This page was originally published in 2007, then re-titled, largely re-written and expanded in 2026.

About teleconverters

A teleconverter, or focal length multiplier, for system cameras is physically built like an extension ring that contains optical elements, equipped with a male bayonet and a matching female bayonet at either end. A teleconverter mounts between a lens and the camera body, and is used to increase the effective focal length of a lens. Typically, teleconverters increase the focal length of a lens by a fixed factor, often 1.4 or 2 times the native focal length of the lens.

Virtually all large camera systems contain at least one or two teleconverter models. For a long time, the Nikon F, Nikon 1 and Nikon Z systems have offered 1.4x and 2x teleconverters, and at times 1.6x and 1.7x models, as well as a couple of special models designed for use on a specific (typically very expensive) lens. Several years ago, Vivitar, Panagor and possibly other brands offered teleconverters with a built-in helicoidat its rear, that could work in macro or close-up mode by increasing the distance between teleconverter and sensor plane. Results were the same as one could obtain by mounting a teleconverter atop an extension ring, i.e. not as good as using a lens designed for close focus.

In addition to the optics, modern teleconverters also contain electronic circuitry, as well as electrical contacts on their front and rear mounts. The number of electrical contacts on the front versus rear of the same teleconverter is not necessarily the same. Some teleconverters, like those made by Olympus for the Micro 4/3 system, have special contacts that mate with additional contacts present only on a restricted number of lenses.

Some teleconverters contain mechanical couplings between their rear and front mounts, that transmit the movement of electromechanical actuators and/or sensors between lens and camera. The same teleconverter may contain both mechanical couplings and electric contacts. For example, all modern teleconverters for Nikon F DSLR cameras have provisions for transmitting electronic autofocus and aperture signals. Some teleconverters (e.g., Kenko) also have a mechanical transmission for the older "screwdriver" Nikon F autofocus controlled by an electric motor in the camera body.

The physical length of teleconverters is much variable, and in general increases with power. In many teleconverters, the front optical element protrudes from the front mount, and this prevents the use of the teleconverters with lenses that do not have recessed rear elements.

A few special-purpose teleconverters do exist. For example, the Nikon TC-16 series of teleconverters have a built-in focusing optical group and a motor to control the latter. This series provides autofocus when used with lenses equipped only with manual focus, but only when the TC-16 is mounted on a specific, legacy model of Nikon F film SLR.

A few brands (e.g. Pentax, Kenko, Vivitar, Soligor, Elicar) marketed 3x teleconverters. Their quality is generally disappointing by modern standards. Kenko also made a 2x-3x Variable Teleconverter Auto Teleplus, consisting of a 2x teleconverter and a separate, variable extension tube. With the addition of the tube, it becomes a 3x teleconverter.

Some professional-grade long telephoto lenses contain a built-in teleconverter that can be inserted into the optical path by manually operating an external lever. Among the current examples are two models in the Nikon Z system and the OM System 150-400 mm Pro.

Teleconverters usually are optimized for a certain range of lens focal lengths. This involves their optical design. In general, it does not make sense to use a wide-angle with a teleconverter. Medium to long telephoto lenses are the most appropriate to use with a teleconverter.

Teleconverters do not affect the working distance of the lens. The depth of field is the same as a lens of similar effective focal length (i.e., lens focal length times the factor of the teleconverter) and effective aperture (diaphragm setting of the lens plus 1, 1.5 or 2 stops, depending on the type of teleconverter).

Nikon teleconverters with electric contacts and mechanical couplings (Figure 3) can provide multiple functions when attached to Nikon AF-I and AF-S lenses. Depending on the specific teleconverter model, these include metering with the aperture fully open, stopping it down automatically during the exposure, and transmitting to the camera the lens focal length and speed (which is necessary for correct functions like exposure and in-camera vibration reduction). In-lens VR and custom buttons on the lens can also be supported.

Autofocus couplings are often present, but in spite of this, autofocus is not supported on several AF-I and AF-S lenses. The reason is often not intrinsic to the teleconverter, but depends on the fact that autofocus in Nikon SLR and legacy DSLR cameras is only supported up to effective lens speeds around f/5.6. Thus, these cameras typically fail to provide a reliable autofocus already with f/4 lenses coupled to 2x teleconverters. Some high-end Nikon DSLRs offer reliable autofocus up to f/8 (i.e. are still reliable with f/4 lenses and 2x teleconverters, but not much slower than this). Contrast-based autofocus, available only on cameras capable of live view, can be able to cope with slightly slower lenses.

