How does a close-up diopter add magnification?

A close-up filter is a positive lens with a focal length in millimetres equal to 1000 divided by its diopter power. Placed in front of the main lens it lets the camera focus much closer, and the magnification gained at infinity focus equals the lens focal length times the diopter power divided by 1000.

How do I stack multiple diopters?

Diopter powers add directly. A +2 and a +4 stacked behave like a single +6 filter. Always put the strongest diopter closest to the lens to minimise aberrations, and expect some loss of edge sharpness with strong stacks.

What is the new working distance?

When the prime lens is set to infinity, the subject in focus sits at the diopter focal length, which is 1000 divided by the total diopter power. A +4 filter focuses at 250 millimetres; a +10 filter focuses at just 100 millimetres from the filter.

Why is there a near and far focus limit?

The lens can still rack its own focus. At infinity setting the subject is at the diopter focal length, the far limit. At the lens minimum focus setting the subject is closer, the near limit. The diopter shifts the entire focus range nearer, giving a usable working band rather than a single distance.

Do close-up filters lose any light?

Unlike extension tubes, close-up diopter filters cause no light loss because they do not increase the lens-to-sensor distance. This makes them attractive for handheld macro, though high-power single-element diopters can soften corners and add chromatic aberration.

Close-Up Lens Diopter Calculator

Close-up diopter filters are the cheapest route into macro: they screw onto the front of any lens and pull its focus range dramatically closer. This calculator tells you exactly what magnification and working distances you will get for any lens and diopter strength, including stacked filters.

How it works

A close-up filter is simply a weak positive lens. Its own focal length in millimetres is:

diopter focal length = 1000 ÷ diopter power

So a +4 filter has a 250 mm focal length, and a +10 filter has a 100 mm focal length.

Working distance. When the camera lens is set to infinity, the in-focus subject sits exactly at the diopter’s focal length in front of the filter. That is the far focus limit. As you rack the lens to its own minimum focus, the subject moves closer — the near limit. The near distance is:

near = 1000 ÷ (diopter power + 1000 ÷ lens MFD)

where lens MFD is the bare lens minimum focus distance.

Magnification gained. With the lens focused at infinity, the added magnification is:

extra magnification = (lens focal length × diopter power) ÷ 1000

A 100 mm lens with a +4 filter gains 100 × 4 ÷ 1000 = 0.4× extra. Combined with whatever the bare lens already does at close focus, this can push a normal lens well into macro territory.

Stacking diopters

Diopter powers add. A +2 plus a +4 behaves as a single +6. Mount the strongest element nearest the camera and expect softer corners as power rises — single achromatic doublets handle stacking far better than cheap singlets.

Notes and tips

No light loss. Unlike extension tubes, diopters do not extend the flange distance, so your exposure does not change.
Depth of field shrinks fast at high magnification — stop down and use focus stacking for usable sharpness front to back.
Front element size matters. Match the filter thread to your lens, or use a step ring.

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