T-Stops vs. F-Stops. Do you know the difference? Why is there a difference?
When calculating the transmission of light through a physical lens, the F-stop is a theoretical value (usually within 1/3-stop), while the T-stop is an actual tested value and spot-on accurate.
So, why the difference? It mostly comes down to cost. One of the reasons cinematic lenses are so expensive is because testing a precision cinema lens for spot-on accurate light transmission is a costly process whereas setting up a lens for f-stop control is less expensive, thus making the photo lens less costly than its cinematic cousin. That cost difference is passed down to you.
How about practical use? Obviously, both T-Stops and F-Stops are valid methods of calculating image exposure, and with the ability for digital cinema cameras to accept photo mount lenses, the F-Stop has long been used in video production. However, in cinematography, consistency is key, and the "fix it in post" approach to scene matching can be expensive (and rather unprofessional). Therefore, in a situation where you want the same exposure over multiple scenes and even multiple days, filming with cinema lenses and calculating exposure using T-Stops is a must.
Budget, time, and desired look always factor into my lens choice on any given project. Like many of you, I use photographic lenses and precision cine lenses for various situations. I own sets of both, and yes... I still carry a light meter!
No matter how you approach lens choice, and scene exposure, it's important to refresh yourself on the basics, know what you need on set to achieve a consistent look. You will go back to that basic well of information on every job and you'll simply be better at your craft.
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Michael Fitzer, This is such a clear and helpful breakdown—thank you for laying it out with both technical insight and real-world application. The distinction between F-Stops and T-Stops can definitely trip people up, especially when transitioning from photography to cinematography. Your point about “fix it in post” being more costly than getting it right on set is so important, especially for indie filmmakers who often don’t have the luxury of reshoots or extensive color grading sessions. And I love that you mentioned still using a light meter. That level of intentionality really separates working DPs from people just chasing a cinematic look without understanding how to build it.