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Lens speed


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#1 Anatole Sloan

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Posted 10 July 2006 - 01:18 PM

Hi,
What advantage does a higher speed lens have when discussing 35mm lenses - e.g. what's the difference between Zeiss superspeeds and normal primes; is there any advantage/disadvantage to having faster lenses?
Cheers.
Anatole
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#2 John Pytlak RIP

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Posted 10 July 2006 - 01:30 PM

Hi,
What advantage does a higher speed lens have when discussing 35mm lenses - e.g. what's the difference between Zeiss superspeeds and normal primes; is there any advantage/disadvantage to having faster lenses?
Cheers.
Anatole


The "speed" of a camera lens is really a function of its widest f/stop, or more precisely T-Stop:

http://en.wikipedia.org/wiki/F-stop

http://www.uscoles.com/fstop.htm

http://www.mir.com.m...er/aperture.htm

As the iris of a lens is opened more, the lens lets in more light. Lenses with T-Stop calibrations take the transmission of the lens elements into account, as well as the geometric iris opening.
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#3 Chris Keth

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Posted 10 July 2006 - 05:11 PM

The "speed" of a camera lens is really a function of its widest f/stop, or more precisely T-Stop:

http://en.wikipedia.org/wiki/F-stop

http://www.uscoles.com/fstop.htm

http://www.mir.com.m...er/aperture.htm

As the iris of a lens is opened more, the lens lets in more light. Lenses with T-Stop calibrations take the transmission of the lens elements into account, as well as the geometric iris opening.



Check out John's links. Essentially for fiction film production, there isn't much difference because you tend to light to a deeper stop anyway and not shoot wide open. For my purposes still shooting student films, however, I like to have fast lenses since I can't always get the lighting equipment necessary to light to a 4 on 200 speed film, for example. Another advantage fast lenses offer is the ability to work with shallower depth of field (a trend recently seems to be the least DoF possible without killing your focus-puller) and in lower light levels.
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#4 David Mullen ASC

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Posted 10 July 2006 - 09:05 PM

Most films don't shoot much at T/1.4, hence why the new primes of the last twenty years (Primos, Cooke S4's, and Zeiss Ultra Primes) were all T/2.0-ish. But because the T/1.4 lenses were old and not very sharp wide-open, Zeiss finally developed a new set called Master Primes.

However, because of the depth of field problems of shooting too wide-open, industry practice still tends to favor around a T/2.8 for most interior scenes involving lighting. Maybe higher for slower lenses or needing more depth of field. Depends on the DP's style. Conrad Hall, for example, liked to shoot wide-open on Primos at T/2.0 all the time. I'd be happier if I could shoot at T/4 more often myself (or T/5.6 on anamorphic lenses.)

The main reason for still needing T/1.4 lenses is to shoot in natural light levels that are really low, like in candlelight, streetlight, etc.
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