Diopters

Diopters or Close Up Filters are positive supplementary lenses. In the most basic sense, this means they’re glorified magnifying glasses. They are positioned in front of an existing lens, allowing that lens to focus on objects that are closer than its normal minimum-focusing distance. Diopters can screw into the lens’ front threads, clamp onto the lens barrel, or fit into a filter tray in a matte box. The glass element is usually a simple meniscus (curved) lens, but in some cases can be two lens elements glued together.

Diopter Strength

Diopters are specified by the reciprocal of focal length, this is called the optical power. Diopters are measured in meters so that a diopter with an optical power of 1 has a maximum focus distance of 1 meter (3.28'). This means that if you have a +1 diopter and you attach it to a lens whose focus is set at infinity, objects at 1 meter will now be in focus. A +2 diopter on a lens set at infinity will reduce the maximum focus to 1⁄2 meter (1.64'), a +3 diopter will reduce the focus to 1⁄3 of a meter (13.12"). It is basically 1/x*1m=maximum focus, where x is the diopter strength. While all of these numbers refer to the maximum strength, If you adjust the focusing ring to a distance closer than infinity, you can get the camera even closer.

Common diopters come in +1⁄2, +1, +2, +3, +4 and +5. Fractional diopters such as +3.2 and +3.5 are also available, and some can go as high as +7. Even low-strength diopters can help to reduce the minimum focus to a more useful range. Schneider Optics, for example, offers fractional diopters in 1⁄8, 1⁄4, 1⁄2 and 3⁄4 strengths for slight adjustments to minimum focus without undue sacrifice of farther focusing distances.

Stacking Diopters

Additionally, diopters can be stacked together for higher-power magnification. Brought together, a +3 and a +4 diopter will equal a +7, making for a focusing distance of 5.5" with the lens’ focusing ring at infinity. Please bear in mind:

• When stacking diopters, the higher power should be closer to the lens.

Each single-lens diopter will induce some chromatic aberration into the image, and stacking them only compounds the issue, so cinematographers need to exercise caution when combining diopters.

• Shoot at a Deep Stop to Mitigate aberrations.

above f/4 or f/5.6, if possible. Stopping down improves the overall performance of the lens and helps to reduce aberrations induced by the diopters.

Aberrations

Most diopters are simple single-lens elements that provide little to no correction of aberrations. Rather, they can induce their own — especially chromatic aberrations, which happen when various color-wavelengths of light are improperly refracted by the lens and don’t meet correctly at a fixed focal point. When that happens, you start to see color fringing around the image. To correct for this, higher-end dual element diopters are made with two lens elements glued together in an achromatic doublet. These diopters are considerably more expensive (and heavier), although in many cases they’re still more affordable than high-end specialty macro lenses.

Split Diopters

These lenses are literally split; they’re cut right down the middle so that the lens holder has half an open space and half a diopter, allowing the cinematographer to apply the close-focus application of the diopter to only a portion of the frame and utilize the camera’s lens, on its own, for the remainder. This results in two distinct planes of focus in the image — one distant (the open area) and one close (the diopter).

To allow for a range of applications, split-field diopters are available in a variety of sizes and shapes beyond the traditional half-circle. For example, there are thin strips cut from diopters that can be mounted in various positions in a holding frame; more exotic variations include center-hole diopters that have a hole bored out of the middle, like a donut.

Focusing with Split Field Diopters

When using a split-field diopter, it is best to set the far distance on the lens first and then slip the desired diopter strength into the matte box, then move the closer item toward or away from the lens until it lands in sharp focus. Bear in mind that the two points of focus cannot be individually adjusted.

Minimum Focus Calculation

When you add a close-up lens to a camera which is focusing at the shortest distance at which the objective lens can focus, and you don't change the focus adjustment, the focus will move to a distance which is given by following formula: X = X / (DX + 1)

X being the shortest distance at which the objective lens can focus, in m, and D being the Diopter value of the close up filter. This is the minimal working distance at which you will be able to take a picture with the close-up lens.

For example, a lens that can focus at 1.5 m combined with a +3 diopter close up lens will give a closest working distance of 1.5/(3*1.5+1)=0.273 m.