Ever since I made my video on f-stops,
I've been getting frequent requests for the T-stop version.
So, that's what we're gonna be covering today.
Let's get Undone.
[offbeat music] ♪ Gerald Undone ♪
♪ He's crazy ♪
What's happening, everybody? I'm Gerald Undone
and without this camera here, I am just a podcast.
So, what are T-stops? Well, they're just like f-stops
in the sense that they inform you of the effectiveness
of your lens, but the calculation is different.
If you remember from my previous video on the subject,
f-stops, or more appropriately, f-numbers,
are merely a ratio of the lens's focal length
to the diameter of its entrance pupil.
Basically, you can figure out how large your entrance pupil is
by dividing your focal length by your f-number.
And your entrance pupil is basically how the aperture
appears when viewed through the front of your lens.
So, a 70mm lens with the f-stop set to f/2.8 would have
an entrance pupil diameter of 25mm.
70 divided by 2.8 equals 25.
As your entrance pupil gets smaller,
less and less light will reach your sensor
and your depth of field will get deeper and deeper
and the same is true for T-stops, but rather than a ratio
based on focal length, a T-stop is a measure
of the transmission efficiency of your lens.
Or more simply, what percentage of the light
makes it through the lens with 100% being a perfect score
and zero light lost. This system is more useful
when you want exposures to match more precisely
and is the reason why cinema lenses used
for movie production are usually measured in T-stops.
You don't want a sudden shift in exposure
when switching angles during a scene,
so you need to be able to fit your cameras
with lenses that have the same transmittance.
F-numbers don't work for this
because even if you were to adjust two
different lenses to have the same entrance pupil size,
it doesn't mean the quality of the elements inside the lens
will be capable of the same light transmittance.
The way a T-stop is calculated is actually pretty easy.
You just take the f-number of your lens and divide it
by the square root of its transmittance score.
So, for instance, let's say that we have the same
70mm f/2.8 lens that we were using in our previous example,
but in this lens, only 80% of the light
that goes into it makes it out the other side.
So, you would take your f-number of 2.8
and divide it by the square root of that 80% or 0.8.
And the square root of 0.8 is .894.
So, you take your f-number of 2.8 and divide it by .894
and you get your T-stop of 3.1-- rounded.
Now, the T-stop of a lens will always be a bigger number
than its f-stop because you can't have a greater
than 100% transmittance. In fact, it's almost impossible
to have 100% transmittance because they're always
will be some light lost when you put that much glass
between the source and the destination.
But high quality lenses can actually get you
surprisingly close to 100%.
Take this lens here for instance.
This is a Sony 90mm Macro and it has a f-stop of 2.8
and a measured T-stop of 2.9. That means it has
a light transmission efficiency of about 94%
and would be considered a great performer.
Now, often when people learn that the differences
between f-numbers and T-stops, they wonder
why we use f-stops in the first place
because it seems like an inferior system.
When it comes to maintaining exposure, it definitely is,
but there are some applications where f-stops excel.
If you need to calculate your depth of field, for example,
then f-stops are much more useful,
because as I said in my previous video,
the diameter of your entrance pupil
is a major contributing factor to your depth of field,
where the transmission efficiency of your lens isn't.
So, the line typically gets drawn between photo and video,
with video requiring a more consistent exposure
between angles and shoot days,
where photographers would require a more reliable sense
of depth of field, because changes in exposure
aren't as jarring from photo to photo as they are with video.
But another advantage to f-stops would be
the reduced production lag and costs.
Because in order to know the T-stop of a lens,
it has to go through a much more rigorous testing process
in order to measure the light transmission
versus simply calculating a ratio based on f-number.
This makes f-stop lenses cheaper on average
and more widely available.
However, this is also the drawback to f-stops
because you can usually safely estimate your f-stop
if you know your T-stop, but the reverse is not true.
Since we know that T-stops are always higher than f-stops,
we can just move down to the nearest stop
in order to take a guess at our f-number.
For instance, in our example of a quality lens
with a T-stop of 2.9 or 3.0, it'll likely have an f-stop
of 2.8 after accounting for lost transmission.
But unfortunately, knowing that a lens
has a f-stop of 2.8 does not allow us
to safely assume its T-stop. That'll have to be measured
precisely in order to be useful.
Finally, when it comes to approaching this topic,
it's important to remember that this isn't a competition.
Often, these two terms are compared
quite adversarially, but as you now know,
they aren't separate concepts, and the T-stop formula
is based in large part on the f-number.
T-stops should instead be looked at as an extra step
that can be taken when exposure precision
is necessary, and as usual, it's just about
using the right tool for the job.
But, that's gonna be it for me. I hope you found this video
helpful or at least entertaining, and if you did,
make sure you leave it the old thumbs up
and consider subscribing if you haven't already.
But, if you did not find this video helpful or entertaining,
feel free to hit the dislike button twice.
Alright...I'm done.