The Nikon TC-14E III in F mount differs from two other teleconverter models from the same series in doing away with most mechanical coupling and replacing them with a rubber weather gasket around the rear mount. This makes it incompatible with over a dozen recent Nikon long telephoto lenses that, however, remain compatible with the two other teleconverters of the same series. Cross-brand and even within-brand incompatibilities between lenses and teleconverters are numerous and abundantly documented. For example, legacy Sigma HSM (electronic autofocus) lenses work well with Sigma HSM teleconverters, but often do not autofocus with Kenko and Nikon teleconverters. Legacy Sigma lenses with mechanical autofocus, on the other hand, are only supported in manual focus with these Sigma teleconverters, but work in autofocus with Kenko teleconverters.

The front element of all modern Nikon teleconverters projects out of its front mount. This means that the rear element of the lens onto which it will be mounted must be deeply recessed. In turn, this restricts mostly to long telephotos the range of lenses onto which these teleconverters can be used.

The bayonet mount at the front of modern Nikon F and Z teleconverters has an extra tab, and accepts only Nikon lenses designed to be compatible with these teleconverters. Third-party teleconverters in Nikon F mount lack this extra tab on their front mounts and, in principle, can accept lenses in ordinary Nikon F mounts (as well as those designed for Nikon teleconverters). However, the front element of some third-party teleconverters does protrude from their front mount, and this prevents their use with some lenses.

It is possible to file away the extra tab from the front mount of a Nikon F teleconverter, to make it accept lenses with normal Nikon bayonets. A second modification of Nikon teleconverters additionally may be necessary with many lenses. Nikon Z teleconverters may be able to recognize certain incompatible lenses and other equipment, even once their front mounts have been modified, and may electronically lock up the camera in this case.

Nikon lenses with mechanical, "screwdriver" autofocus (mainly in the AF and AF-D series) can often autofocus with Kenko teleconverters. These provide both mechanical and electronic (AF-I/AF-S) autofocus transmissions. Tamron teleconverters are re-branded Kenkos, and are identical to these except for the brand markings.

Kenko teleconverters do not have projecting front elements, and accept almost any lens. By providing both mechanical and electronic autofocus, they are also the most versatile. However, these teleconverters are reported to have severe autofocus problems with many Sigma HSM lenses.

In terms of optical quality and in my own experience, Nikon and Olympus (Micro 4/3) teleconverters appear to be among the best.

Teleconverters find a use also in close-up and macro photography, where adding a teleconverter to a macro lens achieves a maximum magnification higher than the native maximum magnification of the lens (by the same factor as the increase in its effective focal length) without shortening the working distance of the lens. However, the effective lens aperture increases accordingly. Only one recent Nikon macro lens (105 mm f/2.8 G in F mount) is however compatible with contemporary Nikon teleconverters.

A qualitative discussion

A teleconverter is not a magic bullet without disadvantages. In practice, a teleconverter magnifies the central part of the image circle of a lens, and therefore it magnifies the lens aberrations by the same amount. No teleconverter can be optically perfect, and even a good teleconverter adds its own aberrations to those of the lens. In addition, a teleconverter cannot be perfectly optimized for lenses of all focal lengths. The manufacturer specifies an optimal range of lens focal lengths that should be used with a specific teleconverter, and outside this range the results may be worse. Certain unusual lens designs may also behave worse than others with teleconverters.

The unavoidable increase in effective aperture caused by teleconverters has numerous undesirable side effects, mainly:

• longer exposures, and an increased risk of motion blurring by subject movement,
• increased ISO, and a proportional increase in high-ISO noise,
• slower and less reliable autofocus, or no autofocus at all,
• less reliable subject recognition.

A further characteristic of teleconverters is that each model has an intrinsic limiting aperture. In practice, this means that a very fast lens, e.g. f/1.2, coupled with a given 2x teleconverter may not behave like an f/2.4 lens, but f/4 or f/5.6. This is less of a problem with modern teleconverters designed for lenses of relatively short focal length, but even these teleconverters do have their own limiting aperture.

