An f-stop describes aperture geometry. A T-stop describes exposure after the lens’s light losses are taken into account. Two lenses set to f/2.8 can transmit different amounts of light, while matching T-stops helps you keep exposure consistent when you change lenses.
For conversion, use T = N ÷ √τ, where N is the f-number and τ is the transmission fraction. To go the other way, use N = T × √τ. You need the lens’s transmission for an exact answer: T2.1 alone does not tell you its f-number.
What Is the Difference Between F-Stop and T-Stop?
F-stops describe the relationship between focal length and the effective aperture opening. T-stops use the same numerical scale but account for light lost inside the lens. Most photography lenses show f-numbers; cinema lenses commonly use T-stops to make exposure comparisons easier.
What Does F-Stop Mean?
The f-number is the lens’s focal length divided by the diameter of its entrance pupil:
N = focal length ÷ entrance-pupil diameter
The entrance pupil is the image of the aperture opening seen through the front of the lens. It is not simply the physical iris or the front glass element. Edmund Optics explains this distinction.
For example, a 50mm lens with a 25mm entrance pupil is set to f/2: 50 ÷ 25 = 2. This is its maximum aperture only if the pupil cannot open any wider at that focal length.
For a given focal length, a smaller f-number means a wider effective opening. You can adjust the aperture of your lens to control exposure and depth of field. The diagram below shows the familiar f-stop scale.
The standard full-stop sequence is approximately f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11 and f/16. Each step multiplies the f-number by √2, reducing the aperture area by half. Those printed values are rounded; f/2 to f/4 is two stops, not one.
© Ben Grant
What Does T-Stop Mean?
The T in T-stop stands for transmission. The number describes the exposure the lens delivers after losses through reflection, absorption and scattering. It is the f-number corrected for the light the lens actually transmits, not a direct measurement of aperture diameter.
As Sony’s T-stop explanation shows, lenses with the same f-number can have different T-stops. Its example is an f/2 lens with 75% transmission, which works out to about T2.3.
Under the definition used here, transmission is greater than zero and no more than 100%. The T-number is therefore at least as large as the corresponding f-number. They are equal for an ideal lens with 100% transmission; losses make T larger.
How to Convert F-Stop to T-Stop
MovieLabs’ camera metadata specification defines T-stop as the f-number divided by the square root of transmittance. You can do this calculation with a scientific calculator:
- Divide the transmission percentage by 100 to get a fraction. For 90%, use 0.90.
- Find the square root of that fraction.
- Divide the f-number by the result.
For an f/2.8 lens with an assumed transmission of 90%:
T = 2.8 ÷ √0.90 = 2.951, or approximately T3.0 when rounded to one decimal place.
Do not put 90 into the formula in place of 0.90. Also, 90% is a hypothetical input here, not a measured value for every f/2.8 lens.
How to Convert T-Stop to F-Stop
Rearrange the same formula to get N = T × √τ.
For T2.1 with an assumed transmission of 90%, N = 2.1 × √0.90 = 1.992, or about f/2. With 80% transmission, the same T2.1 works out to about f/1.88. This is why there is no universal T-stop-to-f-stop conversion chart.
| Known value | Assumed transmission | Calculated result |
|---|---|---|
| f/2 | 90% | T2.11 |
| f/2.8 | 90% | T2.95 |
| T2.1 | 90% | f/1.99 |
| T2.1 | 80% | f/1.88 |
These are worked examples rounded to two decimal places. Use a measured transmission value for the particular lens and setting when accuracy matters.
How Much Light Does a Lens Lose?
If you know both N and T at the same lens setting, calculate the transmission fraction with τ = (N ÷ T)². Multiply that fraction by 100 to express it as a percentage.
For a hypothetical lens that is f/1.8 and T2.0, τ = (1.8 ÷ 2.0)² = 0.81. That is 81% transmission and 19% light loss, compared with an ideal lossless lens at f/1.8.
The exposure loss in stops is 2 × log₂(T ÷ N). For that example, it is about 0.30 stops. Subtracting 1.8 from 2.0 gives a numerical difference of 0.2, not an exposure loss of 0.2 stops.
T-stops follow the same full-stop ratios as f-stops: increasing T by √2 halves the transmitted exposure under otherwise identical conditions. T2 to approximately T2.8 is one stop; T2 to T4 is two stops.
How Can You Find a Lens’s Transmission?
Focal length and aperture alone cannot tell you how much light a lens transmits. Look for a manufacturer’s T-stop specification or a measured test of the exact lens. Check the aperture, focal length and test conditions rather than assuming that one rating describes every setting.
DXOMARK’s lens database reports transmission in T-stops for tested lenses. Its testing protocol describes a controlled light source and calibrated measurements; its overall transmission score averages wide-open measurements across focal lengths. Use the detailed measurements where available, rather than treating the summary score as a rating for every aperture or zoom position.
You can compare the brightness of two lenses at home with fixed manual settings and stable lighting. That can reveal a relative exposure difference, but it is not an absolute T-stop calibration unless the reference and test conditions are known.
Do Prime Lenses Always Transmit More Light Than Zooms?
No. Lens design, glass, coatings and internal losses all matter. Counting elements or looking at the price does not establish a transmission percentage. Kodak’s filmmaking reference guide identifies several of these factors in its explanation of T-stops.
Compare measured values for the lenses you intend to use. A larger gap between N and T indicates more transmission loss at that setting; it does not, by itself, tell you which lens is sharper or better for your work.
Why Do Cinema Lenses Use T-Stops?
When you are switching angles or swapping lenses between takes, matching T-stops helps keep exposure consistent. With the same scene brightness, camera, shutter time, ISO or gain, and filtration, two accurately calibrated lenses set to T2.8 should give similar exposure.
Check the image or waveform when precision matters. A matching T-stop does not promise identical colour, corner brightness or results from cameras with different settings.
© ShareGrid
Does Matching T-Stops Give the Same Depth of Field?
No. Depth of field depends on aperture geometry, focal length, focus distance and the capture and viewing conditions. Transmission loss affects exposure; it does not directly change the size of the entrance pupil.
For two lenses with the same focal length on the same camera, focused at the same distance, matching their geometric f-numbers is the relevant aperture comparison for depth of field. Matching their T-stops alone does not establish that their f-numbers are equal. Changing the iris to reach a particular T-stop can still change depth of field, because the opening itself changes.
Do You Need T-Stops for Still Photography?
For most still photography, f-numbers and the camera’s meter are sufficient. A digital camera can account for light reaching its meter when it chooses an automatic exposure. In manual exposure with fixed shutter speed and ISO, a lens swap can still change brightness.
Use f-numbers to plan aperture and depth of field, then check exposure. T-stop measurements help when you need to compare lens transmission or match exposures under fixed settings; their usefulness is not limited to professional filmmakers.
The photograph below shows the different markings used on a Canon cinema lens and a still-photography lens. The labels identify different measurements, so do not use them as a universal conversion rule.
Conclusion
Use f-stops for aperture geometry and T-stops for exposure after lens transmission losses. To convert between them, you need transmission: T = N ÷ √τ and N = T × √τ, with τ entered as a fraction rather than a percentage.
Matching T-stops helps you match exposure when other conditions stay fixed. It does not guarantee matching aperture size or depth of field. If transmission is unknown, use the lens specification, a measured test or an exposure check instead of guessing a conversion.
