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Another lens and format question


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#1 Davon Slininger

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Posted 05 June 2009 - 11:44 AM

Why can't you get wider apertures on longer lenses?

I've been looking at still camera formats and a 50mm T1.4 equivalent 35mm camera lens on an 8x10 is about a 300-360mm. Largest aperture I can find on a 360mm 8x10 camera lens is T5.5.

It had me thinking about cine lenses too. A set of S4's are T2 but why can't I get a T2 360mm?

Whats the science?
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#2 Tom Jensen

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Posted 05 June 2009 - 01:21 PM

Longer lenses have less light gathering ability. I believe it's because it enters from a smaller angle. On long lenses you are usually out of doors so it's not a problem plus you get the extra depth of field needed for sharper focus. Zoom lenses are generally faster at the wider stops and slower at the longer end. Wide open is usually set for the longest end of the lens. At least that's how I misunderstand it.
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#3 Chris Keth

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Posted 05 June 2009 - 02:01 PM

Why can't you get wider apertures on longer lenses?

I've been looking at still camera formats and a 50mm T1.4 equivalent 35mm camera lens on an 8x10 is about a 300-360mm. Largest aperture I can find on a 360mm 8x10 camera lens is T5.5.

It had me thinking about cine lenses too. A set of S4's are T2 but why can't I get a T2 360mm?

Whats the science?


I answered a similar question here, if you're interested in a little experiment that demonstrates the point.

You can get a T2 360mm. The catch is that the entrance pupil must be 180mm (~7 inches)in diameter, which would make the front element larger than 180mm. All other glass in the lens would have to be larger to keep the large entrance pupil. An f/2 360mm lens would probably have a front element the size of a dinner plate and would be 40 or 50 lbs.

Edited by Chris Keth, 05 June 2009 - 02:02 PM.

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