F-stops vs. t-stops: Understanding what this means

- If you've got enough of an interest in photography that you're watching this video, then you are probably already familiar with the term F stop. And you probably know that when you set the aperture on your camera's lens, you do so using an F stop scale. You probably know that changing the F stop changes the size of the aperture inside the lens which allows more or less light to pass through the lens. Changing the aperture makes for a brighter or darker image and because of the physics of light, changing the aperture alters the amount of depth of field in your image. You may not, though, have ever learned exactly what the F number itself means.

It's derived from a simple ratio, the ratio of the focal length of the lens to the size of the lens' aperture. That second number, the aperture, is a circle of course, and that's why F stop numbers are so strange. Since the calculation of the F stop involves the area of a circle, we end up with fractional numbers. 5.6, 3.5, those end up in our F stop scale. With F stop numbers it's important to know that the F number is not a measure of a specific level of brightness. Instead it's a measure of the relative change in light from one aperture to the next.

What this means is that F5.6 on one lens might not yield the same level of brightness as F5.6 on another lens. Different types of glass absorb more or less light than other types of glass so there's simply no guarantee as to how much light will actually transmit through the lens. For most situations, this will never matter to you at all. If you're trying to shoot the exact same image with different lenses though, this can create troubles. You won't necessarily get the same level of brightness from two different lenses set to the same aperture.

And if you change aperture to get equivalent brightness levels, then you'll get a change in depth of field. Now I can't think of any kind of everyday situation where you would ever encounter this. The only time I've ever run into it was when I was trying to compare image quality from two different lenses for a magazine article. Otherwise this has never been a problem for me so I doubt this is an issue that will ever cause you any photographic trouble. However, if you shoot video with your camera, differences in brightness from one lens to another might make trouble for you if you work with prime lenses.

When shooting video, you'll often change lenses to get different fields of view but because you'll be cutting shots from those lenses against each other, you want to ensure that you have equivalent brightness levels from shot to shot, from lens to lens. For this reason, the T stop, or transmission stop, was invented. T stops still keep the same F scale as F stops but a T stop of 5.6 means a specific quantified level of brightness. Any lens with a T stop of 5.6 will yield the same level of brightness as any other lens with a T stop of 5.6.

Even if you don't shoot video, you might encounter T stops when you're lens shopping. If you're considering a lens that has T stops rather than F stops, know that you'll use the lens in precisely the same way, you'll just also have the advantage of being able to switch to another T stop enabled lens and get the exact same exposure values for a given set of exposure parameters. This is the Canon CNE 50 millimeter T1.3 LF lens. It's a 50 millimeter lens with a maximum aperture of T1.3.

That means a very wide aperture, making this ideal for shooting in low light and offering the promise of very shallow depth of field. It's got a standard EOS mount lens, so this will work on any Canon EOS SLR or cinema camera, but because it uses T stops, I know that any aperture on here will yield the exact same brightness as the same aperture on any other T stop based lens. I look right down here, I see the T stop scale it looks exactly like an F scale, except that there's a letter T right there.

There's some other interesting features about this lens, though. The aperture ring, this one right here is not notched. It rotates smoothly, enabling me to choose apertures in between the normal third stop aperture settings. It also means that I can change apertures silently or smoothly change apertures while rolling. The lens also has these toothed rings, one for the aperture ring and one for the focus ring. These can be fitted to geared mechanisms that provide remote control of these rings that allows the focus puller or camera operator to manipulate aperture and focus without having to get their hands on the lens.

The focus ring itself is a really large diameter ring and it's got a tremendous amount of travel in it. This gives me an extremely refined degree of focus control. I can make very, very, tiny changes in focus because this can open all the way to F1.3. That's very important, my depth of field can be very shallow, I need to be able to precisely place my point of focus. Because this is a L lens, I know this is extremely high quality glass and that's part of what accounts for its 5000 dollar price tag.

Looks a little ridiculous here mounted to a 5D, and it is very heavy, but if I'm shooting video the extra features on this lens will be well worth it. Because T stops are generally only used on lenses intended for video shooting, you probably won't encounter them during normal lens shopping. If you do, know that they will work just like the lenses you're already used to, but with some added benefits that you may or may not need.