T-stops

aholdmeyer · Oct 13, 2007

I've been searching for a clear answer as to what a T-stop stands for, and what they are used for.
I've read places where they are true stops that will give you an accurate exposure, while other places say it is a transmission stop that will determine the amount of light the was lost through the glass in your lens. If someone could clear this up for me that would be great.

Kevin_Zanit · Oct 13, 2007

If you think about the two things you just said, they are the same thing.

A T stop stands for Transmission stop. An F-Stop is derived from a mathematical formula that does not take into account light loss through the glass elements of a lens.

The manufacturer of the lens will measure how much light is lost through the glass and then correct the markings on the lens to be accurate. The end user (you and me) treat it like a regular F-stop. If my meter reads F5.6, I set the lens to T5.6.

Kevin Zanit

David Mullen ASC · Oct 13, 2007

Yes, a T-stop mark is just a "corrected" f-stop mark on the lens (corrected to make the exposure more accurate), just that they can't call it an "f-stop" anymore since that's a physical position based on the focal length and size of the aperture.

It's funny how often this question comes up - I picked up a 1978 issue of "American Cinematographer", which used to have a section where beginners could ask questions. The first letter began: "I'm still confused about T-stops." 30 years later and the question still gets asked.

Kevin_Zanit · Oct 14, 2007

Thats funny, I am sure that question will remain for many years to come. It was good seeing you today btw.

henry1406 · Dec 9, 2007

T Stops

T Stops

Your question about the difference between T Stops and F Stops comes up time and again. The thing to know is that in practical terms, they are so close in value as to not matter, especially if you're shooting on negative film. I've been developing a web site to help clear up some of these types of questions and you can find more info here: http://www.eyefish.tv/dictionary/t-stop I hope this helps.

PanavisionT2 · Jan 18, 2008

They do matter actually. If you watch Cinematographer Style, one of the DP's interviewed recounts how in his early days, an old school cinematographer taught him how to distinguish between a 1/16th of a stop. If you want to be a professional, you need to be able to see the difference between the f-stop and the t-stop.

henry1406 · Jan 18, 2008

T Stop

T Stop

Yes, it's probably true that in the early days film stocks were far less tolerant and required near perfect exposure,a bit like the difference between reversal and negative stock. Nowadays, motion picture negative stock has a wide latitude and can tolerate under or overexposure by 1 to 2 stops without any significant problem. I agree that your exposures should always be well considered and on the button but to worry about 1/16 stop is an unecessary stress and your producer won't thank you if your wasting time on such a small matter.

PanavisionT2 · Jan 18, 2008

The fact that stocks today are more sensitive then their earlier counterparts make the importance of seeing the difference in exposures even more prevalent. The inability to attain consistency in your images will limit your field of opportunity as a cinematographer.

I can't think of any cinematographer who would tell his producer, "I'm trying to correct for a difference in a 16th of a stop." Not only would such a comment made out lout be incredibly desultory, it would invite your immediate dismissal from the production.

It's not something that is spoken, it is something that is planned far in advance, and executed by way of knowledge and experience on the set. It's something that is part of the internal dialog a cinematographer has with himself, knowing that the correction of that errant 16th of a stop is for him and him alone. To speak about it out lout on a set would be considered pompous.

I cannot see the difference between a 16th of a stop, but it is something as a learning cinematographer that I hope someday to learn. If I regard f-stops and t-stops as a 'negligible difference', I can expect my career to become just as negligible.

henry1406 · Jan 18, 2008

T Stops

T Stops

Glad to hear that you are so passionate. Thanks for your comments and good luck.

David Mullen ASC · Jan 20, 2008

This issue isn't underexposing by 1/16th of a stop -- I can't see that small a difference, you can't adjust an f/stop ring to that level of precision, and that's less than a single printer light point of correction in post.

The issue is consistency and matching between lenses, so that a T/4, for example, is the same for all the lenses being used, especially for multiple cameras looking at the same subject.

But considering many f/2.8 lenses become T/3.1 lenses once changed to T-stop markings, I'd say that the difference is more significant than 1/16th of a stop on many lenses.

PanavisionT2 · Jan 20, 2008

I was guessing at the stop difference. I couldn't remember if he said 1/10th, 1/12th or 1/16th. I just remember it being extraordinarily slight and realizing that I couldn't rely solely on f-stops if I wanted to attain as close a consistency as possible.