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Questions regarding T-Stops

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John Bauwens
Questions regarding T-Stops
on Jun 17, 2012 at 2:47:24 am

So I come from more of a post production background, but I find myself having to shoot stuff more and more. I have just started using DSLRs, and with that I have started working with better and better lenses. I am trying to figure out how to use T-stops, but I can't seem to find any information about this. I find information on what they are, why they are better, and why they are important, but nobody really goes in to how to determine the proper T-stop or how to use these in filming. I feel kinda dumb asking this, but any help would be much appreciated. I have not used any lenses with T-stops, but I would like to be prepared and ready if I get the chance to step up to it.

Thanks again guys.


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Rick Wise
Re: Questions regarding T-Stops
on Jun 17, 2012 at 3:30:41 am

Professional older lenses, especially older zooms, are marked in both T-stops and f/stops. We use f/stops to calculate depth of field, and T-stops to set the iris. Example: Your lens is an f/2.8, the widest it can go. Your meter tells you to expose at 2.8. Looks cool. But when you look at the lens you see that when you set the f/stop at 2.8, the T-stop is 4.5. What does that mean? The "true" stop of the lens is 4.5 when the f/stop is 2.8. So you don't have enough light.

Example 2: the meter tells you to expose at 5.6. You set the T-stop to 5.6, and discover that the f/stop is f/4. What does that mean? That you depth of field is what you will get at this focal length and at this distance with an f/4 and T-stop of 5.6.

But on most modern lenses there is little difference between T and f/stops. So if you have a professional lens you may find both T and f/stops. They are likely to be nearly identical.

(The T-stop is the actual amount of light coming through the lens, while the f/stop is the calculated amount.)

Bottom line, stop worrying....

Rick Wise
Cinematographer
San Francisco Bay Area
http://www.RickWiseDP.com


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John Bauwens
Re: Questions regarding T-Stops
on Jun 17, 2012 at 3:55:13 am

Thanks a Rick. The lenses I am looking at are mostly the Zeiss CP2, which seem to only have T-Stops. So these are just more accurate, geometrically based measurements of light rather than the F-stops which might be a bit off.

Thanks again


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Angelo Lorenzo
Re: Questions regarding T-Stops
on Jun 17, 2012 at 3:34:06 am

While f/stops are a simple ratio of the iris size to focal length, t/stops are calibrated "light transmission" stops. T/stops take into account the amount of light lost in lens design.

In other words, a t/stop of t/2.8 will transmit the same amount of light as a "perfect" lens at f/2.8. This may mean the actual aperture might be slightly larger to account for the loss of light (we're talking 1/10-2/10 of a stop or a similarly small difference for modern lenses).

While not that critical in a single lens, it does come in wildly in handy if you think of a set of prime lenses. Every lens set to the same t/stop has the same exposure which aids in exposure consistency.

t/stops are calibrated during construction.

Angelo Lorenzo
Fallen Empire - Digital Production Services


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Rick Wise
Re: Questions regarding T-Stops
on Jun 17, 2012 at 6:39:24 pm

Since the lenses you are looking at have ONLY T-stops, you can be comfortable knowing that the f/stops are almost identical and therefore there is no significant difference between them. Angelo's post lays it all out very nicely.

Rick Wise
Cinematographer
San Francisco Bay Area
http://www.RickWiseDP.com


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