What is a T-stop vs f-stop?
An f-stop is a theoretical calculation based on the lens focal length and aperture diameter. A T-stop (Transmission stop) measures the actual amount of light transmitted through the lens, accounting for light lost to glass absorption and reflections. T-stops are always slightly higher than f-stops.
Why the difference matters
Light passes through multiple glass elements inside a lens. Each element absorbs and reflects a small percentage. An f/2.0 lens with 15 elements might only transmit as much light as f/2.2 — the missing light was lost in the glass.
For photography, this difference is handled by the camera meter — it measures actual light hitting the sensor, not theoretical transmission.
For cinema and video, consistent exposure between different lenses is critical. If you cut between two f/2.0 lenses with different transmission, brightness shifts visibly. T-stops solve this.
Where you see T-stops
Cinema lenses are always rated in T-stops. A T2.1 lens transmits exactly the same light as any other T2.1 lens, regardless of brand or design.
Photo lenses are rated in f-stops. Two different f/1.4 lenses may transmit slightly different amounts.
High-quality lenses with advanced coatings have T-stops very close to their f-stops — a well-coated f/1.4 lens might be T1.5, meaning very little light is lost.
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Should photographers care about T-stops?
For most photography, no. Your camera meter compensates automatically.
For video where you switch between lenses, understanding T-stops helps maintain consistent exposure.
The practical takeaway: a lens with better coatings and fewer elements will transmit more light — one more reason quality optics matter.
If you are building a cinema lens kit, matching T-stops across your set ensures exposure consistency between cuts. This is a key reason cinema lenses are sold in matched sets.