What is a Diopter?

by Dwight Lindsey

A Close-Up Lens is NOT a Diopter!

but everyone calls them that . . .

OK, it's time for me to rant a bit about "Diopters"

Close-up lenses or close-up filters are magnifying lenses mounted in front of photographic lenses and cine lenses to enable focusing closer than the lens will do by itself. They are quite useful to quickly obtain close-up images of small objects. Because they are usually simple single element plano-convex lenses, there will be some quality loss, but they're quick and easy and quite well accepted

These lenses are commonly referred to as "Diopters".

This makes me crazy, because it's simply not correct.

Diopter is a Unit Of Measurement

Diopter is a unit of measurement, much like other units of measurement such as millimeters, meters, inches, etc. It's not a physical thing that you can pick up and hold. It's a unit of measurement.

We don't call lenses "Millimeters", We call them lenses, with a notation of focal length measured in millimeters. So for example we could describe a specific lens as a "50mm f2 lens" we wouldn't call it a "Millimeter".

Likewise, we should refer to close-up lenses just that way, as close-up lenses. So one description would be: "138mm Round +1 Diopter Close-Up Lens". That describes the size of the lens (138mm diameter), the power of the lens (+1 diopter) and the type of lens (Close-Up Accessory Lens).

Now all of that said, it seems that "Diopter" has become the standard name for these things and despite some annoyance, we've used it too, because it's standard. Stupid, but standard . . .

Now that I've got that out of my system, let's have a look at why they are useful and some other ways to approach the problem.

Let's Get Practical - What's the Problem? Why do we need diopters?

The problem that needs solving is that many lenses simply don't focus close enough to get in tight on small objects. It's not uncommon for lenses to have a minimum object distance (MOD) of 3 feet. Even on lenses that focus closer, it's often not possible to get in tight enough on the subject. For example with a long focal length lens focused at 3 feet, about 1 meter, you might be able to frame a head shot, but capturing just a person's eye isn't possible without additional tools. That's where the "diopter" . . . cough, cough . . . Close-Up Lens, comes in handy.

Close-Up Lenses . . . I'm trying hard not to call them diopters . . . usually come in a variety of strengths such as: +1/4 diopter, +1/2 diopter, +1 diopter, +2 diopter and +3 diopter. Lenses can be made with negative diopter numbers . . . but we don't usually use them in photographic or cinematography applications.

We'll get a bit deeper technically in a minute, but in a practical way larger positive diopter numbers mean stronger magnification. For example +3 diopter is stronger than a +1 diopter. The +3 will let you focus closer and on a smaller object than the +1.

The most common type of Close-Up Lens . . . still trying not to call them diopters . . . is a plano convex lens. The lens is flat on one side and convex (bulges out) on the other side. These are relatively simple and cheap to make even in large sizes, as are required by large diameter cine lenses.

Unfortunately, plano convex Close-Up Lenses . . . note the caps on Close-Up Lenses . . . I'm striving for emphasis . . . come with penalties. They reduce resolution and worst of all, introduce chromatic aberrations (lots of pretty colors).

Plano convex Close-Up Lenses with strengths greater than +3 are certainly possible technically, but not generally useful in cinematography because image quality suffers. Image quality that might be acceptable in a photographic lens where small prints are the goal, just doesn't cut it when images are projected on the big screen.

Smaller diameter Close-Up Lenses with greater magnification as large as +10 are available in the photographic market however the image quality penalties are severe.

If you're considering using a diopter stronger than +3 for cinematography . . . just don't . . . the resulting image quality just won't be good enough. Instead you should consider using Achromatic Doublets or the Lindsey Optics Brilliant Macro Attachment Lenses, which are better than Apochromatic.

Let's Get Technical

If you're still with me, here's a short discussion of what diopter really means.

Lenses have optical power, which for multi-element photographic and cinematography lenses is usually expressed as a focal length measured in millimeters.

Just as one can convert from feet to inches and from inches to millimeters, it's easy enough to convert focal length measurements from millimeters to diopters. More on that in a bit.

