Close up lenses are simple and light. There are lots of cheap and shabby ones available. You really want a two element one like the Canon 250D or 500D. Two element (or achromatic) closeup lenses are corrected for chromatic aberration.

Many people refer to these lenses as "diopters" and that is perfectly fine with me. The word diopter does have a perfectly good technical definition, and it is a measure of the strength of the lens itself. Which sense of the word we are using will hopefully be perfectly clear from our context. (Your opthamologist specifies what kind of lenses need to be in your glasses using these units of diopters). I will explain the precise definition later, but for now, just know that a bigger number is stronger, which may not mean better.

The Canon 250D (4 diopter) is recommended for lenses from 30 to 135mm and is available in 52mm and 58mm. The Canon 500D (2 diopter) is recommended for lenses from 70 to 300mm and is available in 52, 58, 72, and 77 mm. (The 52mm Canon 500D goes for $72.00, The 77mm Canon 500D goes for $140.) (There is also a single element "Canon 500" that I am not interested in.)

Note that the Canon numbers (250, 500) refer to the actual focal length, in millimeters of the lens. See the section on diopter theory below. This is consistent with the 250 being a 4 diopter lens, and the 500 being a 2 diopter lens.

Since these things screw into standard filter threads, you are free to use Nikon closeup lenses on your Canon EOS lenses, if you can find them. The problem is that Nikon stopped making them back in 2006 or so. This is too bad, but keep your eyes open on the used market.

The Nikon diopters were available in strengths of 1.5 and 2.9 (who knows how these things are decided and why), whereas the Canon diopters are available in 2.0 and 4.0 diopter strengths. The Nikon 3T is a high quality 52mm two element 1.5 diopter gadget for $40.00. The Nikon 4T is a 2.9 diopter in 52mm, the 5T and 6T were 62mm and 1.5 and 2.9 diopter. It doesn't seem that Nikon ever made anything bigger.

I have been encouraged to try the Canon 500D diopter on the end of my 70-200 f/2.8L lens, and set it to 200mm for lots of working distance.

In March of 2011, I purchased a 77mm Canon 500D. The spec sheet that comes with it says that it is recommended for 70mm and longer lenses, but shows that it will work on both the 17-40L and the 24-70L lenses (when these lenses are zoomed to their longer focal length). And of course it will work on my 70-200/2.8L.

Diopter Theory

Let's dive into a little mathematics and define exactly what a diopter is, and what effect putting it on a known lens will have. A closeup lens, is itself a converging lens, and has a focal length. The shorter a lenses focal length is, the stronger it is, and the more curved (and tricky to manufacture) it's surfaces are.

If we have a closeup lens in hand and know it's strength in diopters, we can calculate its focal length by f = 1000 / d, which works as follows:

A 1 diopter lens has a focal length of 1000 millimeters (1 meter).
A 2 diopter lens has a focal length of 500 millimeters.
A 4-diopter lens has a focal length of 250 millimeters.

(Yes, I am told if you fiddle around with a closeup lens all by itself, you can make images and even take pictures with it).

To a close approximation, if you stack two diopter lenses, their diopters add. In other words, if you take two Canon 500D closeup lenses (which are 2 diopter used alone) and screw them together (not that this is recommeded), you would have a 4 diopter lens.

What happens when you put a 2 diopter closeup lens onto a 200 mm telephoto? First of all, the telephoto is itself a 5 diopter lens, although we don't usually think of it this way. If we add two diopters, the combination is a 7 diopter lens, which has a focal length of about 143 mm. We can boil this down to a general formula, where d is the closeup lens diopter, and F is the original prime lens focal length:

       N = 1000 / ((1000/F) + d)

Now if we do this to a 70-200 zoom set at 200, we have just changed the 200 focal length from 200 to 143, which we could have done by just zooming. However, if the 200 was set to focus at infinity and we added the closeup without changing the focus setting, it would now be focused at a closer distance, namely 500 mm away. With the new focal length of 143mm, this would be a magnification of 0.40.

My particular 70-200 lens is the Canon 70-200 f/2.8L (in the non-IS version). It has a close focus of 1.5 meters (4.9 feet), with 0.16 magnification. Let's look at what happens with a 2 diopter closeup added to that.

First a side note. If we add 12mm of extension, we get to 0.22 magnification, but if we add 25mm of extension, we get 0.41 magnification. Now that we have mentioned that, let's forget about it and go back to the diopter.

Without the closeup lens mounted, and set to closest focus (1500 millimeters), a 200 mm lens will be 231mm from the sensor. Lets hold that focus position, and mount the closeup, which makes it a 143 mm lens. Now we are focused on an object 375 millimeters away, and have a magnification of 0.62

Physics of lenses

All this is basic physics using the thin lens formula:

      1/f = 1/s1 + 1/s2

      M = f / (f-s1)

Note that s1 is greater than f to yield a real image on the opposite side of the lens. The real image will be inverted (and the calculated value of M will be negative). When s1 is less than f, we have positive values of M, but a virtual image and we are no longer talking about photography.