Introduction
The Lens Speed Calculator compares the f-stop, T-stop, and light transmission percentage for any two lenses. Enter the f-stop or T-stop for each lens and the tool returns the transmission percentage for each, the stop difference between them, and the equivalent ISO adjustment needed on one camera to match the exposure of the other. Use it when building a multi-camera package, switching from photo to cinema glass, or evaluating vintage adapted lenses without published T-stop data.
What This Tool Calculates
T-stop is derived from f-stop using the lens transmission percentage: T = f / sqrt(Transmission). A lens at f/2 with 75% transmission has a T-stop of 2 / sqrt(0.75) = T2.3. The stop difference between two lenses follows standard EV arithmetic per the ANSI/ISO 12232 exposure standard: Stop Difference = 2 times log2(T1 / T2). Worked example: a Zeiss Supreme at T1.5 versus a vintage Leica R at f/1.4 estimated T1.9. Stop difference = 2 times log2(1.9 / 1.5) = 0.68 stops. To match exposure, raise ISO by two-thirds of a stop.
The Formula and How It Works
On a horror feature choosing between a Cooke S4 50mm T2 and a vintage Leica R 50mm f/1.4, the calculator showed the Leica was only one-third stop faster than the Cooke — far less than the raw f-stop comparison suggested. On a two-camera documentary with a Canon C70 (Canon 35mm f/1.8, est. T2.0) and Sony FX6 (Sony 35mm f/1.4 GM, T1.6), the tool showed the Sony was 0.67 stops faster, allowing the DIT to offset ISO before grading. On a commercial, the DP used T-stop data to justify a premium Zeiss Supreme rental by showing a consistent two-thirds stop advantage over house Sigma Art glass.
Real-World Examples
Lens Transmission Reference
Modern cinema lenses with multi-coated elements lose 0 to 0.3 stops versus their f-stop. Vintage uncoated glass loses 0.5 to 1 full stop at maximum aperture. Zoom lenses at maximum aperture lose 0.3 to 0.6 stops. The Zeiss Supreme Prime family is rated T1.5 with effectively 100% transmission at T1.5. Cooke S4/i lenses are rated at their T-stop with close to 100% transmission. Always source T-stop data from the manufacturer data sheet, not review sites.
Pro Tips and Common Mistakes
| Detail | Value |
|---|---|
| Always match T-stops across cameras, not f-stops, on multi-camera shoots. | |
| Two cameras at f/2 may have different exposures if their lenses have different transmission values. | |
| When using vintage adapted glass without T-stop data, expose a gray card at your native ISO and note the required f-stop — that is your practical T-stop for that lens on that body. | |
| T-stops matter most in the T1.0 to T2.8 range. | |
| Above T4, nearly all modern cinema glass reaches 99%+ transmission and the difference is negligible. |
Pro Tips and Common Mistakes
Pro Tips
- Lens speed affects every exposure decision a DP makes.
- Knowing the real T-stop — not just the marked f-stop — prevents exposure mismatches between cameras, helps justify lens package costs to producers, and speeds up the gaffer-DP conversation on set.
- This tool runs in your browser in seconds with no account required..
Common Mistakes
Frequently Asked Questions
What is the difference between f-stop and T-stop?
f-stop is the geometric ratio of focal length to aperture diameter. T-stop is the effective aperture after accounting for light lost to glass elements and coatings. A T-stop is always equal to or larger than the f-stop number. A perfect lens would have identical f and T values.
Why do cinema lenses use T-stops?
Because T-stops are exposure-accurate across different lens designs. A T2 on a Cooke S4 and T2 on a Zeiss Supreme produce the same exposure on the same camera at the same ISO. f-stops do not guarantee this consistency across brands and designs.
How much does transmission typically differ from f-stop?
Modern cinema glass: 0 to 0.3 stops. Vintage uncoated glass: 0.5 to 1 stop at maximum aperture. Zoom lenses: 0.3 to 0.6 stops. The difference is most significant at wide-open apertures on older designs.
Can I use photo lenses on cinema cameras?
Yes, with the right adapter. Photo lenses are calibrated in f-stops. You will need to estimate or empirically measure the T-stop. The calculator can work backwards from f-stop and an estimated transmission percentage.
Start Calculating
Use the calculator above to run your numbers before your next production.