Certain lenses of very long focal lengths may vignette in the corners when used with teleconverters, or with specific teleconverter models.

Better use a teleconverter, or crop and magnify?

The above question is like asking whether a lens with teleconverter can produce finer real image information than that produced by the same, unaided lens. The test images can be moderately processed e.g. to improve contrast, but I exclude from this test the use of AI generative techniques to improve the image quality, because these techniques do create detail not originally present in the image, and often not present in the subject, either.

The suitability of teleconverters has been tested by multiple photographers, and the overwhelming result is that, with a lens of good quality and a teleconverter of good quality well matched to the lens, the image shot in the best possible conditions with the teleconverter does visibly contain more real fine detail than the image shot in optimal conditions with the unaided lens. A necessary condition for this to be true is that the unaided lens must outresolve the sensor by a sufficient amount to still provide a high-quality image even with the teleconverter. In other words, should you fail to obtain a better real detail with the teleconverter, versus the unaided lens, and assuming your technique is not at fault, then you are likely using a lens and/or teleconverter of too limited quality and/or mismatched to each other.

In this context, it must be kept in mind that a lens of limited speed, e.g. an f/5.6 supertelephoto, with a 2x teleconverter becomes f/11, and diffraction blurring begins to be observable. If this lens must be stopped down to f/8 to allow recording fine detail and minimizing aberrations, this is feasible without teleconverter. With the teleconverter, however, the lens after stopping down would become effective f/16, which is well into diffraction domain. A high-megapixel sensor places further demands on lens quality and speed. While effective f/11 may still be acceptable on a full-frame 24 Mpixel sensor, it produces detectable diffraction on a 47 Mpixel sensor, and f/16 on 47 Mpixel already blurs fine detail quite visibly.

Naturally, a lens with a native speed of f/4 and good enough to be used fully open without visible losses offers more "headroom" for the use of teleconverters. On the other hand, in the past decade we have seen most major camera system brands introduce relatively cheap f/5.6 (and even some f/8 and f/11) super-telephoto zooms or primes. There is clearly a market for these cheaper lenses among photographers unable to purchase top-of-the-line super-telephotos or unwilling to carry them in the field, and they tend to be optically much better than the cheap catadioptrics of the past. However, keeping their prices reasonable comes at the expense of a little optical quality (compared to top-of-the-line models), and an important part of these savings is a reduction in lens speed. This makes these more affordable lenses inherently less suitable for use with teleconverters, albeit often excellent without teleconverters, and buyers of these lenses should not expect miracles. However, the price difference between e.g. a 600 mm f/5.6 and an f/4 is massive, and the f/4 lens also weighs substantially more.

Teleconverters compared to other methods

More in general, one may rank teleconverters among the alternatives for obtaining "more reach". A common opinion among experienced photographers, which I also share, is:

1. Best of all is a longer telephoto lens/zoom.
2. Second best is using a high-megapixel camera, which allows more cropping in post-processing than e.g. a 24 Mpixel camera of the same format (when coupled with a sharp lens).
3. Alternatively, use a camera with a smaller sensor, which often provides a higher pixel density. The main alternative in this case is using an APS-C camera instead of a full-frame one, since APS-C generally offers smaller sensels than even a high-pixel-count full-frame, and often can use the same lenses as full-frame. The top-of-the-line Nikon DX versus FX DLSR cameras are a good example. In the Nikon Z system, however, the current DX cameras are beginner-level and lack much of the sophistication of professional and semi-pro FX cameras.
4. Teleconverters come third, except in cases where the preceding alternatives are not available, or not sufficient. It also depends on how well one's longest telephoto lens works with teleconverters. Know your equipment.
5. The last resort is cropping and sharpening (without using generative AI) in post-processing, which could be regarded as "cheating", but sometimes is the only remaining way to get a reasonably sharp picture.

The above techniques are not mutually exclusive. Technique 1 and 2 can often be combined. A judicious amount and type of sharpening, without introducing obvious artefacts, is often used by experienced photographers.