The concept of "dioptre" was first introduced by a French Ophthalmologist in 1872. Internationally the word is still spelled "dioptre".

Dioptre just sounds so French . . . so elegant

For reasons not clear to me, in the U.S. we spell it "diopter". It's the same word, just more prosaic.

The most common use of measurements in diopters today is still by eye care professionals, optometrists and opthamoligists use the diopter as a unit of measurement because it is quite convenient for them. Prescriptions for corrective eyeglass or contact lenses are done with simple quick notations of positive and negative diopter units.  

In photography and cinematography, the diopter is also used as a unit of measurement for simple single element close-up lenses, because it is simple and convenient. When simple single element lenses are combined or stacked, the diopter strength of each lens can be added together. For example adding together 2 close-up lenses with strengths of +2 diopter and +3 diopter results in a combined strength of +5 diopter. It makes the math simple.

It's simple to convert from focal length in millimeters to focal length in diopters. The diopter value is the reciprocal of the focal length in meters.

So: 1/1 Meter = 1 Diopter

1/0.33 Meters = 3 Diopter

1/2 Meters = 0.5 Diopter

1/4 Meters = 0.25 Diopter

So when used by itself as a single elment lens, a close-up lens with a power of +1 Diopter will focus an image at about 1 meter.

When you're using a close-up lens in combination with a camera and camera lens . . . and you set the camera lens at infinity, you'll get focus when the front of the close-up lens is 1 meter from the object. You can and will adjust the focus of the camera lens to focus closer.

So, perhaps obviously, with the camera lens set at infinity, a +1 Diopter close-up lens will be 1 meter away from the object that's in focus. A +3 Diopter lens will be 333 mm away from the object.

If for some reason you've got a single lens that's not marked for diopter strength, or perhaps you just want to confirm the makring, you can easily get a quick measurement of the focal length of a single element close-up lens just by imaging a bright light . . . like a window . . . onto a flat surface . . . like a wall. You'll find that easier if it's daytime outside and you turn the room light off . . . When you've got an image in focus, measure or estimate the distance of the lens from the image . . . that's the focal length.

So if the image is in focus and there's about 1 Meter or 1000 mm between the lens and the image on the wall, you've got a lens in your hand with an optical power of +1 Diopter.

Here's a list of common diopters and their focal lengths:

+1 Diopter = 1000 mm focal length

+2 Diopter = 500 mm focal length

+3 Diopter = 333 mm focal length

I hope you've found some of this discussion interesting. If you've read this far you'll know that a "Diopter Lens" is really a close-up lens with an optical power expressed in units of "diopter". If you have any further questions, feel free to contact me from the website, or direct to my email address:

Dwight Lindsey
Lindsey Optics, LLC

And now a word from our sponsor:

At Lindsey Optics we offer single element close-up lenses in the common cine sizes of 4.5" Round Drop-In and 138mm Round Drop-In. These can be mounted in relatively expensive and heavy Rod Mounted Matte Boxes. Typically "lightweight" clip-on matte boxes don't accommodate these easily.

Tray Mount Close-Up Lenses

We also offer our Brilliant² Tray Mount Close-Up Lenses that have the glass permanently mounted in a matte box tray, for quick and easy insertion into any 4x5.65 matte box. They fit nicely in lightweight clip-on matte boxes. These have the same single element plano-convex design as our 138mm Round Drop-In Close-Up lenses. They're just faster and more convenient to use.

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Brilliant Macro Attachment Lenses

Our Brilliant Macro Lenses don't have any of the image quality issues that accompany single element plano convex diopters (there's that word again). The Brilliant Macro lenses are every bit as sharp as the cine prime or zoom that you attach it to and don't exhibit any chromatic aberrations.

Brilliant Macro lenses can be stacked to achieve even greater magnifications, without sacrificing image quality. If you stack a Brilliant Macro Lens +2 and a Brilliant Macro Lens +3, the resulting combination has an optical power equal to +5 Diopter, with very high quality.

For a list of known compatible cine-prime and cine-zoom lenses, see our Brilliant Macro Lens Compatibility Guide

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