Free calculator
Close Focus Calculator
A 25mm extension tube on a 100mm lens gives 0.25× magnification and costs 0.6 stop, so f/2.8 works like f/3.5. A +2 diopter on a lens that focuses to 3 ft brings closest focus in to 1.06 ft. Enter a lens and tube, or a minimum focus and a diopter.
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This web tool refuses a negative diopter too strong for the lens to focus through, and shows quarter-diopter strengths as +0.25. Save, share and get the Pro tools in the app.
How it works
m = extension ÷ focal lengthThe magnification the tube adds with the lens set at infinity. At 1× (1:1) the subject is the same size on the sensor as in life.
Where
- extension
- tubes or bellows between lens and camera, added up(mm)
- focal length
- of the lens(mm)
effective stop = N × (1 + m) light loss = 2 × log₂(1 + m) stopsMoving the lens away from the sensor spreads the same light over a larger image. At 1:1 the loss is 2 stops: f/2.8 exposes like f/5.6. A camera that meters through the lens accounts for it; a light meter reading does not.
1 ÷ new minimum focus = 1 ÷ old minimum focus + DDiopter strength D is in 1 ÷ meters, so work the distances in meters. At its infinity mark the lens focuses at 1 ÷ D meters. Stacked diopters add their strengths. The magnification gain is roughly old minimum focus ÷ new minimum focus.
Where
- D
- diopter strength: +1, +2, +0.5…(1/m)
Diopter or extension tube?
A diopter screws onto the front like a filter, costs no light and keeps the lens's stop, but the lens can no longer focus far away. Extension tubes keep the lens's own optics and can reach 1:1 and beyond, but every millimeter of extension costs light and pushes the infinity mark out of reach. A diopter suits a close-up that keeps the exposure; tubes or a macro lens suit true macro.
How this compares with the CineMath app
The extension tube figures and positive diopters match the app. With a negative diopter the app reports distances even when no subject can be focused (a −2 on a lens that focuses to 0.9 m shows a closest focus of 1.12 m). This page says so instead, and gives where the lens now reaches infinity. It also shows quarter-diopter strengths as +0.25, not +0.2.
Worked example
50mm at f/2.8 with 25mm of extension, Super 35
- m = 25 ÷ 50 = 0.50×
- effective stop = 2.8 × (1 + 0.5) = f/4.2
- light loss = 2 × log₂(1.5) = 1.2 stops
- frame width at the subject = 24.89 mm ÷ 0.5 = 49.8 mm
Half life size: the frame covers a 50 mm wide subject, and the exposure needs 1.2 stops more light.
Quick reference
Extension tubes at f/2.8
| Lens and tube | Magnification | Effective stop | 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 + 36 mm | 0.72× | f/4.8 | 1.6 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 |
| 100 mm + 50 mm | 0.50× | f/4.2 | 1.2 stops |
Diopters on a lens that focuses to 1.5 ft
| Diopter | New minimum focus | At the ∞ mark | Magnification gain |
|---|---|---|---|
| +0.5 | 1.22 ft (37.2 cm) | 6.56 ft (2.00 m) | 1.23× |
| +1 | 1.03 ft (31.4 cm) | 3.28 ft (1.00 m) | 1.46× |
| +2 | 9.4 in (23.9 cm) | 1.64 ft (50.0 cm) | 1.91× |
| +3 | 7.6 in (19.3 cm) | 1.09 ft (33.3 cm) | 2.37× |
Save, share and get the Pro tools in the CineMath app
CineMath is a free download for iPhone, iPad and Mac, with this calculator built in. In version 1.3, coming soon to the App Store, CineMath Professional saves calculations to your history, shares them as links, and adds the Camera Database and Data Planning.
Questions
How much light do extension tubes lose?
Two times log₂(1 + m) stops, where m is the magnification. A 25mm tube on a 50mm lens (0.5×) loses 1.2 stops; enough extension for 1:1 loses 2 stops, so f/2.8 exposes like f/5.6. The lens still focuses closer than it could before, and depth of field gets very thin.
What does a +1 diopter do?
It lets the lens focus closer. Add the diopter strength to 1 ÷ the lens’s minimum focus in meters: a lens that focuses to 2 ft (0.61 m) with a +1 focuses to 1.24 ft, and its infinity mark now focuses at 1 m (3.28 ft). The image is about 1.6× larger at closest focus.
Can I stack diopters?
Yes, their strengths add: a +1 and a +2 work as a +3. On a lens that focuses to 0.45 m, a +3 brings closest focus to 19.1 cm.
Does a diopter cost light like an extension tube?
No. A diopter changes where the lens focuses, not the distance from the lens to the sensor, so the stop stays the same. Extension tubes move the lens out and cost light in proportion to the magnification.
Where does the lens focus at infinity with a diopter on?
At 1 ÷ D meters. With a +2 the infinity mark focuses at 0.5 m (1.64 ft), so nothing farther away can be sharp. Keep that in mind when a shot pulls focus from a close-up to the background.
Is the extension tube magnification exact?
It assumes the lens is set at infinity. Focus the lens closer and the magnification grows further, and lenses whose entrance and exit pupils differ in size change the effective stop. Treat the figures as the starting point for a test.