Today we’re talking about diopters. What do they do? How do you choose? How do you use them correctly? Let’s start with a simple explanation of what exactly a diopter is.
What are Plus Diopters?
You’ll find Plus Diopters on your equipment list among filters because they normally go in a matte box in front of a lens, but technically (because they don’t change any color, contrast, effect, or light quality/amount) they are lenses. A Plus Diopter operates like a magnifying glass. It is a convex piece of glass that goes in front of a lens and makes the image larger in the frame. In doing so, it also shifts the entire focus scale closer to the lens. This gives you the ability to focus on a closer subject than the minimum focus of the lens otherwise allows but also takes away the lens’ ability to reach infinity. The shortening of minimum focus makes them a very popular companion to a set of anamorphic lenses, which often have further minimum focus abilities.
Diopter Types
Plus Diopters come in different strengths, designated by a number. The typical range you might see is between “Diopter + ⅛” and “Diopter + 6”, with most applications landing you in the range of +½ to + 2. The higher the number, the closer you can focus your lens (and the closer to your lens the maximum focal distance becomes). Also, with higher magnification comes more distortion, so a longer lens with a low-value diopter will be less distorted than a wide lens with a high-value diopter.
Proper Use
There is a lot more to learn about what diopters are, but let’s get into how to use them without breaking them. Diopters often come in circular 138mm format. These would typically go into a bellow on your studio matte box, but with 2-stage diopter trays, you have the ability to get diopters into a 2 or 3 stage clip-on matte box. Once you’ve got it clamped into the diopter tray, take a minute before you slide it into the matte box to ensure it will fit without scraping. The higher the magnification of your diopter the more convex it is, and the more room it will need to fit in the stages of a matte box. For this reason, you need at least 2 stages back to back with no dividers on the matte box, try to keep the diopter as close to the lens as possible to ensure clearance. If you have to use a diopter with a filter, a good practice is to put the tray with the filter behind the tray with the diopter. Sometimes it’s easier to hold them together and slide them in simultaneously. If you are trying to use a diopter with a convex that seems like it will pass the front face of the diopter tray(the trays will say you shouldn’t use anything past a +2), you’ll need an empty 3rd tray in front and hold them all together before sliding all 3 in as a unit. This will allow at least one stage width of clearance for the convex portion of the diopter.
FYI
Once you’re absolutely sure you have clearance, lock it in and enjoy the new close focus of your lens! Keep in mind the focus scale you were using for that lens is now essentially meaningless as your values have shifted, so make sure your focus marks are appropriately changed.