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Teleconverters – are they worth having?

For years I have carried, but rarely used, a couple of teleconverters. Many people get a high proportion of unsatisfactory results when using these (including me) and I have been wondering why. I have long suspected that the lack of sharpness usually experienced is not an optical shortcoming but may instead be largely due to difficulty in achieving focus, or maybe even just poor shooting technique.

With this thought in mind I set up my Canon 100-400mm lens on a tripod, zoomed in to 400mm, and pointed it at an old plastic ruler I found in a drawer. I don't have a testing lab so hey, some impromptu drawer searching will have to do. The ruler was my choice of subject because I could place it at a slight angle and clearly see where the point of sharpest focus lay. Using all the usual landscape photographer’s techniques to eliminate camera shake in a long exposure,  I photographed this, first without any converters, then with my 1.5x (600mm), and then with my 2x (800mm). Finally, just to be silly, I tried a shot with both converters fitted, so effectively turning my 400mm lens into a 1200mm. Wow!

I say “Wow!” because I have just recently seen an advert for a second-hand, exceedingly rare Canon 1200mm prime lens for a mere snip at £99,000 – ouch.

The results I got surprised me. I have displayed below a 750 x 600 pixel section from each image, shown at full-size (100% in Photoshop) so you can compare them and draw your own conclusions.

Test image 1
f5.6, 1/2 second, ISO 100, 400mm (no teleconverters)

Test image 2
f5.6, 1 second, ISO 100, 400mm with 1.5x teleconverter (600mm)

Test image 3
f5.6, 1.6 seconds, ISO 100, 400mm with 2x teleconverter (800mm)

Test image 4
f5.6, 4 seconds, ISO 100, 400mm with BOTH teleconverters (1200mm)

I have done no processing to these aside from using exactly the same settings for each in the conversion from RAW files.

Frankly, I was stunned. I really did not expect anything remotely useable with both teleconverters fitted. However, it must be realised that I was photographing a static subject indoors, so I could focus accurately (zooming to 10x in Live View to do so) and that I was able to eliminate camera shake (without using the lens's built-in Image Stabilisation which was swiched off). Even a light breeze would have made this extremely difficult if done outdoors.

Bear in mind that the scale down the right edge of the final shot is in 1/16th inch divisions. Also remember that these are shown at “actual pixel” resolution, so the same as you see at 100% magnification in Photoshop. Normally your images will not be enlarged this much so should appear sharper.

The eagle-eyed among you will have noticed I shot these images at f5.6. That’s wide open for the lens and so sharper results could be achievable by using a smaller aperture. I believe that a Canon extender will not allow the lens to be set to its wider apertures. If that is so and you own a Canon extender, you might also be wondering how I could shoot at f5.6 at all! Well, my “extenders” are made by Kenko and do not impose any restricted aperture range on us. The Kenko teleconverters are also about a third the price of Canon’s extenders. For something I use so infrequently, I’m very happy to make that cost saving.

Much has been written about how, for example, a 2x teleconverter effectively turns a 300mm f2.8 lens into a 600mm f5.6 lens, and how the depth of field is affected. In my opinion, most of the information presented is confusing to anyone without a degree in physics. So, in my usual pragmatic way, I would like to take this opportunity to throw out all that technical gobbledygook and try to explain it in simpler terms.

Depth of field is our perception of what is sharp (often referred to as "acceptably sharp"). A teleconverter effectively works like a magnifying glass between your lens and your camera. It does not change how the lens itself behaves. Without a teleconverter, let's say that everything from point A to point B appears acceptably sharp. Without altering the lens's settings, introducing a teleconverter simply magnifies that image. Because it is magnified, what was "acceptably sharp" before may no longer appear so sharp because we're seeing it enlarged. In other words, the depth of field will appear shorter for the same lens settings. By using a smaller aperture, the depth of field can be increased. Without going into any mathematics, let's just accept that closing the aperture by a couple of stops, say from f2.8 to f5.6, will do that for us. I hope that clears things up a bit!

One final observation from my tests, which is not apparent from the images I've shown above, is that the 1.5x teleconverter displayed some pretty severe vignetting. (That is probably to do with the teleconverter I own as opposed to 1.5x teleconverters in general.) The 2x displayed only a little. When the two were stacked, there was none. Go figure!

My conclusion is that some teleconverters are physically capable of producing some decent results, far better than I had expected to be honest. However, they come with some quite severe difficulties and limitations, and their performance will depend on the lenses used with them. A lens with a large maximum aperture (say f2.8) will be a better choice. A high quality lens will also be a better choice. Teleconverters will not only magnify the image, they will magnify any optical shortcomings of the lens. Any failings in technique will also be greatly magnified. Camera movement is exaggerated, as you would expect with any long telephoto lens. Teleconverters also seem to slow down the camera's ability to auto-focus and might even cause some difficulty in locking focus at all. You might have to rely on manual focusing. They will cut down the amount of light reaching the camera's sensor, meaning you will need to use a slower shutter speed, all else being equal. Of course, that flies in the face of the faster shutter speed you'll need to combat the increased effect of camera shake!

So, are teleconverters for you? Well, I would not recommend them for sports photography. Granted, I have managed to take an occasional sharp sports image while using a teleconverter but the ratio of good shots to bad was pretty poor. It was more by luck than skill or judgement and I was desperate! I would not recommend them for landscape photography unless there is no other way to get the shot. Their use for wildife photography will also be limited to more static subjects in strong light and possibly only with the best lenses such as Canon's 500mm/f4 L IS USM. So, for all but a few people and a few circumstances I feel they just don't deliver.

© David McHutchison (Deemacphotos)
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