Diopters and Digital Photography

CDC (Compact Disc Count) is a measurement used throughout this page. 1 CDC ≈ 0.4 inches (1 cm). A lower CDC means you can fill the frame with a smaller subject — which is what you want for small butterflies like blues and skippers.

Why Diopters?

If you own a camera with a fixed lens (not an SLR), you can't attach a macro lens. A diopter (close-up filter) is the solution. It does two things:

Adds modest magnification — a +5 diopter adds about 20% more image size at a given setting.
Changes your focus range — more importantly, it lets you focus much closer. The rule: divide 100 cm by the diopter power to get the maximum focus distance. A +5 diopter focuses at 20 cm or less; a +2 focuses at 50 cm or less.

Comparison Photos

Photo taken at full zoom without a diopter — wide coverage, about 14 CD-widths across — Full zoom, no diopter

Photo taken at wide angle with +5 diopter at maximum focus distance — still wide at 16 CD-widths — Wide angle, +5 diopter (far)

Photo taken at full zoom with +5 diopter at 7.5 inches — very tight at 3.5 CD-widths — Full zoom, +5 diopter (close)

Without diopter at same distance — requires zooming back to half zoom, giving 8.5 CD-widths — Half zoom, no diopter (same distance)

No diopter zoomed all the way out at 2cm — 3.5 CD-widths but fisheyed and blurry at corners — Wide angle, no diopter (closest focus)

Diopter Power Trade-offs

+5 Diopter (Hoya)

Focus range: 15–19 cm (6–7.5 inches) at full zoom
Coverage: as tight as 3 CDC — frame-filling blues and skippers
Requires manual focus; autofocus unreliable
Very shallow depth of field; brilliant photos or nothing

+2 Diopter (Canon 500D)

Focus range: 31–50 cm (12–20 inches) at full zoom
Minimum coverage: ~5.5 CDC — fine for medium and large butterflies
Autofocus usable; larger focus range is more forgiving
Easier to use consistently, but less extreme magnification

Depth of Field

Close-up work is brutal on depth of field — you may have only a few millimeters in focus. This is determined by your f-stop and image size, not the diopter. To counter it, use a small aperture (large f-stop number, e.g. f/8). This costs light, so you'll need flash — but the results are worth it.

Tip: if you're struggling to focus, zoom out slightly. A smaller image size gives you more depth of field without moving the camera.

What to Buy

Stick to two-element diopters — single-element diopters will hurt image quality even when advertised as "multi-coated." You've spent hundreds on your camera; don't save $50 on the filter.

Hoya — high quality +3, +4, and +5 two-element diopters
Canon 500D (+2) and Canon 250D (+4) — good quality at a somewhat lower price
Diopters are stackable: +4 on top of +4 gives the effect of +8 (with some quality loss)