Cinematography & Cameras

T-Stops vs F-Stops

[mks_tabs nav=”T-Stops vs F-Stops“]
[mks_tab_item title=”Summary“]
F-stop is the number next to f such as f/2.8, f/4 and so on.  F-number is just the number without the f, written as 2.8, 4.0, 5.6 and so on.  The f number is a theoretical measure of the light hitting your lens based on your focal length.   T-stop is a more precise measurement and accounts of the light transmission that is lost in f-stop.

Lenses are either marked with a f-stop or a T-stop.   Photography usually uses f-stop and cinema uses T-stop.  T-stops require a lot more testing and usually more manufacturing cost, hence you don’t find T-stops in photography or less expensive lenses.

Video by Wolfcrow

[/mks_tab_item]
[mks_tab_item title=”Transcript”]

Hi I’m Sareesh Sudhakaran and in this video, I’ll explain as simply as I can, what T-stops are, how they are different from F-stops, and more importantly, when you should you pick which.

If you haven’t already, please watch the video on aperture and f-stops first. F-stops come from the f-number, where f typically stands for the focal length. F-number is the number itself, like 2.8, 5.6 and so on; while f-stop is the number written as the denominator to f, the focal length. So f/2.8 or f/5.6 is F-stop, while just the number is f-number.

I’ve explained in the earlier video where the numbers come from. The important thing to know here is that the f-number is a theoretical number.

Let me explain that with a simple example. You want to drive from your home to a movie, and you fire up Maps to find out how much time you might take.

The app gives you a number, say half an hour. Now, nobody actually drove that route to find the time for you. It used some data and some analysis to figure out its best guess. It’s a theoretical result based on physics and math. Same with the f-stop. The f-number is a theoretical measure of the light hitting your lens based on the focal length.

When you actually drive to the movie hall, you’re bo und to be off the app’s estimate.

Sometimes, you’ll be close; sometimes it’ll be completely inaccurate. T he light that hits your lens loses energy, is reflected a bit, is refracted a bit and absorbed by the insides of the lens. So it’s not going to be as bright at the other end.

This means, the actual light transmission is always lower than the f-stop. This new result, more accurate, is the T-stop – where T stands for transmission.

Whatever the lens is calibrated to is clearly mentioned on it. If it uses f-stops you’ll see f, if it is T-stops, you’ll see a T. A T-stop is typically written in upper case without the slash. I’m not sure why that is, but one theory I have is if it was in lower case, it might be confused for an f upside down, so maybe they decided to keep it simpler.

Now here’s a fun fact. Let’s say you have two lenses from the same manufacturer, say a 55mm and 85mm Zeiss Otus. Both are designed for f/1.4. Do both lenses let through the same amount of light? No, as it turns out. The 55mm lets through T1.5 and the 85mm, lets through T1.7. So if you have a set of lenses from a manufacturer and decide to shoot a scene from different angles with different focal lengths, they might be slightly different in terms of the exposure they’ll require.

However, the older Zeiss Planar versions have the same T-stop of T1.6, so you really can’t generalize here. Somebody has to measure the results for you. Dxomark is one company that consistently measures T-stops, and I’ll link to their database in the description. Now, the question everyone wants to know is, why is f-stop mostly found in photography lenses and why T-stops are predominant in cinema lenses. The answer is pretty simple actually.

With photography, you meter through the in-camera meter. Mostly you turn a dial, the aperture, shutter or ISO till the meter reads 0.0, and you click. So whatever difference there is in exposure due to light transmission, it is eliminated.

Secondly, even if you ignore exposure and make a mistake, the difference is only about a third of a stop at worst, and with today’s cameras that’s an easy fix in Lightroom. In fact, most people won’t even notice the exposure difference in still images unless they are specifically looking for it.

Lastly, in order to mark each lens with the T-stop, the lens manufacturer must test every lens individually, and mark them precisely. That adds a lot of time and effort to manufacturing, so it’s going to cost a whole lot more.

Through decades of professional photography experience, major camera and lens manufacturers have decided it’s not worth the extra expense to mark each lens with the T-stop, and f-stops are good enough for even the most demanding kinds of photography. Now, cinema is different. You have a rapid succession of shots on screen taken at different times or even days, and it’s critical to nail exposure. It’s expensive and time-consuming to correct it in post. Back in the days of film, there was no in-camera meter or waveform monitor. The lighting was set with a light meter and you really needed a rock-solid standard to get the exposure correct throughout.

Today, with the availability of different exposure tools like the waveform monitor and Zebras, you could make a case for using f-stops, which is why the DSLR video revolution was made possible. However, for major productions every minute costs thousands of dollars, so even if you save 15 minutes a day away from scopes, that’s enough justification. This is why, if you’ve watched my cinematography videos, you’ll know major cinematographers decide on a T-stop for each scene and stick to it. So if they decide to change focal lengths, a T2.8 should be the exact same across the board, at least for the same manufacturer.

Now major warning. This might not apply to cheaper pseudo-cinema lenses. But it is true of the best cinema lenses made by Zeiss, Cooke, Angenieux and so on.

So, the takeaway here is, if you’re using lenses with f-stops for video, use waveform monitors or the in-camera meters to get your shots to match exposure. If you’re using light meters, they typically have either f-stop and/or T-stop ratings. If they don’t, match it to whatever lens you have, what else are you going to do? For most productions, you really don’t need T-stops nowadays. Camera sensitivity has improved, and exposure correction is available in your editing software, so whether or not the savings in price is worth the time spent correcting it in post is for you to decide. For major productions like cinema and commercial work, you mostly like will benefit from T-stops. That’s all there is to it.

If you have any questions, please feel free to ask me. Hit the like button if you found this useful. To see more videos like this
one please subscribe. Bye now.

[/mks_tab_item]
[mks_tab_item title=”Shopping”]

[/mks_tab_item]
[/mks_tabs]

Comments