Diopters - Close-up Lenses:
The terms are often interchanged, but, correctly they are called 'diopters'. While they can be purchased singly, I feel it best to purchase a set with varying strengths as it gives a great deal of flexibility in their use. The picture to the right is of a set of 3 Hoya diopters in strengths +1, +2, and +5 - while it's not apparent from the photograph, there is optical glass inside the rings. .These lenses, essentially like filters, are varying in strengths and effectively are the same as used in eyeglasses to correct far-sightedness. In this case, the allow the lens to properly focus at a closer distance than would otherwise be possible. The numbers express the fraction of a meter that you should be away from the subject - so a +2 diopter should allow you to get about 1/2 meter from the subject. The diopters may be "stacked" one onto another to add up to various strengths - using the whole set as displayed above would add to +7 - 1/7 of a meter distance - theoretically. In practice the actual distance may vary a good bit.
If you only have one lens, diopters can be the lease expensive way to get into close-ups as they are relatively inexpensive. However, if you have multiple lenses with differing filter sizes, you'll need diopters to equal the filter sizes. I use mine on both a 50mm lens (52mm filter) and 80-210mm lens (58mm filter size) by having a set of 58mm diopters and a step-up ring so I can mount the 58mm diopters on the ring and the ring to the 52mm threads of the 50mm lens.
Generally, I believe extension tubes perform anywhere from equally to much better - depending on circumstances. But, diopters are a lot quicker and handier if you're going to do much switching around between normal and close-up shots. I use both tubes and diopters, depending on the circumstances.
The following pictures show the effects of using diopters.
The following equipment is used: Canon EOS 10D Camera, Canon EF 50mm f/1.8 lens, Canon 80-210mm macro/zoom lens, Hoya diopter set - +1, +2, and +4. A dime was placed on white felt and continuous lighting was used for illumination. The pictures are taken with the camera handheld. The depth of field can get very shallow on these close up shots so it is very easy to get a portion of the image somewhat out of focus. Generally, when diopters are "stacked" the larger number diopter should attach to the lens and the next smaller one added to it, etc.
Theoretically, AutoFocus will work with diopters. As a practical matter, I found it works OK with the 50mm lens, but with the 80-210 after getting past the +1 or +2 diopter manual focus is a must. The diopter allows the lens to focus closer to the subject than it otherwise would. Some of the pictures below are VERY close to the subject; theoretically, the 50mm lens used should focus as close as 18 inches; in practice I was less than 2" with some of the combinations below.
NOTE: The Canon EOS 10D is a digital SLR camera. Since the chip which the light strikes is somewhat smaller than 35mm film would be, the effect "magnifies" the effective focal length of any lens used. In the case of the 10D the effect is 1.6x - so a 100mm lens becomes, effectively, a 160mm lens on this camera. Most digital SLRs have the same effect and generally are about 1.5x. Other than the "multiplier", the same enlargement effect holds true whether you are using film or digital, though.
Diopters, just as extension tubes, significantly decrease the depth of field. All of the photos below were shot freehand (as opposed to focusing very carefully on a tripod) and that works most of the time, but for extreme close-ups, it can be tedious.