There’s a reason why you can’t put a teleconverter on a cheap lens

If you’re into big zooms for sports and wildlife you may have come across an accessory called a teleconverter and thought it might answer all of your prayers when you need to zoom in even closer on the action. However, it may not be quite as straightforward as it sounds.

For those uneducated on teleconverters, also called ‘extenders’ by Canon, these are an accessory you can put between the lens and the camera body. The best teleconverters contain extra glass elements which magnify the image produced by the lens making it appear bigger and taking you closer to the action.

But, any time you introduce extra glass – you will inevitably impact image quality. For this reason teleconverters work best when paired with really high quality lenses that are super sharp and can get away with taking a small hit to the image quality. Lenses such as Canon’s top flight L-series Canon RF 100-300mm F2.8L IS USM or Canon RF 70-200mm F2.8L IS USM Z I recently tested out with a Canon RF 2x Extender at the annual deer rut in Richmond Park as the pro on Digital Camera Magazine’s recent apprentice feature.

Many teleconverters are designed so that they physically will not work with those cheaper optics that could produce poor results. Many manufacturers choose to give their teleconverters a front element which protrudes outwards and needs space in the reciprocal lens to be able to fit inside of. This rules out sticking a 2x teleconverter on your cheap and cheerful $300 70-300mm f/4.5-6.3, hoping for a 600mm focal length. – the image quality would be very poor and the aperture would become unusably slow, too.

This is because the extra glass inside a teleconverter reduces lightflow – how much depends on the teleconverter you’re using. On a 2x teleconverter it reduces lightflow by a full two stops of light, turning a fast f/2.8 lens into a more sluggish f/5.6. While a 1.4x teleconverter doesn’t take you quite as close to the action it does only reduce light by one full stop, turning an f/2.8 aperture into f/4. 1.7x teleconverters fall halfway between and reduce light by one and a half stops.

This isn’t to say teleconverters are a bad tool by any means. I’ve found them to be an amazing way to access more powerful focal lengths, both more affordably than buying another lens, and is also more convenient with a teleconverter taking up less space and weight in your kit bag. The key is to invest in a solid optic which is compatible with teleconverters in the first place.

When I was first cutting my teeth in wildlife photography, I spent pretty much all of my budget on a good quality Nikon 70-200mm f/2.8E AF-S FL EF VR – so I definitely didn’t have any extra cash to spend on a 300mm or 400mm f/2.8 prime. But, I was able to get most of the shots I wanted using this zoom lens and my 1.4x and 2x extenders and I’m still very happy with the results taken with this setup to this day.

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Want to know more? Check out the best 70-200mm lenses and the best telephoto lenses for wildlife.