Photo Review Tech Editor Margaret Brown looks into the benefits and consequences of using focal length extenders.
Teleconverters are popular with birders because they provide an affordable ““ and relatively space-saving ““ way to achieve very long effective focal lengths. This shot was taken with a 2x teleconverter on a 200mm lens fitted to a ‘full frame’ DSLR camera. Good ambient lighting and effective optical stabilisation were required to allow for the reduction of two f-stops in the light reaching the image sensor.
Ever since the days of 35mm film photography, photographers have been looking for affordable ways to extend the capabilities of their cameras and lenses. Among the most popular products are teleconverters, or focal length extender lenses, which have been popular since the introduction of SLR cameras and continue to be used in the digital age.
Wide-angle extenders and extension tubes for macro photography are other products that fit into the ‘extender lenses’ category. Both continue to be used, with Metabones (see below) providing some technological advances in the former category with its Speed Boosters.
However, teleconverters still garner the largest number of actual sales, largely because they represent an affordable and handy way to obtain more ‘length’ from existing lenses without adding significant weight or cost. But they aren’t necessarily compatible with all cameras and lenses and they do have significant downsides. This article will explain some of the plusses and minuses of extender lenses.
All teleconverters are secondary lenses for magnifying the central area captured by the primary lens. They come in two types, the most common being mounted between the camera body and the imaging lens. Front-mounted teleside converters are also available (see below).
A typical teleconverter lens, the AF-S Teleconverter TC-20E III from Nikon, shown with the diagram of its optical design. The single aspherical lens element is shaded in blue. (Source: Nikon.)
Conventional teleconverters work like a group of lens elements that together act as a magnifier. The image produced by the light passing through them is focused further away than the main lens would normally focus, thereby enlarging it.
Teleconverters are normally defined by their multiplier effect, with the most common multipliers being 1.4x and 2x. A 1.4x teleconverter on a camera with a 35mm sensor will magnify a 25.7 x 17.1 mm area in the centre of the lens’s field of view to fill the 24 x 36 mm frame, while a 2x teleconverter will enlarge the central 12 x 18 mm area to fit the 36 x 24 mm frame.
While this can be handy, there are some important downsides to using teleconverters, the principal ones being highlighted below:
Light loss (lens speed): While increasing magnification doubles the usable focal length of the primary lens, it also reduces the intensity of the light reaching the image sensor by the square of the teleconverter’s magnification. For a 1.4x converter, this equates to one f-stop (1.42 = 2), which means half the light that would normally be transmitted to the sensor is lost when the converter is fitted. A 2x converter reduces the light intensity by a factor of four, which is equivalent to two f-stops.
DSLR users will see this light loss through the optical viewfinder and it may become more difficult to focus (see below), particularly when a 2x converter is fitted. Photographers who use cameras with electronic viewfinders (EVFs) won’t be as disadvantaged since they can boost the brightness of the EVF’s screen.
Focusing speed: Reductions in light transmission can slow autofocusing speeds, regardless of which type of AF system is used. Both contrast and phase-detection systems rely on adequate light.
Some camera/lens combinations will be unable to focus at all, because the AF sensors don’t receive enough light. Most lose the ability to autofocus at a maximum aperture of f/8. Consequently, fitting a 2x teleconverter to a 70-200mm f/4 lens will reduce the maximum aperture by two stops to f/8, making it unable to autofocus.
With a 1.4x teleconverter the same lens would have a maximum aperture of f/5.6 and be just able to maintain autofocus. But even then, you could be unable to use most of the camera’s autofocus points and have to switch to manual focusing.
Camera shake: When you extend the focal length of any lens, camera shake becomes more noticeable. Adding a teleconverter magnifies both your subject and any camera shake. The problem becomes worse with longer telephoto lenses where the magnification is greater from the start.
Quality loss: No matter how well-designed the teleconverter lens is, any additional glass inserted in the light path can reduce image quality ““ although it may not be enough for you to notice. Lens aberrations and other optical problems in the original lens will also be magnified by the teleconverter and are likely to be noticeable. For this reason, 3x teleconverters are almost never used.
