Close Focus: Diopters, Extension Tubes and Minimum Focus

Updated

What minimum focus distance means on a cine lens, how far a +½ to +3 diopter brings it in, what an extension tube does to magnification and exposure, and how little depth of field is left up close.

10 min readIntermediate
  • Close Focus
  • Diopters
  • Extension Tubes
  • Inserts
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Minimum focus distance

Minimum focus distance (MFD, or MOD for minimum object distance) is the closest a lens can focus. On cine lenses it is measured like every other focus mark, from the sensor plane, the ⦶ mark on the camera body. Many makers also list the distance from the front of the lens, which is what decides how close the subject can actually get once the matte box is on.

ARRI Signature Prime close focus, from ARRI's published data.
Focal lengthFrom sensor planeFrom lens front
25mm0.35m (14″)0.13m (5.1″)
47mm and 58mm0.45m (18″)—
75mm0.65m (26″)0.43m (17″)
95mm0.85m (2′9″)—
125mm1m (3′4″, ARRI's rounding)—

For an insert of a ring, a watch face or an eye, the lens's own close focus is often not close enough. Then you add something.

Three ways to get closer

  • A close-up diopter screws or drops in front of the lens. No light loss and no change to the camera build, so it is the usual choice for cine primes. Focus marks no longer apply.
  • Extension moves the lens away from the sensor with a tube or bellows. It gives more magnification but costs light, and it is mostly used with stills-mount lenses.
  • A macro lens is designed to focus close, often to life size, with its own marks. The cleanest result, if the package has one.

Close-up diopters

A diopter's strength is its power in diopters, 1,000 divided by its focal length in millimeters: a +1 is a 1,000mm lens, a +2 a 500mm lens. It shifts everything the lens can focus on closer by that power.

Focus range with a diopter
1 ÷ new distance = 1 ÷ lens distance + D | at the ∞ mark: distance = 1 ÷ D

Distances in meters. Measure them roughly from the diopter, not from the sensor mark. Stacked diopters add their powers.

Where

D
Diopter power (+½, +1, +2 …)(diopters)
lens distance
Where the lens is set, for example its minimum focus(m)

Example: Lens focusing to 0.45m, with a +1

Calculation:
1 ÷ (1 ÷ 0.45 + 1) = 1 ÷ 3.22 = 0.31m | ∞ mark: 1 ÷ 1 = 1m
Result:
The lens now covers 31cm to 1m (about 1 ft to 3′3″)

Where each diopter focuses

With the lens at its infinity mark, the subject is sharp at 1 ÷ D.
Diopter+½+1+2+3
At the ∞ mark2m (6′7″)1m (3′3″)50cm (1′8″)33cm (1′1″)
New closest focus with the lens at its own minimum focus, 1 ÷ (1 ÷ MFD + D), meters and feet.
Lens MFD+½+1+2+3
35cm (1′2″)30cm (12″)26cm (10″)21cm (8″)17cm (7″)
45cm (1′6″)37cm (1′2″)31cm (1′0″)24cm (9″)19cm (8″)
65cm (2′2″)49cm (1′7″)39cm (1′4″)28cm (11″)22cm (9″)
90cm (2′11″)62cm (2′0″)47cm (1′7″)32cm (1′1″)24cm (10″)

Two things follow. The weaker the diopter, the more of the lens's focus range you keep: a +½ still reaches 2m at the infinity mark. And a diopter helps a long lens more: Canon gives the magnification at infinity as the lens's focal length divided by the close-up lens's, so a 100mm with a +4 (250mm) close-up lens works at 0.4×.

Extension tubes

Magnification, effective stop and light loss
m = extension ÷ f | effective stop = N × (1 + m) | loss = 2 × log₂(1 + m) stops

Magnification is for the lens set at infinity; focusing it closer adds more. The effective stop is the thin-lens result and assumes a symmetrical lens; ZEISS notes that how far a real lens deviates depends on its construction.

