Common questions amongst indie filmmakers who are learning the ropes are: Why do some lenses have T-Stops instead of F-Stops? What is the difference? How significant are the differences and how will they help me as a filmmaker?
I personally had the same questions in mind when I worked on my first big budget production earlier this year. The lens package used was a set of Carl Zeiss CP.2 Compact Primes which seem to be one of the most accessible cinema lenses around, price-wise, as compared to the gold standard of ARRI Master Primes and Cooke Cine Lenses. These lenses are classified as cinema lenses, to differentiate them from the regular widely-used photographic lenses.
What else sets cinema lenses apart from a photography lenses? Very briefly, cinema lenses are expensive! They are so because they are designed to make our jobs as filmmakers easier. Filmmaking is already a tough and stressful thing to do so there are many features on cinema lenses that help you focus more on getting the perfect image.
Firstly, a cinema lens is manual only (that means manual focusing, manual aperture control, no electronic connections to your camera), rugged and durable (often machined from high quality steel), has gear teeth built around the focus ring for your follow focus (bye bye zip gears!), and have a consistent build (same weight, same length, same diameter) across all focal lengths.
The T-Stop is (in long form, transmission stop) yet another “life made easy” feature found on cinema lenses. So, what’s the difference between a T-Stop and an F-Stop? Both of them are measures of light, but…
An F-Stop is the measure of light coming through the lens while a T-Stop is the measure of light exposed onto the sensor of your camera.
What does this mean? It simply means that T-Stops are a much more accurate measure of light exposure as it is adjusted to account for the light lost while coming through the barrel of a lens.
HOWEVER, the difference is usually quite minimal (A f2.8 lens may have a T-Stop of T3.0, for example) but in top level filmmaking where time is a lot of money, it is a critical because all cinema lenses can be calibrated to the same T-Stop without having the adjust the aperture again every time there is a lens change. This means consistent lighting for each shot is easier to attain – A lot of money saved in crew fees!
That being said, knowing the T-Stop of a particular lens may be useful when comparing the speed of lenses, even if it includes photographic lenses, because all lenses have light loss, and therefore, have a T-Stop.
For example, when comparing between two lenses that have the same focal lengths and F-Stop number, such as the Canon EF 24mm f1.4L II USM, and the Nikon AF-S Nikkor 24mm f1.4G ED, the Canon has a T-Stop of T1.6 while the Nikon comes in at T.17. Therefore the Canon is the faster lens. And cheaper as well, at USD1749, compared to the Nikon which is USD1999.95 (Prices from BHPhotoVideo)
Calculating T-Stop is a complicated mathematical affair, for those who can, good for you. For the rest of us, some kind folks have submitted nice reviews and lens performance statistics at DXOMark.com. There’s even other details like chromatic aberration, vignetting and sharpness. It’s always great to know public opinion before you place your trust in a product.
So the next time you choose a lens, use the site as a reference and take its T-Stop into consideration!