A brief quantitative discussion

A teleconverter cannot avoid reducing the effective speed of the lens by a fixed factor, in the case of 1.4x and 2x teleconverters by one stop and two stops, respectively. The speed of a lens is generally given by the formula:

$$\mathit{s}=\frac{\mathit{f}}{\mathit{d}}$$

where s is lens speed, f focal length, and d the diameter of the front lens element. In some types of optical design, especially wideangles, it is common for the front lens element to be wider than specified by this formula. In practice, however, it cannot be smaller.

The above formula also shows why it is not possible for a teleconverter to increase the lens focal length without increasing its f/ speed accordingly. The Nikon TC-20 teleconverter series, for example, has a focal length multiplier factor of 2, and the formula becomes

$$\mathit{s}=\frac{\mathbf{2}\mathit{f}}{\mathit{d}}$$

For example, by adding a TC-20 to a 300 mm f/2.8 (which has a front element with an open diameter of 107.1 mm, and therefore an f/ speed of 300 mm / 107.1 mm = 2.8), one obtains a 600 mm f/5.6 (i.e. 600 mm / 107.1 mm = 5.6 ).

A teleconverter magnifies the optical aberrations of the lens it is attached to, and adds a small amount of aberrations of its own. Therefore, teleconverters of good quality and low magnification factor (e.g. 1.4x) are more likely to provide good results. Even a good teleconverter, however, if mounted on a poor or marginal lens cannot correct the aberrations of the latter.

In practice, a way to describe how a teleconverter works is that it takes the central 50% (1.4x teleconverter) or 25% (2x teleconverter) of the image area projected by the lens, and magnifies it onto the whole sensor.

Speed boosters

A speed booster is a device that physically resembles a teleconverter, and like a teleconverter mounts between a lens and a camera. However, the optical result is essentially the opposite of a teleconverter: a speed booster decreases the effective focal length of the lens and makes the lens effectively faster.

To achieve this result, the speed booster must decrease the size of the image circle projected by the lens. Thus, it is necessary to use a speed booster, for example, with a full-frame lens on a crop-factor (APS-C or Micro 4/3) camera, because reducing the size of the image circle projected by this lens still leaves it large enough to cover the whole sensor. For this reason, the two bayonets at either end of the speed booster are of different types. As far as I am aware, all commonly available speed boosters accept a full-frame SLR or DSLR lens and mount on a smaller-format mirrorless camera (most often, Micro 4/3). The focal-length reduction factor of a speed boosters is limited, often between 0.8x and 0.7x.

As a result of the shorter effective focal length, a combined lens and speed booster is usually faster than the native lens speed. Like a teleconverter, however, a speed booster also has an intrinsic maximum speed, which limits the effective speed achievable with any lens mounted on the speed booster.

Teleconverters for fixed-lens cameras

A number of teleconverters are available for cameras equipped with a non-interchangeable (typically zoom) lens. These teleconverters mount at the front of the lens, are designed roughly like a Galileian telescope (i.e. with a converging group as objective and a rear diverging group as eyepiece, producing an afocal, non-inverted image). These teleconverters are either limited in magnification and physically rather small, or remarkably expensive, in the latter case because they use large optical elements. As a rule, the camera lens should be zoomed to its maximum focal length to avoid vignetting with these teleconverters.

As an example, the legacy Nikon TC-E3ED teleconverter for certain Nikon Coolpix cameras has a 3x focal-length magnification factor.

Conclusions

Teleconverters are an effective way to increase the focal length of a system camera lens. The total cost is typically less than purchasing a second lens, but teleconverters have intrinsic limitations, foremost a "loss" of lens speed (1 and 2 stops for 1.4x and 2x teleconverters, respectively) relative to the native speed of the lens.

Using a teleconverter typically allows the capture of more fine detail than cropping and magnifying in post-production an image shot without teleconverter.

A teleconverter of good quality typically gives good results when coupled with a compatible lens of good quality. This is especially true of low-power teleconverters (e.g. 1.4x). Most teleconverters are optimized to work well with lenses of a specific interval of focal lengths.

All teleconverters magnify the aberrations of a lens by the same amount they magnify its focal length. A poor lens on a good teleconverter, or a good lens on a poor or non-optimal teleconverter, cannot make miracles. In addition, teleconverters result in higher effective apertures, which in turn result in increased diffraction blurring even in the absence of other aberrations. Higher effective apertures also decrease the speed and reliability of autofocus and subject recognition, and force longer exposures and/or higher ISO.