Using teleconverters

Teleocnverters are really accessories to have. These two models are from Kenko: the 3x Teleplus Pro 300 (left) sells for £220 and the 2x model costs £200.  A review of these converters will be published tomorrow.

Teleconverters are compact, optical accessories that fit between the lens and the camera body, and effectively multiply the focal length of the lens they are fitted to. Several magnifications are available: 1.4x, 1.5x, 2x and 3x. Fit a 2x teleconverter on a 70-200mm zoom and you have an effective 140-400mm or a 1.5x converter on the same lens changes it to a 105-300mm. Not only do you get greater magnification or 'pulling power' but the lens's minimum focusing distance remains unchanged so a converter benefits close-up shooting too. However, depth-of-field does decrease – by one-half for a 2x converter.

Click on the thumbnails below to view high resolution images
The image on the left was taken at the 200mm end of a 70-200mm zoom lens. Adding a 3x teleconverter to the lens thus giving a focal length of 600mm gets you a more tightly framed image without you having to move at all.

On the face of it, it is all win-win with teleconverters. They are compact, lightweight, reasonably inexpensive gadgets that expand the range of focal lengths at your disposal.

However, there are important downsides. One, they are optical accessories so they can have a negative impact on image quality – resolution and sharpness can suffer and adding extra air/glass surfaces means flare can be more of a problem in backlit conditions.

Two, they cause a light loss. A 1.4x /1.5x teleconverter causes a one stop light loss; while 2x and 3x models lose two and three stops of light loss respectively. This might not sound much but in practice it is significant and means that you need to keep an eye on shutter speeds to avoid camera shake and adjust the ISO accordingly. For example, fitting a 2x teleconverter to a 70-300mm f/4-5.6 turns it in to a 140-600mm f/8-11. On cloudy days when there is less light around you could try shooting at slower shutter speeds but the longer focal length does mean camera shake is a serious risk. Or increase the ISO to allow faster shutter speeds at the expense of image quality. Just remember the reciprocal rule, ie with an equivalent of a 600mm lens set a shutter speed of at least 1/600sec.

The light loss also means the viewfinder image is much darker than with the lens on its own. This makes composition trickier but it also means that the camera's autofocus system will be less efficient or it might not work at all.

Generally speaking, you get the better optical performance with 1.4x or 1.5x teleconverters because their optical design is less complex and the 2x and 3x models should only be considered if you need the extra focal length. For general ‘just in case’ use, buy a 1.4x or 1.5x converter. Lenses of at least 100mm are recommended for 1.4x/2x converter use and 200mm for 3x models. It also makes sense to buy the best that you can afford.

Whatever you use, for the best possible image quality close the lens a couple stops down from maximum aperture ensure the best possible quality.

Finally, make sure that you use a decent monopod or tripod such as a Manfrotto to ensure maximum camera stability too.

Teleconverters are available from many camera brands and from independent brands too – Sigma and Kenko.

Find the tripod to suit your needs at www.manfrotto.co.uk.

Don't forget to enter our exclusive competition where you can win one of six Manfrotto 190XPROB tripods!


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