December 31, 2016

What's the Difference Between F-Stops and T-Stops (And Why Does It Matter)?

If you've never bothered to learn about the difference between f-stops and t-stops, now might be the time.

Most filmmakers and photographers are familiar with what an f-stop is and how each measurement on your camera affects your images, but how many know about t-stops? Sareesh Sudhakaran of wolfcrow puts it all in terms anyone can understand in the video below, explaining the differences between the two, as well as why it might be more helpful to use t-stops for filmmaking.

F-stops and t-stops both represent a certain value, one that is determined by the focal length of a lens divided by the diameter of the aperture. However, while f-stops are a "theoretical" measurement, t-stops are actual measurements that are tested when the lens is calibrated. This is why lenses that show t-stops tend to be so expensive, like the cinema lenses of Cooke, Zeiss, Angenieux, and Leica, because manufacturers have to put each lens through a lengthy and costly testing process.

So, what's the big deal? "My lenses have f-stops—should I find ones with t-stops? Are my images suffering because of this?" Hold your horses—and no, probably. Chances are you will be able to do your job just fine without the added power of an expertly calibrated lens with t-stops, but Sudhakaran shares three things you should think about now that you know more about f/t-stops.

  • For lenses with f-stops, use scopes or the camera meter.
  • If using light meters, stick to either f-stops or t-stops.
  • T-stops only make sense financially if the time saved makes up for it.

You'll probably find this information on t-stops more useful when you get a job on a big budget feature film or commercial that shoots with lenses that use them, but until then, you can probably just put this in the "trivia" compartment in your brain.      

Your Comment

7 Comments

Oh, come on... You too? What he says is simply wrong. Many of the technical stuff he posts are simply dead wrong. He is not a cinematographer, not even AC. He only guesses things based on his engineering background.
You should recommend the ASC Manual instead of this pool of misinformation.

December 31, 2016 at 1:51PM

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False Information Warning.
".....while f-stops are a "theoretical" measurement......"

No.
The f-stops are mathematical CORRECT calculations based on the lens itself.
The f-number is given by: f-number = focal length/diameter of the entrance pupil.
Further details: https://en.wikipedia.org/wiki/F-number

Jeff

January 2, 2017 at 3:26PM

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JeffreyWalther
Steadicam Operator
1488

Interesting.
Everytime I criticize (quite rightly) a post by V Renée
I lose a lot of my NFS score.

You know what? I don't care :-)

January 3, 2017 at 11:22AM

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JeffreyWalther
Steadicam Operator
1488

This video is VERY misleading, I'd rather remove the article to save the knoobs.

For more info on "why is it misleading" - just read the comment section on youtube.

January 3, 2017 at 7:14PM

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Linking to a Youtube video with false and misleading information is not doing the nofilmschool viewers a service. Many negative comments by viewers on Youtube are correct about this content. The links should be pulled.

January 6, 2017 at 4:29PM

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After reading the YouTube comments and looking this up on my own I've lost a lot of respect for this site. Worst, is this wanker's response to the comments. I thought No Film School was better than this.

January 7, 2017 at 3:17PM

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Brian
85

Measuring t-stops are not why cinema lenses are more expensive. I mean, sure, they have to calibrate them (you know, measure the light going in and compare it to the light coming out the other side), but that's really no different than making sure your 25mm lens isn't really 24.9mm or your 114mm front diameter (or 104mm, 95mm, 80mm...etc...) isn't mismeasured. It's just how they are marked for cinema.

As far as why still lenses use f/stops...who cares? This is a film blog. It sounds crazy to me to use a light meter, but *know* that if you set it to that f/stop, that you aren't feeding the sensor or film enough light because there is no compensation for the light loss...especially since there aren't markings on the side of the lenses to help you compensate for the light loss the way filters will tell you the filter factor. If you're using slide film with has so much smaller a latitude that it could really affect your exposure. I guess more still shooters are measuring through the lens more often, but still...plenty of people use light meters in the stills world. Using t-stops just means you don't have to worry about it or think about it at all. You don't have to know that one lens takes more light than another or spend any time calculating and compensating for the light loss.

January 7, 2017 at 3:33PM, Edited January 7, 3:38PM

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Daniel Mimura
DP, cam op, steadicam op
2200