What Is an Extension Tube?
An extension tube is a spacer that sits between your lens and the camera body to alter the MFD (minimum focus distance). The MFD of a lens is a measurement of the closest point a subject can be from the camera’s sensor, whilst still being able to focus. If a subject is closer to the sensor than a lens’ MFD, you cannot focus on that subject. The thicker the extension tube used on a lens, the higher the lens magnification becomes as you’re able to move the lens much closer and still achieve focus. What all this means is that you can turn a normal lens into something with much a much higher magnification factor to achieve macro-like images, without needing a dedicated macro lens.
Later in the article I will discuss the knock-on image quality issues caused by extension tubes, but the other important thing to realise is that when extension tubes are in place, you can no longer focus all the way to infinity. This has few practical implications because most people are using extension tubes to try to focus on something at a very close distance, but it’s worth knowing so that you don’t think your lens is broken when you find it’s no longer able to focus on something further than a few feet away. It’s for this reason that you can’t practically leave an extension tube in place all the time, so it’s something you need to carry in your bag and use when it makes sense to do so.
How Do Extension Tubes Affect Image Quality?
Unlike teleconverters, extension tubes have no optics in them at all so in some circumstances they have very little effect on image quality. The tricky thing about point is that every lens reacts very differently to using extension tubes, so it’s hard to deliver a sweeping answer to this question. When manufacturers design the optics inside a lens, they take into account things like barrel distortion and pincushion distortion, and try to correct for it as much as possible. Focusing a lens does move the optics inside a lens, so that means that the amount of distortion varies depending on how far away your subject is from the lens. Lens designers try as hard as they can to correct for distortion at the most important points in a lens’ focus range, but essentially what it means is that some lenses are sharper than others when they are used at the minimum focus distance. Take a macro lens for example; The designers know that it’s going to spend much of its time being focussed at the absolute closest focus point, so they correct distortions for that point. Often a macro lens is at its absolute sharpest when used at the MFD. Conversely, super telephoto lenses tend to be used for focussing on objects that are quite some distance from the lens, so the opposite is true.
What all this means is that every lens performs differently at the MFD, and you can’t necessarily trust that an expensive, well regarded lens is going to be super sharp at the MFD. Extension tubes have the effect of magnifying these imperfections on the sensor, so if you start with a lens that performs poorly at the MFD, you’re going to notice it pretty quickly once you start using the tubes. A great example of this is found in the section further down this guide where I tested the $130 Canon 50mm f/1.8 STM with extension tubes, and the $2300 Canon 24-70 f/2.8 L II with the same tubes. The “nifty fifty” actually performed much better in terms of overall sharpness with extreme extension. Close examination of my 24-70 images showed considerable distortion and loss of fine detail. I would regard this very same lens to be one of the sharpest Canon have ever made, so it just goes to show that not all lenses take well to being operated outside of their designed parameters.
For many people, the whole point of using extension tubes is so that you don’t have to buy a new (macro) lens, meaning you’ll most likely have to use whatever lenses you already have. After purchasing the extension tubes though, make sure to test them out on all your lenses to figure out which ones respond best to this kind of usage. If you find a particularly great combination, please do share it in the comments at the bottom of this article as well!
The other side effect of extension tubes is that they can cause vignetting when used on lenses that are set at, or close to, their widest aperture. The amount of vignetting is somewhat dependant on the lens, but also very dependant on the thickness of the tubes you are using. 12mm extension tubes rarely prove to be much of an issue in this regard, but once you start getting into the 20mm or greater thicknesses, you’ll want to pay attention to this, and stop your lens down to counteract it.
Extension Tubes and Effective F-stop
The other thing you’ll notice if you are manually exposing your images is that there is significant overall light loss when extension tubes are used. This occurs universally across the image, so it’s distinct from the previously mentioned vignetting. The process of shifting the lens further away from the sensor actually increases the aperture of the lens, so your image gets darker and your depth of field will increase just as if you had changed the aperture setting of the lens via the dial on your camera. The difference is that this change in aperture isn’t actually shown on the camera, because the camera doesn’t know how much you have extended the lens. If you aren’t deeply familiar with f-stops then I’d recommend brushing up by reading my Understanding F-Stops article.
The important point that’s hidden within that previous article is that “The diameter of an aperture is equal to the focal length, divided by the f-number”.
This means that f-stop is directly related to the focal length and since focal length is related to various dimensions in the lens design, extension tubes also change the effective focal length of a lens when you add them, and this in-turn causes a change in effective f-stop. In practice, you probably wouldn’t notice it if your camera was set to some sort of automatic exposure mode because the camera will compensate for it. What you should remember, as a rule of thumb, is that by the time you get close to magnifications of 1:1, the effective f-stop of the lens will have changed by about 2 stops. That means you, or your camera, are needing to compensate for this with a higher ISO, or a shutter speed that’s 4-times longer than you’d need without those tubes. It could make the difference between being able to hand-hold the lens, or not. And it could also make the difference between being able to get a sharp shot of a small moving subject, or not. In low light, if you are struggling to get the exposure settings required for a sharp shot, then it might be a good idea to use a shorter extension tube to gain back some of that lost light, and therefore maintain a faster shutter speed.
