LensMath (speedboostercalculator.com) is a set of free calculators for photographers, videographers and cinematographers. It exists to take the lens maths out of your head so you can focus on the shot.
Our primary tool, the Speed Booster & Effective Aperture Calculator, shows the effective aperture, effective focal length and full-frame equivalent when you use a focal reducer (speed booster) or teleconverter on cameras with different crop factors.
Whether you are adapting vintage glass to a modern mirrorless camera or planning exposure for a film shoot, you get instant results without doing the multiplication yourself.
Who builds LensMath
LensMath is built and maintained by Mubeen. Mubeen builds and runs independent websites and web tools, and created LensMath to give photographers and filmmakers fast, accurate lens calculations without spreadsheets or guesswork.
How the calculator works
Every result comes from standard optical relationships, shown in full on the calculator page:
- Effective aperture = original f-number × booster ratio
- Full-frame equivalent focal length = focal length × booster ratio × camera crop factor
- Light change in stops = −2 × log2(booster ratio)
The maths runs in your browser and nothing you type is sent to a server. The results assume ideal optics, so they are theoretical values. They do not include light lost in adapter glass, vignetting or the quirks of a specific adapter. You can read the full explanation in our guide to speed booster effective aperture, and see the limits on our disclaimer page, Read our full methodology
Found a mistake or want a feature?
If a number looks wrong or you want a new camera, ratio or calculator added, tell us. Email mubeen.icit@gmail.com or use the contact page. Every message is read.