So you're getting into making DSLR video and shopping for a new lens. You done some research and narrowed down the choices. There's one lens that is f/1.4 and another that is T/1.5, but which one is better? It's hard to compare one lens to another when they aren't measuring the same thing. So what's the deal?
In brief, the f-stop is a measure of the size of the aperture in a lens, though it's commonly used by photographers as a shorthand for ability of a lens to transmit light. The T-stop is a measure of the lens's actual ability to transmit light. It might sound like splitting hairs, but the distinction has some very real practical implications for making video.
The f-stops Here: Focus
The f-number of an aperture, or f-stop, is a measurement of the size of the aperture, and is given by N = f/D, where f is the focal length, and D is the diameter of the entrance hole in the lens.
Depth of field and the appearance of out-of-focus elements are both determined by f-stops. The actual light transmission characteristics of the two lenses might be very different, however, including the look of the images they create depending on the glass elements the lenses contain. An f/2.8 from one lens will have the exact same depth of field as an f/2.8 on any other lens with the same focal length.
In practical terms, f-stops are useful for calculating depth-of-field. Using f-stops allows you to accurately predict the appearance of sharpness with any lens at any distance, a technique called focus calculation. This isn't vitally important for day-to-day picture-making, but for many types of photography, like landscape, architecture, and portraiture it is very useful to know exactly how each lens draws space.
Begin Transmission: Finding Exact Exposure
As light passes through the lens it gets reflected and diffracted. A little bit of light is lost every time photons pass from air to glass to air and between glass elements.
The T-stop is the transmission value of the lens, or the amount of light that actually reaches your camera sensor. All of the glass elements inside your lens eat up a little bit up light, and even the length of the lens barrel can affect how much light actually reaches the camera. So the T-stop starts by measuring aperture just like an f-stop, but it adjusts to account for the light transmission efficiency. A T-stop can be measured by T-stop = f-stop/Lens-Transmittance Percentage. So for example, an f/2 lens with 75% lens transmittance is a T/2.3.
It is impossible to have 100% light transmission through glass. For example, the Canon 85mm f/1.2 has a transmission value of T/1.4. (For those of you counting, that is a 1/2 stop of light!) "But if I'm only getting 1.4, then why not buy the f/1.4 lens?" Of course, that lens is going to lose light as well. For example, the Sigma 85mm f/1.4 has a transmission value of T/1.5.
Remember, the T-stop measures light, not any of the other picture qualities that are associated with aperture. A lower-quality lens with a lower transmission value could be upwards of 1/2 stop darker under the same lighting conditions, simply because it has a higher T-stop.
So why are cinema lenses rated in T-stops? With video, you are viewing 24 images or more every second. If you're trying to replicate a look between two lenses, even a small fluctuation in exposure value between shots will be very noticeable. A T-stop doesn't let you calculate focus, the way and f-stop does, but it does tell you exactly what your exposure should be. This is especially useful when using a hand-held meter instead of a sensor inside the camera.
Are T-stops Right for You?
Cinema lenses are more expensive than photography for several reasons, including de-clicked aperture and a long focus throw, but the main reason is the T-stop. If you are buying more than one lens, and using a hand-held or spot meter, knowing the T-stop of your lenses can save a lot of time and expense correcting your footage in post-production. Depending on your purpose, having T-stops on a lens might be completely worth the extra expense. This is especially true for larger productions like commercials and films.
Hopefully this helps you make sense of T-Stops and can inform your next lens purchase or rental decision. Looking at effective T-stops can sometimes make all the difference between buying a higher quality (more expensive) lens versus a cheaper knock-off. And, finally, if you'd like to know the T-stop for any given lens, you can look up the ratings on www.dxomark.com.