Another consequence of altering the f-stop of a lens in this way is that less light is getting to the camera’s AF sensor, so you may find that in low light, AF performance is decreased when using extension tubes. Most people tend to use extension tubes with static subjects so you can always tun on live view and focus manually, but it’s another thing you should be aware of. This light loss will occur with any brand of extension tube as it’s simply a mathematical function, so that change in AF performance isn’t linked to the brand of extension tube in any way.
Calculating The Change In Magnification from an Extension Tube
The magnification of your existing lens can usually be found in the lens specifications from a manufacturers website. Once you have that magnification, you can calculate the new magnification with an extension tube, using the following formula:
New magnification = Native lens magnification + (extension amount/focal length)
Example 1: The Canon 50mm f/1.8 STM has a native magnification of 0.21x. If we use a 12mm extension tube on it, our new magnification will be 0.21 + (12/50) = 0.45x
Example 2: Using the same Canon 50mm lens with stacked 12mm and 25mm extension tubes would give us a magnification of 0.21 + ((12+25)/50) = 0.95x
0.95x magnification is very close to life-sized 1:1 that you would get in a true macro lens!
Extension Tube Examples
In order to demonstrate how these tubes work, I set up a test subject and then photographed it with several different lenses, both with and without extension tubes. Note that each time a new photo was taken, the camera has been physically moved closer to the subject. The tubes themselves do not “zoom in” in the same way that a teleconverter would do.
Canon 50mm f/1.8 STM
I particularly wanted to include this lens because it’s by far the most cost-effective way to use extension tubes for macro-like photos. This lens costs around $120 and has a minimum focus distance of 1.15 ft./0.35m. This is already quite a close focus distance, and that makes it a great candidate for use with extension tubes. In general, 50mm prime lenses are great options for extension tube usage, so if you’re not a Canon user, be sure to check out the equivalent lens for Nikon, Sony, Fuji or whatever brand you use.
Canon 40mm f/2.8 STM
This Canon 40mm pancake lens is another great option for Canon users because it already has an MFD of just 0.3m. Results are fairly similar to the Canon 50mm f/1.8 STM, but the 40mm pancake is so small, you could call this a pocket-sized macro lens! Pretty awesome!
Canon 24-70 f/2.8 L II
I’ve included this lens because it demonstrates the reason why smaller prime lenses are more suitable for usage with extension tubes for macro work. I performed the same test as with the previous two lenses, but ran into troubles when attempting to use 25mm tube. The problem is that the minimum focus distance is a measurement from the camera’s sensor and NOT the front element of the lens. That means that with physically longer lenses, particularly ones that extend while zooming like this one, you can actually reach the point where the MFD is inside the lens and you can never focus at that point. Of course you could move the subject back away from the lens, but then you might as well not bother with the extra extension tube at that point.
The other problem is the physical proximity of the subject to the lens. The lens eventually blocks light from falling on your subject, and if you’re trying to shoot small insects, good luck getting a lens this close to them!
Canon 100-400 f/4.5-5.6 L IS II
The reason I’ve chosen to include this lens in the examples is to demonstrate the difference between using extension tubes on shorter and longer focal lengths. Compare these images to those from the first example of the 50mm lens, and you’ll see that the difference in subject size varies much more greatly with shorter focal lengths. For the 50mm lens there is a drastic difference between what you can capture with the native 50mm and what you can capture with the 37mm of extension tubes. The same can’t really be said for that on this longer focal length zoom. Yes, there is a noticeable difference, but it’s not what I would call drastic, so it’s worth understanding this difference as you get into longer focal lengths. Despite the 100-400 lens having an admirably short MFD for a 400mm lens, the end result isn’t nearly as “macro like” as the results you get with the shorter prime lenses. For this reason, many people choose to use a close-up lens with longer focal length lenses, rather than using extension tubes. Extension tubes work best for adapting short focal lengths, close-up lenses work best for adapting longer focal lengths.
Of course one advantage this longer focal length setup does have is that you aren’t so close to the subject, making it a good option for small animals and insects where a closer presence could cause them to flee.
Types of Extension Tube
Extension tubes come in several varieties but they are usually discussed in three distinct categories.
OEM Extension Tubes
These are the most expensive kinds of extension tubes and they are manufactured by the manufacturer of your camera, like Canon, Nikon, Sony or Fuji. They allow full autofocus when used with compatible lenses, and you can communicate with the lens via the camera in order to change the aperture or engage image stabilization.
