Macro Magnification info regarding extension tubes, diopters, reversing rings and extension tubes

Macro Magnification Details

Focal Length and Magnification

Any shooter looking to get into macro lenses quickly discovers that there are 3 general sizes of macro lenses: 50-60mm; 100-105mm; and 180-200mm. And the prices are proportionate to the focal lengths! The important thing to know is that all of these lenses provide the same 1:1 reproduction ratio. As for which focal length is best, like so many things in life, the middle ground is likely the most versatile for multi-purpose. A 105 gives best usability in the broadest array of conditions and subjects, and isn't insanely expensive. As far as I have found, as an owner of four macro lenses, there are no bad ones. The benefit of the longer focal length is greater subject distance, which keeps calm nervous subjects and a longer working distance makes flash illumination easier, especially as you approach 1:1.
A secondary benefit of a longer focal length is the ability to restrict potential distraction from background elements, since the field of view is tighter.

As you get into bug work, you might be inclined to get closer and closer, quickly hitting the 1:1 wall. Here are some of the commonly employed solutions, all of which can be used with macro and non-macro lenses:

Extension tubes: Popularized by Kenko and the original camera manufacturers, these tubes—which contain no optics—are positioned between the lens and body.

So how do they work? Notice how when you focus most lenses to a closer focus distance, the front element pushes away from the body? Tubes simply provide a greater distance, and thus a closer focus. Tubes are a very effective and versatile solution, since they can be used with any lens, but some users don’t like having to remove a lens from the body so that the tube can be mounted. Some extension tubes couple fully (mechanically and electronically), but others are simple dumb optical spacers without mechanical or electronic connectivity. Extension tubes are typically used for focal lengths of 50mm and greater, although the longer the focal length, the more extension needed to provide the same degree of magnification. Note that when one or more tubes is mounted, the lens cannot focus to infinity. But you sure can get close. No optics!

Diopters: Essentially, these are magnifying lenses that mount onto the front of the lens. Whatever lens you mount the diopter to, allows that lens to focus closer than before. The most popular high-quality solution is the Canon 250D and 500D diopters. The downside of any diopter is that it is a filter of a specific size, so unless one bothers with stepping rings, the diopter must be fitted to lenses of a matching filter diameter. A quality diopter is not a cheap item, and the better ones, like the Canon D-series, are actually a matched set of two diopters sandwiched together.

Reversing Rings: These are perverse little tools do something that just seems wrong: A reversing ring provides a means to mount the lens backward. Mounted in reverse, the lens can focus very closely. In fact, the wider angle of the focal length used, the closer you can focus. That said, there are significant downside to these, most notable being that the lens aperture must be manually stopped down prior to exposure, which is all but impossible on many modern electronically-controlled lenses. So they're rather hard to use compared to other solutions available.

Here's a good idea if you work with reversing rings a lot. It'll protect the backside of your lens and even function as a sort of makeshift hood. I used a Dremel to bore out the plastic.

Coupling Rings: like the reversing rings, these just seem wrong. What you do with these is mount a shorter focal length (normal or wide angle) backward and in front of your primary lens, essentially using the shorter focal length like a diopter (see above). A real plus about coupling rings is they allow a decent short telephoto, if coupled to a normal lens or moderate wide-angle, to achieve about 2:1 reproduction without even buying a macro lens (but the rig looks a bit goofy). Other downsides: awkward to work with, and possible vignetting issues.

Here’s a quick formula for determining the resulting magnification of lens coupling. If your primary lens (the one mounted to the body) is 100mm, and your reversed lens is 50mm, then you’ll achieve 2:1 reproduction. If your primary lens is 150mm, then you’ll obtain 3:1 reproduction.

I don't have a picture of a coupling ring, but it looks similar to any filter (such as a UV filter), except there is no glass and both sides of the ring feature a male thread to couple two lenses together, face-to-face.

Bellows: A bellows may be thought of as a stepless extension tube, providing infinite variability as to the amount of extension. Downsides: Beyond awkward, the bellows is a studio tool only. Also, auto-aperture is not maintained so you have to focus with your aperture wide open, and then stop down to your shooting aperture prior to exposure.. However, for variable and extreme magnification, bellows are best.

Quirky feature: Some bellows allow swing and shift adjustment for DOF gain and creative effects:

Notes regarding all of the above bullets:
Using the various solutions, I make reference to one tool or another being able to “focus closely.” Actually, little or no turning of the focus ring is involved; you mount the tool to your rig, and you’re able to move the rig closer to your subject, but there is typically little variability in the degree of magnification (which is generally not a good thing!). Of all the bulleted items, the reversing ring is the least flexible.
Note that all of these solutions, when applied, do not allow the macro lens to focus at infinity, so if you want to quickly switch from a bug to a bird, you’ll face a frustrating moment!
Regardless of the solution employed, you’re better served by using non-zooms for these methods. There are dozens of web sites that provide more info regarding these close-up tools. Google is your friend.
If you’re the old-school type, go to your neighborhood bookstore and order a copy of John Shaw’s Closeups in Nature. It is the standard reference for most macro enthusiasts.

The Magnification Wall

For practical purposes, you will find it very difficult to hand-hold the camera and capture an image beyond 2:1. This assumes you’re using flash to freeze motion. Even at a small opening (such as f22), if your magnification is 1:1 or greater, usable depth of field is a millimeter (!) or less.
Greater magnification can be achieved, and good images captured, if careful technique and proper equipment is applied. However, the more magnification you want, the tougher it gets: motion, focus, stability, focus, optics…everything starts to fall apart.

Cropping, or Cheater's Macro

Of course, you can crop, especially if you have a camera with a high pixel count. Some macro posters on websites crop everything, and some see that as cheating, even deceptive. The bottom line is whether you’re happy with your images for whatever purpose you intend.

If you crop to some lovely detail on a bug, always save it to a separate file. And remember that the nice crop may look terrific on your PC’s screen or the web, but if it’s a severe crop, you probably won’t get a good quality 8x10 out of it. Assuming you need a minimum of 200 dpi for printing, an 8x10 would dictate the image be 1600 by 2000 pixels, which likely exceed the dimensions of a severe crop. Here are a couple severe crops. Again, great on the web, but dimensionally lousy for living room art:

Next: Focusing