What is a teleconverter?

Canon EF 1.4x III Extender
Canon EF 1.4x III Extender

A teleconverter is also referred to as an extender, multiplier or doubler. It is a magnifying secondary lens housed in a tube that has bayonet attachment interfaces on both sides. A teleconverter is typically attached between a camera body and a compatible main lens to extend the focal length of the main lens and increase magnification. While similar in construction and appearance to an extension tube, it’s function is completely different.

Due to the way a teleconverter works, it can only be used with interchangeable lens cameras. As an alternative, many manufacturers produce telephoto conversion lenses that screw in front of the primary lenses, much like a filter. These conversion lenses can also be used with compact cameras with non-interchangeable lenses.

Canon EOS 1Dx Mark II Camera + EF 1.4x Extender + EF 400mm/f2.8 L IS Mark III Lens
Canon EOS 1Dx Mark II Camera + EF 1.4x Extender + EF 400mm/f2.8 L IS Mark III Lens

Teleconverters are labeled according to the magnification factor they provide, with the most popular models being 1.4x, 1.7x and 2x. By multiplying the main lens’ focal length and the teleconverter factor you receive the effective focal length of the system. The light loss factors can be found in each teleconverter’s specifications. For modern units, it is usually 1 stop for 1.4x models and 2 stops for 2x models.

As an example, attaching a 1.4x teleconverter to a 200mm/f2.8 prime lens would result in effective parameters of the system equal to a 280mm/f4.0 lens. The combination retains the prime’s minimum focus distance while allowing a greater magnification. The same prime lens with a 2x teleconverter would yield results similar to a 400mm/f5.6 lens.

Virtually all modern teleconverters have electronic contacts to facilitate communication between the camera, lens and teleconverter. These contacts are essential for functions like autofocus, aperture control and image stabilization. On the used market there are older pre-AF teleconverters that are manual focus only. While physically compatible with certain modern lenses and cameras, these teleconverters might severely limit the functionality of the system.

How to use a teleconverter

You first need to establish that the teleconverter, lens and camera you have are all compatible with each other before attempting to put them together. The following chapters give all the information you need to do this. Attempting to assemble incompatible components can be disappointing or frustrating at the least. It can also lead to expensive optical or mechanical damages to the equipment.

Once you have established you have the correct kit, using a teleconverter is pretty straightforward. As you will be exposing the inside of your camera, it is advisable to do this in a dry, relatively dust-free environment. It goes as follows:

  1. Dismount the primary lens from the camera body if mounted and put on a body cap.
  2. Remove the rear cap of the primary lens (if it was not attached to a camera) and the front cap of the teleconverter and mount them together.
  3. Remove the body cap and the teleconverter rear cap and install the teleconverter/lens combo on the camera as usual.

It is not always necessary to follow the above order. It is good practice to mount the teleconverter to the lens first and then attaching the combo to the camera. This minimizes the risk of electronic glitches and is generally the approach recommended by manufacturers.

Why and when to use a teleconverter

Using a teleconverter gives you more flexibility at the cost of reduced light transmission, sharpness and autofocus performance. A high-quality teleconverter matched to the main taking lens is a way to extend the magnification capabilities of your camera system without sacrificing too much image quality or convenience. It can add some of the flexibility of a zoom lens to your primes and give you the additional reach without the weight and monetary penalties of larger lenses.

There are several typical teleconverter usage situations:

Teleconverters and telephoto lenses

By far the most popular application of teleconverters, and the one they are designed for, is extending the reach of telephoto lenses in sports, wildlife and other long-distance photography. This is why virtually all teleconverters are optimized to work best with telephoto and super-telephoto primes.

Moreover, teleconverters increase magnification while preserving minimum focus distance. This further expands the creative possibilities and flexibility of a telephoto lens.

Teleconverters and macro lenses

Another common application for teleconverters is in macro photography. Almost all dedicated macro lenses are of fixed focal length, and teleconverters are the only way to alter magnification while preserving the focus distance. Higher magnification can also be achieved by the use of extension rings, bellows, reversing rings or close-up lenses, but all of these methods rely upon shortening of the focus distance, i.e. bringing the camera closer to the subject. When this is impossible or undesirable, a teleconverter is the only option, provided you find one compatible with your macro lens.

According to lens and camera manufacturers, using a teleconverter on a macro lens is not a common usage scenario. You would rarely find macro lenses on an official teleconverter manufacturer’s compatibility list. Research and experimentation are key in such a situation.

Teleconverters and Tilt-Shift Lenses

A rather exotic combination, but one not unheard of. As every lens manufacturer only offers a very limited tilt-shift lens lineup, one way to achieve more flexibility is through teleconverters. Although not a combination officially confirmed by Canon, various sources report that Canon’s TS lenses (17, 24, 45 and 90mm) all work well with the brand’s 1.4x and 2x teleconverters. As always, any such reports should be taken with a grain of salt and actual testing should be performed before trusting a project to an unofficial setup like this.

Things to consider before buying or renting

There are several things to consider before committing to purchase a teleconverter:

Lens and camera compatibility

The first thing to do before purchasing or using a teleconverter is to check whether it is compatible with the lens and the camera you plan to use. As the teleconverter goes between the camera and the lens, it needs to have the correct bayonet mounts – Canon EF, Nikon, Sony A-mount, etc. Generally, all interchangeable lens cameras support the use of teleconverters, however, this is not the case for lenses. Due to physical and optical constraints, each lens manufacturer has a list of lenses that can be used in conjunction with a teleconverter.

