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T-stop?

t stop t-stop f-stop

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#1 Richard Plucker

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Posted 24 February 2014 - 01:36 PM

Hello all,

I'm kind of new to cinematography in general, so excuse my ignorance... I'm looking into buying a lens for my DSLR and one lens says that it has an aperture of T1.5-22. I looked up some general info about what T-stop is, but could someone give me a better explanation of it and how it is different from f stop? Is it better to have a higher or a lower number? 

This is the lens, if interested. http://www.amazon.co..._pr_product_top


Edited by Richard Plucker, 24 February 2014 - 01:38 PM.

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#2 Adrian Sierkowski

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Posted 24 February 2014 - 01:40 PM

Ok.. here's the best way to think of it, F is the Focus Stop-- e.g. the stop which calculates DoF, and T is the "True Stop" e.g. the stop which tells you how much light is getting through. Normally, the two numbers are the same (or so similar it's a wash, such as on my T1.3 lenses which are an F1.2) It only really applies to cinema glass and some stills zooms and the reason why the numbers differ is that the glass elements in the lens reflect and refract a certain amount of light (e..g they can't transmit 100% of it through), which obviously effects exposure. However the DoF is just a mathematical thing-- not related to the amount of light in the scene.

As to which is better, It's best to have a lens with a low T stop, e.g. a T1.3, which even if you never want to open up that far (for various reasons) it gives you the option in a pinch... and there will be pinches sometimes. All lenses, however, perform best right in the middle of their T/F Stop range, normally around a 4/5.6


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#3 Richard Plucker

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Posted 24 February 2014 - 01:51 PM

Awesome, thanks for the help and quick reply! 


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#4 Mark Dunn

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Posted 24 February 2014 - 01:53 PM

'T' actually stands for 'transmission' as Adrian explained. So the 'T' stop is the effective 'f' stop seen by the film.

It's not significant in stills photography because, among other reasons, automatic exposure usually compensates for it, but in cine you need to be sure that every lens you might use has the same transmission at a given stop for consistent exposure between setups. SoT8 is always T8, whichever piece of glass you pick up.

As Adrian said, the T and f stops are very similar on primes, but  zooms typically lose 1/3-1/2 stop, so the classic 12-120 f2.2 is T2.5.


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#5 Richard Plucker

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Posted 24 February 2014 - 01:58 PM

So f stop is the amount of light that enters the lens and t stop is the amount of light that actually passes through it  and reaches the sensor? 


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#6 Zac Fettig

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Posted 24 February 2014 - 02:28 PM

A f-stop is a ratio of the focal length of the lens to the diameter of the aperture. A T-stop is an F-stop that has been calibrated at the factory, to adjust for absorption in the glass and variation in the blades of the aperture, since an aperture is not perfectly circular.

 

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

 

Both measure the amount of light falling on the sensor. An f-stop is a theoretical value (what it should be), and a t-stop is a laboratory calibrated value (what it actually is).


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#7 Adrian Sierkowski

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Posted 24 February 2014 - 02:48 PM

I'm sorry zac, but a t and f stop are two entirely different things. F stops have nothing to do with the quantity of light hitting the film but only with a ratio of sizes. While it effects the amount of light let through it is not a measurement of such light as a transmission (t) stop is factory calibrated or not
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#8 Zac Fettig

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Posted 24 February 2014 - 03:18 PM

I'm sorry zac, but a t and f stop are two entirely different things. F stops have nothing to do with the quantity of light hitting the film but only with a ratio of sizes. While it effects the amount of light let through it is not a measurement of such light as a transmission (t) stop is factory calibrated or not

 

I'm pretty sure that they are basically the same, and not entirely different at all! :)

 

http://en.wikipedia....F-number#T-stop

 

A T-stop is a F-stop adjusted, at the factory or afterwards (calibration) to compensate for the actual transmittance (hence "Transmission" stop) of the lens of a calibrated, white light sample. An F-stop assumes 100% transmittance, and that is the rub; some light is always lost to heat. A typical cine/camera lens will have a transmittance in the 70-85% range, with the remainder lost to heat.

 

http://en.wikipedia....i/Transmittance


Edited by Zac Fettig, 24 February 2014 - 03:20 PM.

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#9 Adrian Sierkowski

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Posted 24 February 2014 - 04:11 PM

That does not make them equivalent-- one is what is used to calculate depth of field, and the other is used to calculate exposure. These are two different concepts basedon different variables only one of which is based on the actual amount of light in a scene. And while the overall transmission of a lens is in part based on the size of the iris, this doesn't mean that the T stop and F stop are more than just linked at a fundamental level. Trying to link them both to the overall concept of exposure muddies what the two are used for.


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