T-Stops and F/Stops

[Sanjin Jukic]

"T-Stops and F/Stops

When discussing lens aperture size,
cinematographers usually refer to T-stops
while still photographers refer to f/stops.

• The f/stop is a physical measurement of the theoretical ability of an ideal lens to pass light.
By dividing the focal length of the lens by its iris diameter, the f/stop can be determined.
Light, which is lost through the lens itself, is not considered in the f/stop calculation.

f/stop = Lens Focal Length / Diameter of Lens Aperture

f/1.4 = 50mm / 36 mm

• The T-stop is a measurement of the actual amount of light a lens will pass after allowing for
transmission losses due to absorption, internal reflections and light scatter. T-stops are determined
for each particular type of lens since lens design, quality of the glass, number of elements and lens
coatings do vary. Therefore, T-stops are more accurate than f/stops.

What relates the two is the lens' effciency in transmitting light. If the lens could transmit all the light entering it,
its T-stop and f/stop would be the same. Light meters mathematically calculate exposure in f/stops. Once you
have determined the proper exposure, you can set the lens to the correct T-stop.

Stops relative to exposure:

• the smaller the number, the bigger the hole
• the difference between the actual light that passes through the hole represented by
any two consecutive stops is either double or half."

Everything you ever wanted to know about filmmaking find at

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[Felix K.]

The question is now how big the differences are in numbers between F-stop and T-stop in reality.


Also: the DOF is calculated with the F-stop. So both numbers are important.

 

[Stephen Williams]

The question is now how big the differences are in numbers between F-stop and T-stop in reality.
.

Hi,

It depends on the no of glass elements & the quality of their coatings. Newer coatings are significantly better in this respect.

Stephen

 

[Jeff Kilgroe]

Glass composition, thickness, etc.. all play roles too.

As a hypothetical example; Due to light loss through the optical path of a lens, a large zoom at T3 may actually have an F number of 2.2.

 

[Stephen Williams]

Not all companies always tell the truth with Tstops. Never known Cooke to lie

 

[Alexander Ibrahim]

The question is now how big the differences are in numbers between F-stop and T-stop in reality.


Also: the DOF is calculated with the F-stop. So both numbers are important.

It is instructive to compare Zeiss ZF lenses with Zeiss Compact Primes.

Its the exact same glass, according to Zeiss. The Compact primes have been rehoused and marked in T-stops though. Also the compact primes have been changed from a 9 blad iris to a 14 blade iris for the Compact Primes.

As an example the 50mm and 85mm SLR lenses are f/1.4 and t/1.5 according to Zeiss.

Also, as an exercise, you can establish your own "effective t-stops" for lenses marked in f-stops.

Its the same work as establishing an EI for your camera... but this time we are taking the EI as a known and adjusting the f stop. This sort of test works best with a known sensor- for example film.

Here is the process, in brief, for reference.

Light a gray card, using your lightmeter, to a fixed footcandle level.

Look up the correct exposure for the ISO/EI of your camera in the ASC manual. ( 9th edition volume 2 page 790)

So for an ISO 320 camera, with a subject at 64 footcandles, the correct exposure is t/4.

So, set your lens to f/4.

Now have a look at a waveform monitor. Is your gray card reading at 50 IRE? It won't be, so adjust the iris until it is reading 50 IRE.

That new position is t/4.

Adjust the light level and test for every full stop. On most SLR lenses, the difference between f stop and t stop is not constant throughout the range.

Zooms, especially ramping zooms, require testing to see if the "t stops" you just set hold throughout the zoom range.

Hope that helps.

 

[Stephen Williams]

On most SLR lenses, the difference between f stop and t stop is not constant throughout the range.
.

The marks are inaccurate, not been an issue for SLR stills work for the last 30 years as cameras had internal meters, not to mention film is very forgiving with exposure.

 

[Sanjin Jukic]

The marks are inaccurate, not been an issue for SLR stills work for the last 30 years as cameras had internal meters, not to mention film is very forgiving with exposure.

Ansel Adams on the way to the mountains. Photo by David Vestal, Leica M2, 35mm Summaron f/2.

Leica glass was always pretty accurate with marks and it's one of several reasons why is expensive.

I heard that marks are pretty inaccurate at the most of the cine glass except Cooke but as you said film is forgiving with exposure.

 

[Stephen Williams]

Leica glass was always pretty accurate with marks and it's one of several reasons why is expensive.

I heard that marks are pretty inaccurate at the most of the cine glass except Cooke but as you said film is forgiving with exposure.

Hi Sanjin,

I think that is correct, higher quality in manufacturing always costs more, Cooke mark each lens individually by hand so you would expect them to be spot on.

Stephen

 

[Sanjin Jukic]

Hi Sanjin,

I think that is correct, higher quality in manufacturing always costs more, Cooke mark each lens individually by hand so you would expect them to be spot on.

Stephen

Hi Stephen,

agree.

Cooke still has more of the handcraft approach even in the age of robotics that produces higher quality and lower cost to the manufacturing industry.

Much more of the industrial approach you'll find with other cine and still lens makers.

 

[Humberto Rivera]

Thanks Sanjin, the web site at; http://motion.kodak.com/US/en/motion/Publications/Film_Essentials/index.htm was excellent. It kinds of reminds all of where we came from; the Cine World, were its origins began. Now we move to the digital world with the new Red Epic S35, as the world changes, we change with it! Now we can do things that were impossible to do just years ago. I was looking at the Nicholas Cage film the other day “Lord of War” and to the second installment of “The Transformers II” there is a world of Difference in the pacing of the “Transformers” compared to “Lord of War”, and the band “Played On”!

Humberto Rivera