The Difference between F-Stops and T-stops. What’s Up with That?
Chatting with one of my editors the other day when he asked me the practical difference between a t-stop and f-stop. I gave him my dumb dumb explanation that F-Stops measure the size of the aperture and T-Stops measures the light. Well maybe it’s time to rehash the details so I can look like I am ‘extra betterer’ than everyone else if asked again, and so I can keep getting paid more.
F-Stops. I know what they are, but what are they?
Ok, so an F-Stop is a calculated measurement of the iris diameter of the lens. It is not a measurement of the amount of light that is transmitted to the sensor or film. It represents the size of the aperture related to focal length. One stop difference doubles the light, or cuts it in half.
Since it is measuring diameter, and not recieved light, a f4 from two manufacturers will transmit differing amounts of light through the rear element. This is due to the lens design, coatings, and simple process variations changing the amount of light that is reflected and refracted before making it out of the back. Most lenses allow 60%-90% of incoming light through the lens. More expensive glass typically results in better light transmission.
T-Stops. The Final Stop in Measuring Light Transmission
On set, you either light the scene using a light meter, or these days using just the camera to expose. The T-Stop is an actual measurement of light transmission through the lens. Which to a cinematographer, makes a lot more sense. A T-Stop is determined by taking the F-stop and diving by the square root of the transmittance number in percentage:
A lens at f2.8 having a light transmission of 80% will result in a T-Stop of ~3.1
So on set, measuring your setup for T-stops and therefore using lenses marked with T-stops, you will not have to compensate for variations in exposure when you swap focal lengths. Think about post: if you are shooting the same setup using lenses marked with F-Stops. You will be close on your exposure, but even just a slight change is going to be one small extra pain in the ass down the road when it comes time to match cuts.