T-Stops vs F-Stops: How great cinematographers know when to pick which

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.

Posts created 3303

10 thoughts on “T-Stops vs F-Stops: How great cinematographers know when to pick which

  1. Man, you are not only so wrong, but also arrogant, ignoring all the comments pointing your misinformation. It's not the first I've seem here…
    It's very sad to see how begginers are being so flooded with uncorrect content. The internet is a wonderful place for study, but so tricky.

    The issue here is far from being an opinion. F-Stop is: the ratio between the focal length and the diameter in mm of the apperture. Period.
    Please, study more before putting this kind of content.
    Be responsible.

  2. I think the little "t" is reserved for time in physics or some time is handled by the shutter, also a "t" on the knob.

  3. Wrong.
    F-stop is not theoretical. Neither is a measure of light. It's a geometric metric of the relation between focal length (f) and the diameter of the iris.
    Light gathering is +/- proportional to the f-stops and that is a consequence of the diameter of the iris (among other less important factors).

  4. question l want to buy samyang 85mm f1.4 or samyang 85mm t1.5 price is almost same just litle expnesive t1.5 mostly l make video but also take photo if l buy t.1.5 is it l lost any fonction for photo and when l m focus from t stop is it work focus sound from dslr 7d still cant decided thank you l wait your answer

  5. The problem is people don't understand what "theoretical" means. Just because mathematics is used to calculate something does not mean it isn't theoretical. Almost all values calculated using mathematical formulas are theoretical because they do not factor in real world losses. These losses usually cannot be calculated using a formula for a number of reasons, and this have to be MEASURED. Formulas are theoretical simplifications of real life and make life easier…

    In this example with tstops, this is why the T stop cannot be calculated with a formula. The f stop however can be and is used as a theoretical approximation of the T-stop. In photography it also has the use of giving information about the depth of field. In terms of measuring light transmission, it is theoretical because it doesn't factor in the real world losses of light.

    Great video!!!

  6. Question- why is it that longer focal length lenses needed a larger pupil to illuminate the film or sensor equally. In other words why does f/2 on a 200mm lenses need to be 4 times larger than f/2 on a 50mm to give an equal exposure given fixed ASA and shutter speed?

  7. f/number = FL/number = # (=aperture diameter) = DoF (reference)

    Aperture Diameters, is purely an IRIS OPENING SIZE = that is RELATED (relatively high vs low values) for LIGHT (Brightness) WITHOUT GLASS OPTICS at all (so, this is why it's ONLY EXACT for DoF, but APPROXIMATE for BRIGHTNESS)

    T-values is F-values for LIGHT (Brightness) WITH GLASS OPTICS included specific to the lens itself (so, here, it's INTENTIONALLY EXACT for BRIGHTNESS via TRANSMISSIVITY of the Lens's Glass Optics, and only APPROXIMATE for DoF)

    If a lens maker (lens testers) LISTS BOTH, then one knows:

    F-numbers better speaks to the EXACTNESS for DoF Only
    (Brightness is Approximate, because it ignores glass transmission inefficiency)
    Aperture Diameter is EXACT (it is how f-value is derived)

    T-numbers better speaks to the EXACTNESS for Brightness Only
    (DoF is Approximate, because actual aperture diameter is unknown)
    Glass Optic Transmission is EXACT (measured via test)

  8. 1:55 – You aren't sure why f-stops are written with a '/' and T-stops aren't?
    Because the first is a ratio of two numbers, and the second isn't.
    f-stop is aperture diameter divided by focal length; T-stop is a measurement of light transmission.

  9. isn't it the case though that T-stop lenses for DSLR and mirrorless cameras have non-clicking aperture rings so you can pull focus and move t-stop w/o out jittery clicks and what not? I'm new to T-stops and been looking at SLR magic among other lenses that would be fun to shoot a more "old school" look for most of my personal filming projects on sony a6400.

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