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T vs. F numbers

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Ron Pestes
T vs. F numbers
on Apr 23, 2011 at 3:50:11 am

Please excuse this question but what does the T stand for in video camera lenses. I am used to the F stop in film, but T? Does it stand for the same thing? Why is it not called an F stop? Thanks for clearing this up.

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Sony EX-3
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Michael Palmer
Re: T vs. F numbers
on Apr 23, 2011 at 4:32:58 am

The T-stop is the true speed of the lens, f-stops are a measure of the aperture of a lens, f-stops tell us how wide open the iris of a lens is.

Good Luck
Michael Palmer


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Ron Pestes
Re: T vs. F numbers
on Apr 23, 2011 at 6:14:57 am

OK, but what is the difference between true speed and aperature? Isn't it the same thing? Is F2.8 the same as T2.8?

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Sony EX-3
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john sharaf
Re: T vs. F numbers
on Apr 23, 2011 at 2:34:39 pm

F2.8 on your light meter equals the exposure on the film or imager of the T2.8 setting on your lens.

JS



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Greg Ondera
Re: T vs. F numbers
on Apr 23, 2011 at 5:16:03 pm

T-stops are used by cinematographers pretty consistently as it is a more accurate measurement tool of the true light coming through a lans.

Greg Ondera
http://www.Plexus.tv
http://www.SurgeonToday.org


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Rafael Amador
Re: T vs. F numbers
on Apr 23, 2011 at 10:43:19 am

Thats marketing.
Nothing new. Kind of the F-Stops for dummies.
To understand the F-stop, you must have some basic knowledge about lenses.
With the T-stop there is no need to waist time in learning nothing.
rafael

http://www.nagavideo.com


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john sharaf
Re: T vs. F numbers
on Apr 23, 2011 at 2:17:56 pm

T-Stop refers to the actual "transmission" factor of film and video lenses, taking into account the amount of light absorbsion that the numerous pieces of glass the light passes through reduces.

F-Stop is a mathematical calculation of the focal length and iris opening, indicating the "speed" of the lens.

When you read the F-stop on a lightmeter, and set the same number as a T-Stop on the lens you'll have the corresponding exposure. If the lens was marked in F-Stops, you would be slightly underexposed.

JS



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Michael Palmer
Re: T vs. F numbers
on Apr 23, 2011 at 2:55:52 pm

The T in T-stop stands for transmission. Transmission of light.
F-stops are not a true measure of light transmission of the lens and that is why John said you maybe slightly underexposing using F-stops.
I think it would be fair to say that t-stops vary to about a half stop faster than f-stops in general and proper exposure to one person may differ with the next.

Good Luck
Michael Palmer


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Rafael Amador
Re: T vs. F numbers
on Apr 23, 2011 at 5:13:10 pm

John&Michael,
Thanks for the insight. The truth is that I haven't had the opportunity to work with any lens set with "T-stops".
I understand that any less could be graded in F-stop and T-stop, isn't it?
rafael

http://www.nagavideo.com


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john sharaf
Re: T vs. F numbers
on Apr 23, 2011 at 5:19:27 pm

T stop is really only pertinent in film cameras, where you are not able to "monitor" the effect behind the lens, and you are more likely to be setting exposure with a light meter.

In video you are always looking at the post lens effect, eithER in a viewfinder or monitor and thus have no reason to know or care what the T-Stop is or isn't. There is no need for grading or anything else if you are consistent in the method you use to set exposure; most likely zebras, waveform or histogram in video.

JS



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Bob Cole
Re: T vs. F numbers
on May 10, 2011 at 5:24:13 pm

The T Stop on some zoom lenses gets larger (i.e. the lens transmits less light) as you zoom in. Always a good idea, if you plan to shoot "wide open," and to zoom in during a shot or an interview session, to make sure you set the lens to the "T" stop of its maximum telephoto setting.


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