a look at 35-mm camera lens as they are the same. Notice how the price increases dramatically when the F-stop is lower. Same problem there I'm afraid. You get what you pay for. If you want a "fast" lens then expect to pay anywhere upwards of 2K after which the sky is the limit.

The lens on the XL2 is "affordable" while still being excellent. If you want perfection (which is practically impossible) be prepared to dig deep ;)

Donie
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