Online Tool Store
· 4 min read
How to Work in Stops Instead of F-Numbers
Heshan Fernando
Co-founder & COO
Going from f/2.8 to f/8 is three stops. Knowing that means you can immediately compensate — three stops slower on the shutter, or three stops more ISO, and the exposure is unchanged.
Not knowing it means arithmetic with the numbers 2.8 and 8, which do not divide into anything helpful.
Why the sequence looks strange
The f-number is focal length divided by aperture diameter. It is a ratio, and the reason it produces awkward-looking numbers is that light gathering depends on area, not diameter.
Halving the light means halving the area, which means dividing the diameter by the square root of two — about 1.414.
Start at f/1 and multiply repeatedly:
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
Each is the previous multiplied by √2, rounded to something readable. Every step halves the light. Once you see that the sequence is geometric rather than arbitrary, it stops needing to be memorised — 1.4 doubled is 2.8, 2 doubled is 4, 4 doubled is 8, and the whole scale is two interleaved doubling sequences.
| Stop | f-number | Relative light |
|---|---|---|
| 0 | f/1.4 | 1 |
| −1 | f/2 | 1/2 |
| −2 | f/2.8 | 1/4 |
| −3 | f/4 | 1/8 |
| −4 | f/5.6 | 1/16 |
Stops are the universal unit
The reason stops matter more than f-numbers is that they apply across all three exposure controls.
Aperture — one stop is a factor of √2 in f-number. Shutter speed — one stop is a factor of 2 in time. ISO — one stop is a factor of 2 in sensitivity.
That common unit is what makes exposure trades trivial. Close the aperture two stops, open the shutter two stops, and the exposure is identical with different depth of field. No arithmetic on the actual numbers is required.
Working in stops also makes flash, filters and lighting ratios speak the same language — a three-stop neutral density filter means exactly the same thing as three stops of anything else.
T-stops measure what arrives
An f-number is geometry: focal length over diameter. It assumes all the light entering the front element reaches the sensor.
None of it does. Every glass-air surface reflects a little and the glass absorbs a little, so a lens with many elements transmits noticeably less than its f-number implies.
A T-stop is measured rather than calculated — the aperture an ideal lossless lens would need to transmit the same light. It is always a higher number than the f-number of the same lens, and the gap grows with element count.
Cine lenses are marked in T-stops because matching exposure across shots and across lenses matters more than the geometric figure. Stills lenses are marked in f-stops because depth of field calculations use geometry, and the transmission difference is usually small enough to ignore.
Common mistakes to avoid
- Treating the f-number sequence as arbitrary values to memorise.
- Assuming two lenses at the same f-number transmit the same light — element count and coatings differ.
- Confusing a one-third-stop click on a modern camera with a full stop when converting.
- Forgetting that stopping down past a lens’s optimum introduces diffraction, so more depth of field costs overall sharpness.
- Applying a crop factor to the aperture when calculating exposure, which is a depth of field conversion rather than an exposure one.
How to do it with Aperture F-Stop Converter
The Aperture F-Stop Converter works in both directions.
- Enter two apertures to get the difference in stops.
- Apply that number directly to shutter speed or ISO to hold exposure.
- Use the T-stop note when matching exposure across different lenses.
- Remember that depth of field and diffraction both change with the aperture, not just the exposure.
Other photography tools are in the tools directory.
Frequently asked questions
Why is the f-number sequence so odd?
Because each full stop halves the light, and light gathering depends on area. Halving area means dividing the diameter by the square root of two, which produces 1.4, 2, 2.8, 4 rather than round numbers.
What is the difference between an f-stop and a T-stop?
An f-number is geometric — focal length over aperture diameter. A T-stop is measured and accounts for light lost in the glass, so it is always a slightly higher number for the same lens.
Does aperture affect anything besides exposure?
Depth of field and diffraction. Opening up reduces depth of field; stopping down past a lens’s optimum softens the whole image, usually somewhere around f/8 to f/11 on full frame.
Final thought
Learn the sequence as repeated multiplication by 1.4 and it stops being a list to memorise. After that, every exposure trade is arithmetic you can do without a calculator.