f-stop vs T-stop: why the same aperture number means different actual light on different lenses
A cinematographer explained why two lenses both marked 'f/2.8' didn't expose identically on set — one was actually letting through noticeably less light due to its specific glass elements, which is exactly what T-stop ra
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
A cinematographer explained why two lenses both marked 'f/2.8' didn't expose identically on set — one was actually letting through noticeably less light due to its specific glass elements, which is exactly what T-stop ratings are designed to reveal.
The factor below is the one that ends the argument — write it once, then stop improvising mid-job.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
T-stop accounts for a specific lens's optical light transmission loss — there's no universal f-stop-to-T-stop conversion factor
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Assuming there's a fixed conversion factor between f-stop and T-stop that applies across all lenses — T-stop loss depends on each specific lens's glass elements and coatings, so it has to be measured per lens, not calculated from f-stop alone.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in f-stop (theoretical).
- Confirm the target unit is really T-stop (this lens, illustrative), not a similar-looking one.
- Apply the factor: T-stop accounts for a specific lens's optical light transmission loss — there's no universal f-stop-to-T-stop conversion factor.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A cinema lens is marked f/2.8, and its datasheet also lists a T-stop rating that differs slightly.
A cinema lens marked f/2.8 might be measured at T3.1 on its own datasheet, reflecting roughly 0.3 stops of light lost to the lens's specific glass and coatings.
That's T3.1 for this particular lens at f/2.8 — a different lens design at the same f/2.8 could measure a different T-stop entirely.
If you want a second check, invert the conversion and see whether you recover the original quantity within normal rounding.
Quick reference table
| f-stop (theoretical) | Calculation | T-stop (this lens, illustrative) |
|---|---|---|
| 1 | 1 × — | — |
| 2 | 2 × — | — |
| 5 | 5 × — | — |
| 10 | 10 × — | — |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
Matching exposure across multiple lenses on a film set requires T-stop ratings specifically, since f-stop alone doesn't account for how much light each individual lens actually transmits.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Always use a lens's actual measured T-stop rating for exposure-critical work rather than assuming a fixed offset from its f-stop — the loss is lens-specific, not a universal constant.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
Try the converters mentioned in this article
FAQ
What is the exact f-stop (theoretical) to T-stop (this lens, illustrative) factor here?+
T-stop accounts for a specific lens's optical light transmission loss — there's no universal f-stop-to-T-stop conversion factor
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Assuming there's a fixed conversion factor between f-stop and T-stop that applies across all lenses — T-stop loss depends on each specific lens's glass elements and coatings, so it has to be measured per lens, not calculated from f-stop alone.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-26 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.