F-Stops vs T-Stops – What are they? and when do you use them?

F-Stops are a familiar term when it comes to photography. As more photographer start to delve into the world of video, though, they’ve started looking at cinema lenses. The familiar F-Stops are gone, replaced by T-Stops. What does it mean? Aren’t they both basically the same thing? Why do two seemingly identical lenses such as the two Samyang 20mm lenses above have different maximum F and T values? What’s the difference?

It’s a little difficult to sum up the difference in a sentence or two, but Sareesh Sudhakaran from Wolfcrow is here to help. In his latest video, Sareesh talks us through the differences between F-Stops and T-Stops. We also learn when it is better to use a lens which uses one measurement over the other, and which may be most cost effective.

https://www.youtube.com/watch?v=jYRJVRMlIe8

Essentially, an F-Stop is a mathematical equation. It’s based on the focal length of the lens relative to size of the opening through which light is allowed. A T-Stop is the actual measurement of light transmitted through the lens.

So, a 100mm lens at f/4 has an aperture opening of 25mm. That same 100mm lens at f/2 has an aperture opening of 50mm. Twice the diameter means four times as much area, so, two stops more light gets in. The focal length and aperture relationship allows a certain overall amount of light through the lens. This is why a 50mm f/1.4 has a much smaller aperture than a 105mm f/1.4 but still allows (roughly) the same amount of light to hit the sensor. In theory.

At one time, this was probably an extremely reliable and consistent measurement. When glass had little-to-no coatings, and lens design was very simple, there probably wasn’t much light lost. Pick up a few different lenses of the same aperture, and they’d probably be virtually indistinguishable.

Today, though, there are so many different lens coatings and strange designs that light gets lost all over the place. Half a dozen different lenses at f/4, even from the same manufacturer, can often yield different exposures at the same settings. The differences may not be huge, but placed side by side, they’re often noticeable.

These slight differences aren’t often a big deal to many photographers. If you’re using the in-camera meter, the sensor will account for these slight differences in light transmission and compensate. It may tell you to shoot 1/125th of a second with one lens, but 1/150th of a second with another, even though the scene, ISO and your aperture haven’t changed. The sensor sees the actual light transmitted through the lens.

The T-Stop, on the other hand, is an actual measurement of the light getting through the lens. While f/4 might meter differently on half a dozen different photography lenses, T4, on the other hand is identical across the board. At least, it is when it comes to quality known brands of cinema lenses.

Unlike photographers, cinematographers are often shooting the same scene with multiple cameras, multiple lenses, and from multiple different angles. Those slight differences are much easier to notice when switching from shot to shot.

And while these differences can be corrected in post, when shooting a 90 minute movie, it’s a lot of extra work. Michael Bay’s Transformers movies, for example, have an average shot length of 3-3.4 seconds. That’s around 1500-1800 individual clips. The time and cost to fix the inconsistencies in those in posts adds up very quickly.

For most of us lower budget types, modern technology can help to solve the problem. Monitors like the recently released Aputure VS5 have built in false colour, RGB parade, and a host of other features to show us what our sensor sees to get consistent exposure from shot to shot. This is why so many people want DSLRs and mirrorless cameras with clean HDMI output, even if they’re not recording externally.

Monitors like these help us to get around the issue of inconsistency. But they do still add extra time to the shoot itself. We simply need to double check and compensate for these inconsistencies as needed. And if we don’t do it at the point of capture, we have to do it in the computer. If you’re a small one or two man crew, you just factor this time in. When shooting personal projects, it just means it might take a little longer to finish. If you’re shooting for a client, you just include the extra time in your quote.

On a higher budget movie, though, those extra few seconds can add up across the course of filming. So, they just need to know that when they put T4 into a lens (any lens), they’re going to get a reliable and consistent exposure. They don’t want to be second guessing their lenses, or have to waste time taking more measurements on-set. They need to know the camera is seeing what they expect it to see.

With photography lenses, the higher end lenses are generally fairly consistent, at least with each other. They’re still not always going to be perfect, but they’re often close enough to not matter. If you get a Nikon 24-70mm f/2.8 and a Nikon 70-200mm f/2.8 from the same generation, for example, shooting at 70mm on either lens will generally yield the same amount of light transmission at a given aperture. But it can’t always be relied upon.

So, if you ever wondered why you’re getting inconsistencies between lenses, especially when shooting video, now you know.

Do you use the photography lenses for video? Or have you invested in cinema lenses with T-Stops to make your life and workflow go a little easier? Will you stick with your photography lenses for video? Have you spotted these inconsistencies in exposure yourself? How do you get around them? Let us know in the comments.