Camera calculator · free in CineMath
Close Focus Calculator
Magnification, effective aperture and light loss for extension tubes; new close-focus distance for diopters.
Close Focus covers the two ways to get closer than a lens allows. Extension Tube mode gives magnification, effective aperture and light loss, so you know the frame size and the exposure change before the insert. Diopter mode gives the new minimum focus distance, where the lens focuses at its infinity mark, and roughly how much more magnification you get.
Worked examples
Extension tubes at f/2.8
| Lens and tube | Magnification | Effective aperture | Light loss |
|---|---|---|---|
| 50 mm + 12 mm | 0.24× | f/3.5 | 0.6 stops |
| 50 mm + 25 mm | 0.50× | f/4.2 | 1.2 stops |
| 50 mm + 68 mm | 1.36× | f/6.6 | 2.5 stops |
| 100 mm + 25 mm | 0.25× | f/3.5 | 0.6 stops |
Diopters on a lens that focuses to 0.45 m
| Diopter | New minimum focus | At the ∞ mark | Magnification gain |
|---|---|---|---|
| +0.5 | 36.7 cm | 2.00 m | 1.23× |
| +1 | 31.0 cm | 1.00 m | 1.45× |
| +2 | 23.7 cm | 50.0 cm | 1.90× |
| +3 | 19.1 cm | 33.3 cm | 2.35× |
What you enter
- Mode: Extension Tube or Diopter
- Extension Tube: focal length (mm, presets 50 to 180), extension (mm, presets 12 to 68) and stop (presets f/1.4 to f/8)
- Diopter: the lens's minimum focus distance, in feet or meters, and the diopter strength (presets +0.5 to +5; negative values work too)
What you get
- Extension Tube: magnification, effective aperture and light loss in stops
- Diopter: new minimum focus distance, focus distance at the lens's ∞ mark, diopter power and approximate magnification gain
- Notes at 1:1 or greater and at two or more stops of light loss
How CineMath works it out
- Magnification m = extension ÷ focal length, with the lens set at infinity
- Effective aperture = stop × (1 + m); light loss = 2 × log₂(1 + m) stops
- With a diopter of D: 1 ÷ new distance = 1 ÷ old distance + D; at the ∞ mark the lens focuses at 1 ÷ D meters; stacked diopters add
- Magnification gain ≈ old minimum focus ÷ new minimum focus
Guides
More Camera calculators
- Depth of FieldNear and far focus limits, hyperfocal distance, and the stop that holds two subjects in focus.
- Field of ViewHorizontal, vertical and diagonal angle of view, and frame coverage at any distance.
- Focal Length MatchFind the lens that gives the same horizontal view on a different sensor width.
- ExposureEnter a stop change and see the iris, shutter angle, ISO or ND that compensates for it.
- AnamorphicDesqueezed aspect ratio and horizontal-equivalent spherical focal length for any squeeze.