The top image was captured with a 400mm telephoto lens on a camera with a 36 x 24 mm sensor. A 1.4x teleconverter was added to produce the image below it. Note the slight increase in vignetting (edge and corner darkening) in the lower picture.
Compatibility issues: Not all teleconverters work with all primary lenses. Lenses with protruding rear elements can’t be used with teleconverters; when the lens glass is flush with the mount there is nowhere for the teleconverter glass to go.
Macro photography: Teleconverters can also be used to extend the magnifying power of macro lenses. Some macro lenses may be able to detect and adjust for the altered f-stops; others can’t and will require you to shoot in manual exposure mode. The main problem with using a teleconverter on a macro lens is the dramatic reduction in depth of field. There is no real fix for this once you’ve passed the point at which stopping down becomes impractical.
Since different cameras have different lens mounts, in most cases, the teleconverter you use should be made by the same manufacturer or a third-party lens manufacturer that produces compatible products. Tamron and Tokina had no teleconverters listed on their websites when this article was produced. Sigma was offering four teleconverters, the TC-1401 1.4x Teleconverter and TC-2001 2.0x Teleconverter for Canon, Nikon and Sigma cameras and the APO 1.4x EX DG Teleconverter and APO 2.0x EX DG Teleconverter for Canon, Nikon, Pentax (longer primes only), Sigma and Sony cameras.
Canon’s built-in teleconverter
Back in May 2013, Canon announced the EF 200-400mm f/4L IS USM Extender 1.4X lens with a built-in teleconverter. This lens remains unique as the only professional telephoto zoom lens with a built-in extender. Being large, heavy (3.5 kilograms) and very expensive (around $15,000 in Canon’s online store when we went to press), it’s mainly used by specialist sports and wildlife photographers.
Its main advantage lies in being able to switch the extender in and out without taking the lens off the camera. This saves time and eliminates the risk of exposure of the camera’s image sensor in environments with dust and high humidity. It also makes it possible to extend the focal length without changing the overall lens length.
Interestingly, switching the extender group into the optical path reduces the maximum aperture by one f-stop, which is the same as fitting a 1.4x teleconverter. But it’s much faster, thanks to a simple lever switch.
Canon’s EF 200-400mm f/4L IS USM Extender 1.4X lens. (Source: Canon.)
Tips for using teleconverters
1. Telephoto lenses are best used on fast (f/2.8 or better) prime lenses. The faster the primary lens is, the less the reductions caused by the teleconverter will matter.
2. Avoid using zoom lenses with teleconverters. Although some zoom and teleconverter pairings are possible, most zoom lenses are too slow to work with teleconverters. Avoid mid-range zooms like 18-55mm and 18-70mm on APS-C DSLRs or 14-14mm on M4/3 cameras, particularly if they are ‘kit’ lenses, which are usually relatively slow.
3. Keep track of the effect the teleconverter has on focal length and aperture. The camera may display an aperture setting but it’s likely to be wrong because it doesn’t account for the effect of the teleconverter. Account for the light loss when estimating what shutter speed and/or ISO setting to use and whether autofocusing is possible or reliable.
4. The magnification of optical problems can be best worked around by using very high quality optics that have fewer problems to magnify. Minor distortions can usually be corrected when images are edited, although more powerful teleconverter lenses will make cropping photos problematic.
5. While it’s possible to stack teleconverters (using two or more attached end to end), this will result in much greater magnification of inherent problems and correspondingly greater challenges to be dealt with. In addition, not all teleconverters are compatible and some teleconverters will vignette with other teleconverters. There is no way to know if there will be problems without trying combinations out.
6. An alternative to using a teleconverter or extender is to crop the images you capture. It’s cheaper than buying an extender lens but you end up with fewer image pixels. You can get away with cropping to a limited degree. But if you need to really magnify your images a lot, a converter is a better option.
