F-Stop vs. T-Stop: What’s the Difference?

Photographer Matt Granger asks the question, “When is an aperture of 2 not really a 2?”

The answer? When the f-number doesn’t match the t-number. But, what the heck is the t-number? And how is it different from the f-number? Below, Granger explains the difference between f-stops and t-stops and compares a few lenses:

Photographic cameras are normally measured in f (e.g., f/2.8, f/4) while cinema lenses are normally measured in t. Basically, the f-stop is the measurement of the opening of the lens; the t-stop is how much light makes it to the sensor.

Understanding F-Stop and T-Stop

F-stop is determined by the size of the diameter of the circle that the blades of the lens allow light to pass through. The numbers in an aperture—f/2.8, f/8—signify a certain amount of light, but that doesn’t necessarily mean that’s how much light is getting to your sensor.

F-stop is measured by the size of the opening at the front of the lens. A t-stop is a little trickier to measure since it is how much light, having passed through the aperture and through the elements in the lens, actually gets to your sensor. You do lose some light along the way.

All of the glass lenses inside a lens, as well as reflections, steal a little bit of light. It’s impossible to have a 100% transmission ratio. Depending on the quality of the lens you are using, the amount of light stolen varies. If you can see the lenses inside, that mean there will be more reflection and loss of light.

The amount of light that’s coming through the lens that you have to work with in terms of your exposure, balanced with your shadow speed and ISO, that’s t-value.

Comparisons of Lenses on the Market

With the Canon 85mm f/1.2, for example, you really can’t see a whole lot in the way of lenses inside and it’s quite an expensive lens, but you’re still losing a little light. This lens has a t-value of 1.4, which means that even though it will open up in terms of aperture, with an f-number of 1.2, you’re losing .2 on the way through the lens.

Here are a few examples:

Sony Planar 85mm f/1.4 has a t-value of 1.6.

what is a lens t-stop

As far as 70-200mm f/2.8 lenses go:

It just goes to show that price doesn’t necessarily determine quality. Watch the video for more examples and comparisons.

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