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Your Lens’ Aperture Might Be Lying to You, Or: The Difference Between F Stops and T Stops


We all understand what an F stop is and how it’s a vital component in ensuring we have a properly exposed image, but have you ever heard of a T Stop? While they might not be as relevant to you in your day-to-day photography as F-stops, knowing what they are will give you a better understanding of how your glass works.

The video above does a good job of explaining the difference, but let’s elaborate a little further. F stop is the measurement of the opening of the lens; T stop on the other hand is the measurement of how much light passes through aforementioned opening and actually makes it to the sensor.

Lenses are made up of multiple elements of glass, pieced together in specific groups. As the light passes through these elements, each of the pieces of glass absorb some of the light, effectively minimizing how much of the light actually reaches the sensor.

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This means that even if you’re shooting at f/1.2, your sensor may only be receiving the light equivalent of f/1.4 – that’s a full half-stop of light.

This also means that not all f/1.2 lenses are created equal. For example The Canon 50mm f/1.8 has a t-stop of 2.1, while the Nikon 50mm f/1.8 has a t-stop of 2.

If all of that seems a bit confusing, the video at the top by Matt Granger does a good job of explaining the process in detail. So, whether you’re just looking for more knowledge or plan on going lens shopping soon, it’s well worth the watch.

And if you’d like to see what the F stop to T stop ratio is on the glass in your bag, head on over to DxO Mark.