Aside from a dedicated macro lens, there are two ways to increase the magnification of a lens. One is to add a magnifying lens to the system, either in the from of a close up lens or a teleconverter. The other option is to use an extension tube, what we’ll be talking about here.
Extension tubes increase the magnification of lens by allowing it to focus closer than it was designed to. The closer the object, the more magnified the object becomes in the image. To make the lens focus closer, it spaces the lens further away from the camera body, hence the extension part of the name.
Extension tubes are not without their limitations and flaws though.
On the positive side, they don’t have any optical elements, so they won’t in and of themselves, degrade the image quality the way an inexpensive teleconverter can. However, because they do increase the magnification of the lens, they do magnify the flaws in the lens’s own design. A mediocre quality lens, or even a really good one with a lot of added magnification, will show in the images made with it.
On the other hand, there are a couple of limitations that need to be understood.
To start with, extension tubes prevent the lens from focusing at infinity while they’re attached. This is not a game ending issue in most cases, but while using extension tubes you can’t just go from a macro image of say a flower, to a portrait of a person like you can with a dedicated macro lens.
Secondly, the magnification gained with each extension tube is going to vary from lens to lens, and from tube to tube. Where a 1.4x teleconverter has the effect of providing 40% more magnification regardless of what lens it’s mounted to. A 12 mm, for example, extension tube will produce a different changes in magnification with 50 mm lens, a 70 mm lens, and so forth. In fact, because the minimum focusing distance of the lens is factor in the resulting magnification, a 12mm extension tube will produce different maximum magnifications from different lenses of the same focal length if they focus to different close focus distances.
Finally, some extension tubes will simply be incompatible with some lenses. As a rule, you cannot use an extension tube that provides more extension than the focal length of the lens. For example, a 25 mm tube won’t work on a 24mm lens.
That said, the advantages of extension tubes are that they’re light, easily packed, and can be used to gain just a little more magnification in the field when it’s necessary. They’re great, for example, for travel and nature photographers who can’t afford the weight of a dedicated macro lens. Or for someone working in a tight space that needs to shave of just a little close focusing distance off mid-telephoto lens to fit in the space.
Since extension tubes are completely hollow, having no optical elements at all, there’s nothing inherent in the optical path of the tube that can degrade the image quality of a lens. As the old joke goes, Kenko air is as good as Canon air.
That said, while there are no optical elements in the tube, that doesn’t mean that they can be made shoddily. If the front and rear mounting surfaces aren’t parallel, and parallel to reasonably high tolerances, the extension tubes will impart a tilt to the lens and that will alter the planes of focus and therefore the respect depth of field.
My ability to test how true the two mounting surfaces is limited to the rather crude use of a 0.01mm digital caliper. Fortunately, this is usually enough to know whether the tube or adapter is close enough to be a problem or not.
My 3 extension tubes tested out as follows.
In my opinion, both the 12 and 20 mm tubes are quite true, within 20 microns out around any side. The 36mm tube fairs poorer, though even at 50 microns it’s still parallel enough to be good in all of the cases where it would be used. Also remember, I’m measuring this with an inexpensive digital caliper not a micrometer or a dial indicator on a lab grade granite slab.
That said, I’m not at all bothered by the increased variance with the 36 mm tube. This is primarily because the error in parallel of the surfaces, becomes more of a factor in critical focus as the focal length decreases. At 36mm the tube is already too long to be used on most wide angle lenses, and where it starts being usable — around 50mm or so — the error isn’t going to matter that much. In this respect this is a different can of worms that a glassless mount adapter for a mirrorless system where it might be used with any possible focal length.
It’s probably also worth noting, that the extension provided by the 20 mm tube is about 1.3mm short of the listed extension, while the other two tubes are quite a bit closer.
Beyond needing both mounting surfaces to be parallel, the tubes introduce joints to the lens-camera connection path. Moreover, we’re talking about still camera lens mounts, which are notoriously less rigid than cine and video mounts. As a result a stack of extension tubes can introduce a noticeable amount of flex into the camera-lens mounting arrangement. For still photographers, this flex likely won’t be of much concern. However, for video users, even the flex of a standard stills lens mount is enough to be noticeable on video, add an extension tube to the mix and it’s very obvious that the camera and lens aren’t one rigid piece.
In my informal testing, I don’t find that a single Kenko extension tube behaves much differently with respect to flex than my Canon teleconverter. In both cases, the tubes them selves are pretty rigid, with most of the apparent flex coming from the play inherent in the additional mounting surface.
In my experience, flex really only becomes a problem when you start stacking many extension tubes to get the desired magnification.
So let me start with my biggest, well really only, dislike, the caps. Kenko doesn’t spend a lot on their front and rear caps for the stack, and while I’m not all OCD about having really nice caps on everything, I also try to avoid keeping caps with cameras or lenses. For example, if I’m changing lenses, I’ll pull the rear cap off the new lens, and put it on the lens I’m taking off the camera. So my first order of business was to order a new Canon rear lens cap, and a Canon body cap to replace the Kenko ones.
So what are the good parts. The biggest to me are price. The Kenko set runs between $110 and $120 for all three tubes. Canon wants $80 for their 12 mm extension tube and $129 for their 25 mm extension tube. There are no optics, so there’s no worry there. The Kenko tubes will mount both EF and EF-S lenses, so there’s no advantage for Canon there either. And neither the Canon or the Kenko tubes aren’t weather sealed, so again no advantage. The only thing you really get with the Canon tubes is a Canon logo on them.
Which leaves the big question, should you bother with extension tubes or do something else for macro photography?
My easy answer to this is that you should probably have a set of tubes anyway. Even if you buy a good dedicated macro lens, you can always squeeze more magnification out of it with an extension tube if you want or need to. Moreover, if you’re traveling light and don’t want to carry the macro lens but still have some close up capabilities, the tubes work really well for that too.