Teleside converters are also known as front-mount teleconversion lenses. They attach to the camera’s primary lens by means of an adapter ring and are designed for use with cameras that have non-interchangeable lenses, such as video camcorders and compact digicams.
Because they simply magnify the image, they don’t change the focal ratio and, therefore, don’t affect shutter speed.
However, being additional glass, they have the potential to reduce image quality and their magnifying power will amplify any aberrations in the converter or primary lens. More powerful teleside converters can also cause vignetting (corner darkening).
Wide-angle Extender Lenses
A few companies produce lenses that extend the wide-angle coverage of lenses. Like teleside converters, these add-ons screw into the filter thread on the lens. They are normally specific to a single lens ““ or a limited group of lenses. In most cases, they are cheaper than buying a new lens with the desired coverage.
The VCL-ECF2 Fisheye Converter for the Sony SEL16F28 16mm f/2.8 and SEL20F28 20mm f/2.8 E-mount lenses.
Sony is a major participant in this area, announcing four new conversion lenses (two for full-frame and two for APS-C) at the beginning of 2015. One set, comprising an ultra-wide and a fisheye converter, is specific to the FE 28mm f/2 lens announced at the same time, while another set was for use with the existing 16mm (SEL16F28) and 20mm (SEL2028) APS-C E-mount lenses.
Olympus offers five conversion lenses for attaching to M.Zuiko digital lenses. They include two wide converters (MC-14 and WCON-P01), a fisheye converter (FCON-P01) and two macro converters (MCON-P01 and MCON-P02). All five are compatible with the M.Zuiko Digital 14-42mm F3.5-5.6 II and M.Zuiko Digital 14-42mm F3.5-5.6 II R lenses. Some can be used with a few other lenses in the company’s line-up.
Extension tubes, like teleconverters, are mounted between a lens and the camera to increase magnification. They’re commonly used with macro lenses or standard prime lenses.
Unlike conversion lenses, extension tubes contain no optical components so they don’t increase aberrations, although they will reduce light levels because they increase the length over which the imaging light must travel. Most extension tubes lack electronic connections and require manual exposure adjustment.
Metabones Speed Boosters
The original Metabones Speed Booster, announced in January 2013, was designed to enable DSLR lenses to be used on mirrorless cameras. It took advantage of the short flange distances (between the back of the lens and the image plane) in M4/3 and Sony’s E-mount cameras, to increase the maximum aperture of the fitted DSLR lens by one stop; hence its name. At the same time, it increased the field of view of the lens by a factor of 0.71x and boosted the lens MTF (modulation transfer function, an indicator of imaging performance).
Since then, Metabones Speed Boosters have been manufactured for a wide range of camera bodies (both still and cine cameras) and most popular lens mounts. Field-of-view magnification ranges from 0.58x for some cine cameras to 0.64x and 0.71x for digital cameras with M4/3, APS-C and full frame sensors.
Some examples of Metabones Speed Boosters, clockwise from top left: Canon EF to Micro Four Thirds, Canon EF to Sony E-mount, Nikon F G-mount to Fujifilm X mount, Leica R to Sony E-mount. (Source: Metabones.)
According to developers (Brian Caldwell of Caldwell Photographic Inc and Wilfried Bittner of WB Design), the Speed Boosters don’t introduce any aberrations. On the contrary, they claim the ‘optical performance of the new Speed Boosters is so good that the MTF of any lens attached to it will be improved’.
In addition, since the Speed Booster essentially compresses the image formed by the objective lens into a smaller size, it helps to undo the cropping effect although it doesn’t completely recover the corners of the original lens. The latest Speed Boosters support autofocusing and include the required contacts for transmitting AF signals between the camera body and the main lens.
Naturally, these advantages aren’t cheap. Local prices range from around $600 to $1150, depending on the magnification factor and camera/lens mount combination. (In some cases it would be cheaper to buy a lens that covered the wanted field of view.)