Canon Nikon Sony Sigma Kenko Tamron

Just because a lens is not listed on a manufacturer’s list of compatible lenses does not mean that it certainly does not work with a teleconverter. Unless explicitly stated otherwise, the absence of a camera or a lens from a compatibility list means simply that the manufacturer has not tested or cannot guarantee proper operation of the combination. In such cases, research and experimentation go a long way. Have a look at the Testing a lens/camera/teleconverter combo chapter for more information.

As far as cameras are concerned, for physical compatibility, all that you have to do is to confirm that the extender has the same bayonet interface. In operation, however, the use of a teleconverter with certain lens/camera combinations can render autofocus unreliable or outright disabled due to the decreased maximum aperture of the setup. You can calculate your effective maximum aperture and then compare it to the camera’s listed autofocus aperture requirements to determine whether autofocus will be operational.

In addition, with some setups the teleconverter fails to communicate its presence to the camera, resulting in the camera reporting incorrect aperture/focal length which can sometimes lead to improper exposure. This also means that wrong metadata is being written in the image files. Again, testing is the only way to confirm that everything is operating properly.

Primes vs zooms

In general, teleconverters are optimized for use with fast prime lenses, and each lens manufacturer has lists of recommended combinations. Because of the light loss, using a 2x teleconverter with an f4.0 zoom lens would take the effective aperture of the system to f8.0, an aperture at which framing can be difficult and some autofocus systems cease to operate. In addition, lower light transmission means longer exposure time, increasing the risk of motion blur, or higher ISO, decreasing image quality. This is why teleconverters are best suited to lenses with f2.8 or larger apertures, which in most cases means prime lenses. However, most teleconverters do work well with fast zooms of the 70-200/2.8 type, provided physical and optical compatibility.

Teleconverter vs cropping

Generally, a high end, well-matched lens and teleconverter combination will almost always provide higher image quality than an equivalent crop from a lens-only exposure. However, there are some situations in which cropping might be a more viable option. For example, in lower light levels the reduced light transmission of a lens with a teleconverter may render autofocus unreliable. This can dramatically reduce the chances of getting a properly exposed, in-focus image. In such circumstances, you might be better off settling for a wider but properly exposed and focused image from which to crop in post.

Buying the right one

Teleconverters are complex optical devices which directly affect image quality, so it is not a good place to make compromises. Low quality or mismatched teleconverter can render the image of a $5000 telephoto lens useless, so make sure you do your research before you purchase. Once attached to a lens, it becomes a part of the optical path, so theoretically lenses and teleconverters by the same manufacturer should yield the best results. However, in practice, you may find that a third-party teleconverter is just as good, if not better than the lens brand offering.

You can use the following checklist to determine whether a certain teleconverter is suitable for you.

  • Does it fit my camera mount?
  • Is it compatible with my lens/es?
  • Will the combination retain autofocus capabilities? If not, am I still willing to use it as a manual focus?
  • Same brand vs third-party?

Specifics by camera system

The pairing of teleconverters and lenses from different generations and systems is more straightforward in some manufacturer’s line ups than others. While all Canon EF lenses and teleconverters use the exact same mount and electronic contacts, this is not the case with other manufacturers. The Nikon lens mount, for example, has undergone a series of changes over the years. As a result, various incompatibilities exist between lenses, bodies and teleconverters of different vintages.

Testing a lens/camera/teleconverter combo

Even though modern camera lenses are high-quality precision optical instruments, lens parameters can vary from one copy to another rather dramatically. For example, one 200mm f2.8 lens can, in reality, be a 198mm lens, while another copy of the same lens can possess an actual focal length of 201mm. Such variations are typical, and can also be observed with other parameters. As teleconverters are stacked behind the lens, they increase the complexity of the optical path and introduce their own variables into the final setup. This means that, unless the lens and teleconverter have been matched as a pair in the factory (as is the case with the Nikon 800mm f/5.6E FL ED VR and it’s dedicated TC800-1.25E ED teleconverter) you don’t really know what results a particular teleconverter/lens combination would yield. The only way to know is through testing. This is valid if the lens and teleconverter are officially compatible, but is especially important for combinations not officially confirmed by the manufacturer.

So how do you start? If you are considering a certain camera/lens/teleconverter combo, chances are someone else has already tried it and posted their results online. As always, any such information should be used for guidance only. Proper testing should be done before you invest in equipment or commit to a project. It is advisable to first borrow or rent the equipment you consider and test it extensively.

In this side view of the Sony FE 2.0x Teleconverter you can see how much of the teleconverter's optics have to actually go inside the lens barrel
In this side view of the Sony FE 2.0x Teleconverter, you can see how much of the teleconverter’s optics have to actually go inside the lens barrel

Physical compatibility

The first and most crucial step in such testing is to ensure that the lens, camera and teleconverter are physically compatible with each other. This starts with verifying that all of them use the same bayonet mount. Then you have to check that no physical contact between the glass elements of the lens and teleconverter will occur once they are mounted together. If you are using a combination officially verified by the lens manufacturer, then you can be fairly confident that this is not going to be an issue.

If you are attempting an untested combination, however, it is imperial to establish the presence of sufficient clearance before assembling the setup to avoid potential damage of mechanics or glass surfaces. PL teleconverters for cine cameras often come with a dedicated measurement tool for this very purpose. However, there is no such practice with still photography extenders, so you will most likely have to improvise.

You can do this by carefully measuring the protrusion or depression of both the lens’ rear element and the teleconverter front (lens mount side) element. Protruding teleconverter lenses are usually surrounded by a rubber bumper which makes measuring easier and safer. Great care should be taken when measuring elements sunk into the barrel of the lens or teleconverter. Bear in mind that in many lenses the rear element moves when focussing and/or zooming. Therefore, clearance should be measured at the element’s most extended position. To avoid damage you can use a wooden stick padded with soft tissue paper or a piece of card as a makeshift dipstick, which you can then overlay on a ruler to get a measurement.