The pros and cons of teleconverters

Teleconverters have been around for a long time and continue to be available because they’re seen as a cheap and convenient way to extend the range of a telephoto lens. Most consist of a magnifying lens (or lens assembly) that is inserted between the camera body and an existing lens, to provide an increase in focal length. A 2x teleconverter will double the apparent focal length, converting a 300mm lens to 600mm focal length. A 1.4x teleconverter will increase the focal length of a 300mm lens to 420mm.

Adding a teleconverter to a long telephoto lens will allow the photographer to capture dramatic close-ups of insects and small animals without ‘spooking’ the subject. These three frames show the field of view with a 300mm lens on a M4/3 camera (top), plus the magnifying effects of 1.4x (middle) and 2x (bottom) teleconverters. (Source: Olympus.)

Buying a teleconverter will cost you less than buying a new lens with the increased focal length. In addition, adding a teleconverter should mean you have less weight to carry than if you bought a longer lens. You can also fit a teleconverter without changing the minimum focusing distance of the lens, which is handy if you like taking close-ups.

However, teleconverters per se have some significant downsides. Recent improvements in optical design technologies have allowed some lens manufacturers to produce lenses with built-in teleconverters perfectly matched to the main optics in an attempt to overcome some of them.

Canon’s EF 200-400mm f/4L IS USM Extender 1.4x lens features a switchable 1.4x teleconverter that can extend its zoom range to the equivalent of 320-640mm.

Canon’s  EF 200-400mm f/4L IS USM Extender 1.4x lens has been available since 2013 and was reviewed for the Photo Review website in November of that year. Designed primarily for ‘full frame’ cameras, it can also be used on Canon’s APS-C sensor DSLRs, where it covers a focal length range equivalent to 320-640mm or 448-896mm with the extender engaged.

The new M.Zuiko Digital ED 150-400mm f/4.5 TC 1.25x IS Pro lens provides a fast telephoto zoom lens with a switchable 1.25x teleconverter for owners of Micro Four Thirds cameras. 

The M.Zuiko Digital ED 150-400mm f/4.5 TC 1.25x IS Pro super telephoto lens for Micro Four Thirds cameras has a basic zoom range equivalent to 300-800mm on a 35mm body, which can be switched to cover a 375-1000mm equivalent range. It is compatible with the Olympus 2x teleconverter, which can further extend that range to the equivalent of 750-2000mm.

Drawbacks

The main downside of teleconverters is a loss of lens speed, which is directly related to the magnification factor of the teleconverter. A 1.4x teleconverter will reduce the maximum aperture of the lens by a full f-stop, while a 2x teleconverter cuts it by two full stops. So if you have a 300mm f/2.8 lens, adding a 1.4x converter will give you a 420mm f/4 lens, while adding a 2x teleconverter transforms it into a 600mm f/5.6 lens.

Adding more glass – and glass/air interfaces – to an existing lens will create additional problems, since no lens is ever perfect. One of the most significant is the reduction of the amount of light reaching the sensor, which is a consequence of the longer reach the teleconverter provides.

Reducing the amount of light entering the lens will affect autofocusing because all AF systems depend upon having enough light. This will slow the speed at which most cameras will focus. Some entry-level DSLRs can even become unable to autofocus at some or all aperture settings because of the reduction in the light reaching the AF sensors.

Extending the effective focal length will also make it more difficult to keep the lens aligned and targeted upon the subject, particularly for sports, wildlife and bird photographers who usually deal with moving subjects. Stabilisation, either in the lens or in the camera body (or both) can help to keep viewfinder images steady.

But stabilisation can be affected by magnification. Fitting a teleconverter will increase the effects of camera shake by the converter’s magnification factor. This is true for both camera shake and subject motion so you’ll need to use techniques like increasing the shutter speed and/or using a tripod/monopod to deal with this problem.

Teleconverters will also reduce image quality because, while magnifying the image, they will also magnify the effects of any aberrations in your original lens. Sharpness and contrast are the most likely to suffer and ghosting and flare can go from being minor irritants you can mostly deal with to serious problems.

Ambient air temperature can also degrade image sharpness, particularly on hot days, where air passing over dark surfaces will become turbulent. With longer 2x extenders, even the best optics will have reduced performance because of the magnification effect.

Teleconverters can be useful with shorter zoom lenses. These three frames show the field of view with a 40-150mm lens on a M4/3 camera set  at 150mm (top), plus the magnifying effects of 1.4x (middle) and 2x (bottom) teleconverters. (Source: Olympus.)

Advantages
While the benefits of the extra reach are obvious, don’t discount the added benefit of reduced weight to carry. Depending on its ‘speed’ (maximum aperture) a 600mm lens for a ‘full frame’ camera typically weighs between 2.5 and 3.5 kg. A fast 400mm prime lens will also be in the 2.5-3.0 kg range.

