iris closing when zooming at DVinfo.net

Tim Commeijne · March 11th, 2005, 04:15 PM

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

Lloyd Coleman · March 11th, 2005, 04:50 PM

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.

Donie Kelly · March 11th, 2005, 06:22 PM

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

March 11th, 2005, 04:15 PM	#1
Tim Commeijne Regular Crew Join Date: Nov 2004 Location: Europe, Belgium, Oostende Posts: 44	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

March 11th, 2005, 06:22 PM	#3
Donie Kelly Regular Crew Join Date: Jan 2004 Location: Ireland Posts: 159	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 __________________ Macbook Pro 2.0Ghz and Canon XL2

March 11th, 2005, 04:50 PM	#2
Lloyd Coleman Major Player Join Date: Oct 2003 Location: Salt Lake City, UT Posts: 256	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.