I was asked a question about extension tubes a couple of days ago- What are they? Do they work? Are they any good? He then asked if there was a formula for figuring out the effect an extension tube will have for a given lens. Now we are talking!
First, let me explain what an extension tube is. An extension tube is nothing more than a spacer between your camera body and your lens- it is a hollow tube. There is no glass, no anything in an extension tube- just space.
The purpose of the extension tube is to push the lens farther from the camera- the farther the lens is from the camera the closer it will focus. This is why most lenses get longer (grow) when the focusing ring is turned to close focus. In lens-speak, when a lens grows it is called extension. Only lenses that have internal focusing don’t physically grow longer with close focus. On these lenses the glass inside the lens travels farther away from the camera when the focusing ring is turned. Either way, in order for a lens to focus more closely when the focusing ring is turned the lens glass must move away from the camera.
Extension tubes are a bit old school these days. Back when I was a starting pro 30 years ago, extension tubes were popular because there were no macro lenses that focused all the way down to life size and the long telephoto lenses had minimum focusing distances that were too far to get close to small critters. In other words, there were no lenses that had enough built-in extension to allow them to focus as closely as photographers wanted. An extension tube solved this problem.
Unfortunately, an extension tube also created problems. For every inch of length of an extension tube a stop of light was lost. For macro work, I often used an extension tube that was 2.5 inches long so when I looked through my lens (2.5 stops darker!) it was really dark! Also, back in the day using an extension tube negated any autofocus or the sophisticated metering modes. Together, this made using extension tubes a pain in the neck.
These days most macro lenses and long telephotos have plenty of extension built-in to make extension tubes pretty much unnecessary. For any lens you might have that doesn’t focus quite close enough for you the far better alternative is to use a close-up diopter. A diopter looks like a thick filter and screws into the front of your lens like a filter would. When on, the diopter changes the optics of your lens to allow it to focus much closer than it otherwise would. You lose your infinity focus when the diopter is on but you gain close focus.
The magic of diopters is that you don’t lose any light and your autofocus and metering systems are not comprised in any fashion. I find that diopters work great on 70-200mm or 70-300mm zoom lenses. When I travel internationally I often will take my 70-300mm lens and pop a diopter into my pocket when I go out shooting. It’s much easier than carrying around a separate macro lens.
So what about the formula to figure out what effect an extension tube might have on one of your lenses? Okay, here goes, now pay attention.
To figure out the magnification ability of a lens take a picture with it when it is at its closest focus. Then measure the dimensions of the area of the photograph. If the area you just photographed is 1” by 1.5” your lens focuses down to life size or 1/1 (one to one). If the area is 2” by 3” then your lens focuses down to one half life size or 1/2 (one to two). Many general lenses focus down to 4” by 6” or 1/4 life size. Many telephoto lenses focus down to 8’ by 12” or 1/8 life size.
Now that we have that straight, the formula for extension is length of the extension over length of the lens. A lens that goes to life size has an equal amount of extension as lens length. So a 105mm macro lens that has 105 mm of extension built-in will focus down to life size. 105/105 = 1/1 = life size. A 105mm lens that has only 52mm of extension will only focus down to 1/2 life size- 52/105 = 1/2 life size.
If you have a 200mm lens that can photograph an area 4’’x 6” (1/4 life size) than you have 50mm of extension in that lens. My 200mm lens is a 1/1 life size macro so it has 200mm of extension built-in.
The actual question I was asked is about using a 12mm extension tube on a 200mm non-macro lens. We now know, using the above mathematics, that this extension tube won’t have much positive effect. Better to get a diopter that fits than use an extension tube that will be little help.
So there you have it, everything you wanted to know and many things you probably didn’t want to know about extension tubes. I’m glad you asked!