Third-Party Extension Tubes (with AF)
Third-party tubes are any ones that are not made by the manufacturer of your camera or lens. Popular third party extension tube manufacturers include Kenko, Vello and Neewer, the most popular ones being the Kenko tubes. I’ve previously written a direct comparison between the Canon extension tubes and the Kenko extension tubes. When it comes to buying lenses, many people really prefer to buy the ones that are made by the brand that makes their camera, and in many cases (but not all), these do tend to be of a higher quality. Since extension tubes are really just spacers, and contain no optical elements, it’s much easier for third-party manufacturers to make competitive, quality products. They are always much cheaper than OEM tubes, typically 50% of the cost or less. As you can see in the comparison between the Canon and Kenko tubes below, it’s very hard to tell them apart.
Third-Party Extension Tubes (without AF)
The next type of tube you can buy is even cheaper than the previous ones, and there are many Chinese brands that sell them for prices as low as $10. These kinds of tubes do not have any electronic contacts in the mount, so you can’t autofocus with the lens, and you can’t even adjust your aperture because the camera has no way of communicating with the lens.
How to Change Your Aperture When Using Manual Extension Tubes
In order to adjust the aperture when using these contactless, cheaper extension tubes, you must follow this procedure:
- Mount the lens directly on the camera
- In aperture priority or manual mode, select the aperture you want to use in your image
- Hold down the DOF preview button on the camera. This causes the aperture to engage to the chosen value
- Whilst holding the DOF preview button, remove the lens from the camera. This causes the aperture to stick at the requested value
- Mount your extension tubes to the lens
- Connect the whole lot to the camera body again
As you can see, this is a real pain in the butt to have to go through this process every time you want to change your aperture value! My first set of extension tubes was just such a set, and I think I used them once before deciding to buy some that included electronic contacts instead.
Which Kind of Extension Tubes Should You Buy?
I have to be honest and say that I do not think the fully manual, contactless, super cheap extension tubes are worth the hassle at all. I tried them and hated the experience! They may be ludicrously cheap, but since the third-party AF compatible ones aren’t even that expensive, I really would recommend saving up for them, or the OEM ones.
So what about choosing between the OEM or the third-party ones? It’s hard to make a sweeping statement in that regard because I haven’t used all the third-party ones that are available. I tested the Kenko ones and came away much more impressed than I expected to be, but I still choose to own the OEM Canon ones myself. My personal reason for that choice is that I’m using these things on lenses that cost over $10,000 when they are paired with my super telephoto lenses. If I think there is even a 1% chance that the Canon ones are stronger or more reliably built than then third-party ones, it makes sense for me to use the Canon ones. In the grand scheme of things, the price difference is a tiny fraction of the cost of my lenses.
Having said that, for most people I do think that good third-party ones like the Kenko set are the best option. There’s certainly no difference in performance between those ones and the Canon ones. My choice to use the Canon’s is specific to the lenses I’m using them with, but I would be much more inclined towards the third-party ones if I was just using smaller, cheaper prime lenses with them. I think the Kenkos, or similar third-party ones are probably the right choice for 95% of people out there.
Look for Light Leaks
Sometimes with cheaper extension tubes stacked together, it’s possible for a small amount of light to creep into the tubes where they are joined together. If you experience areas of low contrast in your image, or odd looking bright sports or artifacts, this could be the problem. It’s easily solved by using some black electrical tape around the joins in the tube stack.
Using Extension Tubes to Stack Extenders
Another useful way to use extenders is to use them for stacking together two extenders that will not natively connect due to protruding optical elements. For example, as you can see in the photo below, Canon’s extenders have an optical protrusion that would physically prevent them from connecting directly to a lens that has a rear element close to the mount. By using a 12mm extension tube between them, they will connect without a problem. It’s also a very convenient way to store multiple extenders in your camera bag!
Summary With Pros & Cons
I hope by now you’ve come to the same conclusion that I did when I first learned about extension tubes; These things are very useful, and highly affordable! If you like nature photography then they can come in very handy for photographing small animals or plants, and if you’re a travel photographer than they are great for picking up detail shots of food using your existing travel zoom lens, or a fast prime. There are very few photography accessories out there that can make such a vast difference to an image for such a small amount of money. When you take into consideration the fact that they also weigh very little, making them easy to carry, you can see why I’m always recommending them to people.
Extension Tube Advantages
- Cheaper than buying a dedicated macro lens.
- Light weight and easy to pack compared to carrying a dedicated macro lens.
- Often minimal loss in quality due to lack of optical elements.
- Stackable to provide magnification options depending on your subject.
Extension Tube Disadvantages
- A lens can’t focus at infinity with an extension tube on it.
- Minimal effect to magnification on longer focal lengths.
- Can cause vignetting at wide apertures and stack heights.
- Increases the effective f-stop of the lens, thereby requiring a longer shutter speed or higher ISO to compensate.