Your Aperture Is Sort Of Lying To You: When Is An Aperture Of F1.2 Not Actually A F1.2?

September 30, 2014 by Tiffany Mueller 16 Comments

When Is An Aperture Of F1.2 Not Actually A F1.2? By the time the light gets to your sensor, that’s when.

One of the first things we learn as photographers are F stops and how we can use them to properly expose a photograph, but there is also such a thing as T stops and we don’t always give them the attention they deserve. Of course, a T-stop may not be essential knowledge on every photo you take, but understanding what a T stop is will give you a better understanding of light, which is never a bad thing for a photographer to have. (It’s also helpful information to have in your bag if you’re going to be lens shopping soon!). And Matt Granger does an amazing job of explaining the difference.

The F-Stop vs The T-Stop

F Stop – The F stop is the measurement of the opening of the lens.

T Stop – The T stop (T= Transmission) is the measurement of how much light has passed through the aperture and actually made it to the sensor.

As we all know, a lens is made up multiple glass optics, or individual lenses inside the lens body. As light travels through the lens, each of those glass optics absorb part of the light, effectivly reducing the amount of light that actually hits your sensor. This happens regardless of your f stop, so even though you may be shooting at an aperture of f 1.2, your sensor may only be seeing an amount of light equivalient to f 1.4. It may not seem like there’s a big difference between a 1. 2 and 1.4, but it’s actually half a stop!

Check out Matt Granger’s video tutorial (below) as he eloquently breaks down the T stop, then you can head over to DXO Mark and check out the t-stop values for all your lenses.

Video Breakdown

[Matt Granger via Reddit | DXO Mark ]

About Tiffany Mueller

Tiffany Mueller is a photographer based in Hawi, Hawaii. You can follow her Twitter here and her personal life here.

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Camera Girl

Actually there is a big difference but not because of the light transmission, the design of the lens determins the f stop and the difference in the design of a cinema lens to a still camera lens is where the difference really is, a cinema lens have greater depth of field at any focal length than any still camera lens of the same value.
- David
  
  No.
  
  F-stop is just the relationship between the aperture to the focal length. It’s the same for all lenses no matter if the lens is a cinema or photography lenses.
  
  T-stop is how much light the lens transmits compared to an ideal lens of equivalent F-stop. The definition is the same no matter if the lens is a cinema or photography lens.
  
  The difference is that cinema is a bit more sensitive when it comes to exposure – why cinema lenses tend to use T-stop, because F-stops are not consistent in their light transmission over different lens designs.
  
  The reason why you get a greater depth of field in cinema is because super 35mm film exposes a smaller area of film than a 35mm photograph does. It’s roughly equivalent to APS.
- Film School Boy
  
  Wrong, the only reason we have ‘T’ stops is to allow film makers to maintain consistant exposure between different lenses….
Amede Phonon

Known by Magic Lantern users for a while already.
- Tiffany Mueller
  
  represent :)
Dan Sulla

Here’s another one did you know a 2×4 ain’t 2″ by 4″?
Marc Pujol

F-stop is the ratio of focal length to aperture size.
- Richard Wagner
  
  Then why is the Panasonic FZ200 a constant f2.8 through out the entire focal zoom range if that is true?
- Marc Pujol
  
  The same reason any zoom keeps a constant f-stop throughout the range. This article is a little confusing in that it crosses f-stops and t-stops. F-stop is a measure of the aperture opening and t-stop is a measure of the light passing efficiency of the lens, both have to be taken into account to measure the amount of light hitting the sensor. DSLR’s meter after the lens so t-stops are compensated for by default.
Mauricio Tamaio

Luis Cruise fyi
Luis Cruise

Thank you Mauricio Tamaio
Andrew Sible

But why did do give the same lens achieve various t-stop values for a variety of sensors? Organize by t-stop, Nikon only, and watch the 50mm f1.4d….. ??? I’m confused.
thebeline

I don’t get it. I mean. An f-stop is a ratio, while a t-stop is a measurement of transmission. They are two wholly (tho similarly conceptualized and having the same effect on exposure) different things.

While, yes, your lens optics may absorb some light, that will not affect the aperture stops, and will only act as though the scene is actually darker. This is a LOT like the fact that ALL digital cameras’ ISOs are not precise. Which would have the same effect.

Now, where F vs T stop gets really interesting is in lenses like the Minolta STF lens:
http://en.wikipedia.org/wiki/Minolta_STF_135mm_f/2.8_T4.5_lens

This lens had an actual aperture of 2,8 wide open, but contained a radial graduated ND filter inside (essentially) that gave it an effective wide open aperture of T4.5. THAT is where things get cool… Not in this whole “my glass is purer than your glass.” Hocus pocus.

Put the lens on the camera, expose, shoot. User aperture the way it is meant to be used, and guess what? It won’t make a bit of difference to you one way or another.
David Negrete

would this then, based on the T value, make it inaccurately using a light meter and perhaps would it be best to use the cameras metering system?
Frank Nazario

So the more i read … the more i see … the more i research …im seriously considering the Sigma Art line of lenses as my workhorses (and save some $$$ to boot!).
AnthonyD42

This really threw me the first few days working on my first feature film. I still don’t know technically exactly what it means but, when shooting a scene with maybe 7 different lenses, any variation in the exposure, contrast or color in the lenses would be apparent on the big screen as they cut from angle to angle. So it’s crucially important for the lenses to match. That’s why cinema lenses are marked with T Stops (and what they are sooooooooo much more expensive).