Where

extension
Tube or bellows length added(mm)
f
Focal length(mm)
N
Marked stop

Example: 50mm at T2.8 with a 25mm tube

Calculation:
m = 25 ÷ 50 = 0.5 | 2.8 × 1.5 = 4.2 | 2 × log₂(1.5) = 1.17
Result:
0.5× magnification, effective T4.2, 1.17 stops of light loss
Lens at infinity, marked T2.8.
Lens and tubeMagnificationEffective stopLight loss (stops)
50mm + 12mm0.24×T3.50.62
50mm + 25mm0.50×T4.21.17
50mm + 36mm0.72×T4.81.56
50mm + 68mm1.36×T6.62.48
100mm + 12mm0.12×T3.10.33
100mm + 25mm0.25×T3.50.64
100mm + 36mm0.36×T3.80.89
100mm + 68mm0.68×T4.71.50

Real lenses land close to the formula. Canon lists 0.24× to 0.39× for its 12mm tube on the EF 50mm f/1.8 II; the 0.24× at infinity is exactly 12 ÷ 50. For its 25mm tube Canon lists 0.53×, a little above the 0.50× the formula gives. Compared with a diopter, extension gives a shorter lens far more magnification, which is why a 25mm tube does on a 50mm what it takes a 50mm tube to do on a 100mm.

Magnification tells you the frame: the subject area is the sensor size divided by m. On a Super 35 sensor 27.99mm wide, 0.25× frames 112mm across, 0.5× frames 56mm and 1× frames 28mm, about the width of a watch face.

Depth of field up close

Depth of field at a given magnification
DoF ≈ 2 × N × c × (1 + m) ÷ m²

For the marked stop N, circle of confusion c and magnification m. Up close, depth of field depends on magnification, not on which focal length got you there.

Where

N
Marked stop
c
Circle of confusion(mm)
m
Magnification(×)

Example: Half life size (0.5×) at T2.8, Super 35 (c = 0.025mm)

Calculation:
2 × 2.8 × 0.025 × 1.5 ÷ 0.5² = 0.84mm
Result:
Under a millimeter of depth; at T5.6, 1.7mm

At life size it is 0.28mm at T2.8 and 0.8mm at T8. That is why inserts are shot stopped down with plenty of light, and why the subject or the camera usually moves on a slider or focus rail rather than the focus ring.

On set

  • Pick the weakest diopter that gets there. It keeps the most of the lens's focus range, so the shot can still move in depth.
  • Forget the engraved marks. With a diopter or extension on, the scale is wrong. Focus by eye on a large monitor, then measure from the front of the diopter or lens to the subject and write your own marks.
  • Tell the gaffer about the stop. Extension costs light, and depth of field up close is so thin that most inserts want T5.6 or deeper. The exposure triangle guide covers where the extra stops can come from.
  • Check the matte box and lights. At a few centimeters from the front element, the matte box, flags and the lamp's heat all get close to the subject.

Close Focus in CineMath

CineMath's Close Focus calculator has two modes. Extension Tube takes the focal length, the extension and the stop and gives magnification, effective aperture and light loss, using the formulas above. Diopter takes the lens's minimum focus, in feet or meters, and the diopter strength, and gives the new minimum focus, where the lens focuses at its infinity mark and the approximate magnification gain. It flags 1:1 and beyond, and light loss of two stops or more. The free web version does both in your browser.

For the depth of field once you are there, use the depth of field guide and the circle of confusion for your format.

Questions

What is minimum focus distance?

The closest distance at which a lens can focus, measured from the sensor plane, the ⦶ mark on the camera body, to the subject. Cine lens makers usually also give the distance from the front of the lens, which is what tells you how close the subject can physically be: an ARRI Signature Prime 75mm focuses to 0.65m from the sensor, which is 0.43m from its front.

How does a diopter change focus distance?

Add the diopter's power to the lens's focus distance in diopters (1 ÷ distance in meters). A lens focusing to 0.45m with a +1 diopter focuses to 1 ÷ (1/0.45 + 1) = 0.31m, and at its infinity mark it focuses at 1 ÷ 1 = 1m. Distances are measured roughly from the diopter, not the sensor mark.

Do diopters lose light?

No. A close-up diopter adds no exposure correction; Canon states its close-up lenses do not reduce the amount of light. Extension tubes do lose light, about 1.2 stops at half life size.

How do you calculate magnification with an extension tube?

With the lens set to infinity, magnification is the extension divided by the focal length. A 25mm tube on a 50mm lens gives 0.5×. Focusing the lens closer adds its own magnification on top; Canon lists 0.24× to 0.39× for its 12mm tube on the EF 50mm f/1.8 II.

How much depth of field do you get up close?

Very little: roughly 2 × N × c × (1 + m) ÷ m² for magnification m. At half life size, f/2.8 and a 0.025mm circle of confusion that is under a millimeter. At life size and f/8 it is 0.8mm.

Next step: before the insert day, work out which diopter or tube gets each shot. CineMath's Close Focus calculator gives the range, magnification and exposure change for your own lenses, and so does the free close focus calculator.

Calculators for this guide