But when you add a teleconverter to an existing camera+lens system, you’ll be increasing your load by less than 250 grams for a 1.4x extender or about 350 grams for a 2x teleconverter.

Precautions
Many teleconverters are only compatible with one – or a small number of – lenses. Always check the manufacturer’s website for compatibility before you purchase.

Some won’t support autofocusing. Manual focusing is often difficult, particularly when working in dim lighting and/or shooting low-contrast subjects. Sigma produces teleconverters with a variety of camera mounts and provides excellent data on the Sigma website to enable users to check the functionality of its converters with different lenses.

In addition, autofocusing may not cover the full range of the main lens. It’s not uncommon for manual focusing to be required at close focusing distances or at specified aperture settings. Firmware updates may also be required to ensure full compatibility with the latest cameras.

Teleconverters vs cropping

Cropping the frame provides an alternative to fitting a teleconverter – and it’s certainly a cheaper option. However, the more you crop, the more the image quality is likely to be reduced.

As we outlined in Issue 82 of Photo Review magazine, there are times when cropping can be appropriate – but because pixels are discarded, it will reduce resolution.

In addition, any problems in the image, such as noise or unsharpness, will be magnified by cropping and subsequent enlargement. However, that must be countered by an assessment of the possible loss of quality introduced by the additional lens plus any blurring due to camera shake caused by the reduced light transmission of the converter lens.

Teleconverters to consider (as at Jan 2020)

Canon Extender EX 1.4x III – features seven lens elements in three groups with Canon’s Super Spectra coating on all elements to minimise ghosting and flare. A new Fluorine Coating on the front and rear elements makes it easy to keep clean and grease-free. Dustproof and Waterproof (JIS2 equivalent). Only compatible with specific Canon EF L-series lenses and with some lenses, functionality will be limited.

Canon Extender EX 2x III – features nine lens elements in five groups with new lens materials for reducing ghosting and lens flare. Dustproof and Waterproof (JIS2 equivalent). Only compatible with specific Canon EF L-series lenses and with some lenses, functionality will be limited.

Fujifilm XF 1.4x WR Teleconverter – designed specifically for GF 250mm f/4 R LM OIS WR lens and features seven lens elements in three groups plus  integrated electronic contacts to make lens functions fully accessible. Weather-resistant construction. Claims no degradation of optical quality.

Fujifilm XF 2.0x TC WR Teleconverter – only compatible with XF50-140mm f/2.8 R LM OIS WR and XF100-400mm f/4.5-5.6 R LM OIS WR lenses. Weather and dust-resistant with nine elements in five groups. Claims no degradation of optical quality.

Fujifilm Teleconverter GF 1.4x TC WR – only compatible with Fujinon GF 250mm f/4 R LM OIS WR lens. Features seven lens elements in three groups plus integrated electronic contacts. Weather and dust-resistant construction.

Nikon AF-S TC-14E III 1.4x – can be used with Nikon FX and DX format F-Mount lenses with full support for autofocus, exposure metering and image stabilisation. Includes fluorine coatings on the front and rear optical surfaces for maximum image sharpness and clarity. Weather-resistant lens barrel. Not fully compatible with all lenses. Autofocus is available only with cameras that offer f/8 support.

Nikon AF-S 2x Lens Teleconverter TC-20E III – features seven elements in five groups plus Nikon Integrated Coating (IC). Utilisation of an aspherical lens element results in a more compact and lightweight teleconverter. Not fully compatible with all lenses.

Olympus 1.4x Teleconverter – MC-14 – designed specifically for the Olympus M.Zuiko Digital ED 40-150mm f/2.8 PRO and M.Zuiko Digital ED 300mm f/4 PRO lenses. Six-element lens construction. Dust, freeze and splash proof.

Sigma TC-1401 1.4x – A dust and splash-proof converter designed for professional use. Available in Sigma, Canon and Nikon mounts. Incorporates one SLD (Special Low Dispersion) glass element for minimising chromatic aberration.

Sigma TC-2001 2x – Similar design to the TC-1401 1.4x  with two SLD (Special Low Dispersion) glass elements for minimising chromatic aberration.Available in Sigma, Canon and Nikon mounts.

Tamron Teleconverter 1.4x – designed for the Tamron 150-600mm f/5.6-6.3 Di VC USD G2 and Tamron SP 70-200mm f/2.8 Di VC USD G2 lenses. Features six elements in three groups plus BBAR (Broad-Band Anti-Reflection) coating. Full electronic communication between camera and compatible Tamron lenses. Moisture-resistant design.

Article by Margaret Brown (see Margaret’s photography pocket guides)

Excerpt from Photo Review Issue 83

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