Teleconverters 101

The problem, I think, is that it seems so simple. “If I add a 2x teleconverter I’ll get that closeup shot I want.” We all know what a teleconverter’s purpose is: it magnifies the image by a factor of 1.4x or 2×. A 300mm lens with a 1.4x converter becomes a 420mm lens. With a 2x converter its a 600mm lens. So cool: why pay $6,000 for a 600mm lens that weighs 12 pounds, I’ll just mount a 2x converter on my trusty 70-300 f/4-5.6.

It sounds pretty straightforward. But the reality many inexperienced photographers find is “my shots were all horribly soft.” Or “the camera wouldn’t focus”. The more experienced among you have been through all this and needn’t read any further. But for those just starting out, who haven’t realized yet that almost nothing affects photography more than using a teleconverter, this is required reading.

Teleconverters do a lot more than magnify the image though:

1. The teleconverter reduces the maximum aperture of the lens by one stop (1.4x converter), 1.5 stops (1.7x converter) or 2 stops (2x converter). An f/4 lens becomes an f/5.6 lens with a 1.4x mounted. An f/5.6 lens becomes f/8.
2. Since most cameras lose the ability to autofocus at f/8, an f/5.6 lens with a 1.4x converter mounted won’t autofocus on most camera bodies.
3. Teleconverters add an extra set of elements to the light path after the lens has done its job focusing the image refracting the image one more time. This will degrade image quality either a little bit or a lot depending on the lens and converter being used.
4. Teleconverters add an extra set of electrical contacts between the camera and the lens. In the case of some third party converters electrical information may not be passed from the camera to the lens (this can be a good thing as well as a bad thing as we’ll discuss later).

So lets look at each of the effects the teleconverter will have on our setup. Then I’ll try to give a summary of when teleconverters are great, when they suck, and when they may be OK. Yeah, I know, most of you are already down at the summary. But maybe you’ll come back later and read the stuff in between.

Effect on Aperture and Autofocus

A teleconverter spreads the image out over a larger surface area. This makes the central part of the image cover the camera sensor (magnifying it) but also discards all the light rays from the edges of the image, so the amount of light reaching the camera is decreased. With a wide aperture lens (say f/2.8) this isn’t too big of a deal as long as the light is good. An f/2.8 lens with a 2x teleconverter is now effectively an f/5.6 lens (two stops lost). Shutter speed will be slower, obviously, which may affect your photography if the light isn’t good, but that usually shouldn’t be too big of an issue.

The effect causes more problems when lens being used isn’t a wide aperture lens. Some people don’t realize that when the camera is autofocusing it opens the lens to its widest aperture, autofocuses, then closes the lens down to the aperture you’ve set (unless you’ve set it to the widest aperture), autoexposes the image, then takes the shot, all in a split second. The majority of consumer and prosumer cameras are able to autofocus at an aperture of f/6.3 or wider but cannot autofocus at f/8. Most professional cameras (Canon 1D series, Nikon D3, etc.) can autofocus at f/8, although only certain autofocus points may be active at this aperture. (BTW—this effect isn’t really about the amount of light entering the camera’s autofocus system, it is more about the focusing system comparing light entering from two different angles. If you’re really fascinated by the optical physics involved you can read more about it HERE )

The addition of a 1.4x teleconverter to an f/5.6 lens, or the addition of a 2x teleconverter to an f/4 lens changes the maximum aperture to f/8. Most non-professional cameras will read the maximum aperture as f/8 and then won’t even try to autofocus. This can lead to one interesting effect with f/4-5.6 zoom lenses when a 1.4x converter is used: the lens may autofocus at the short end where the aperture is f/4, but stop autofocusing when you zoom to the long end where the aperture is f/5.6. Most professional grade cameras can focus at f/8, but usually only at the center point, or with other limitations (single shot autofocus only, etc.).

Anyway, the summary of all this is a 2x teleconverter should only be used on lenses with aperture at least f/2.8, a 1.4x teleconverter (and usually a 1.7x) can be used on a lens with an aperture of at least f/4, and lenses with maximum aperture of f/5.6 or more should not be tried with teleconverters. There is one workaround some people have tried with mixed success: using a third party (Kenko, etc.) teleconverter that doesn’t fully report electronic information to the camera may fool the camera into not realizing the lens / teleconverter combination is an f/8 aperture. The camera will then at least attempt to autofocus and in very good light may actually be able to do so. Sometimes. Slowly and unpredictably. There’s also a trick some have reported with putting tape over the electronic contacts to prevent the camera from recognizing the teleconverter is present that accomplishes the same thing. We really don’t recommend it: tape residue on delicate electronic contacts can ruin them.

Optical Effect on Image Quality

All teleconverters bend the rays of light an additional time after they leave the lens and before they get to the camera sensor. This is going to have some effect on image quality. Now, just like with lenses, a good quality teleconverter will have less effect than a bad quality converter on image quality. But there’s another variable involved. I don’t speak optical physics well enough to explain all the details, but the rays of light from a wider angle lens are affected much more adversely by a teleconverter than the rays of light from a telephoto lens. Similarly, most zoom lenses are affected more adversely than a prime lens (the Nikon 200-400 VR seems to be the one exception to this rule).

The end result of all this is that the best teleconverters are ‘tuned’ optically to work best with supertelephoto prime lenses. They are OK with telephoto lenses, and not very good with standard range lenses. Adding a 1.4x converter to a 400mm f/2.8 lens has almost no effect on image quality. Put the same converter on a 70-200 f/2.8 zoom and the image will be affected a bit more, although it will probably be acceptable. But put it on a 24-70 f/2.8 zoom and the image will be noticeably soft and a bit distorted. 2x converters have more effect on image quality than a 1.4x converter—noticeable on a supertelephoto prime, quite noticeable on a telephoto zoom. The manufacturers are so aware of this that Canon and Nikon have made their teleconveters and lenses in such a way that the converter will only mount to certain lenses, trying (without much success) to keep people from putting a teleconverter on their 18-200 superzoom and getting horrible images.

One note about image quality: its always subjective. I’ve had people who told me they got great images from a 50-500 zoom with a 1.4x teleconverter mounted. They are pleased with them, which is the important thing, while I would have found them unacceptable. Some of the difference may be comparison experience. I’m thinking about the shots I got with a 500mm prime lens, printed at a 16×20 inch size. They may be using their images primarily to show on the internet or to make smaller prints from. What is not OK to me, may be perfectly acceptable to them. Some of it may be technique. I’m not the steadiest hand with a long telephoto zoom. A shot that might have awful motion blur when I took it might be much better shot the same way by another photographer.

Magnification of Technical Errors

The other issue that can be important when using a teleconverter is technique to eliminate camera shake and vibration, which can cause motion blur. The standard rule of thumb for eliminating motion blur is shutter speed less than 1/focal length. So basically a 500mm lens requires a shutter speed of 1/500 second to get a sharp picture. But if our camera is a crop camera the effective focal length is really 750 mm. Add a 2x teleconverter and the effective focal length is now 1,500mm so shutter speed should be at least 1/1,500 of a second. But since we’ve taken away two stops of aperture with our 2x converter, it may be very difficult to get this fast of a shutter speed. If the lens is image stabilized, the stabilization will still work with the teleconverter and will help us enough that we may get by with a slower shutter speed, perhaps 1/500. At any rate, if you’re going to shoot with a really long lens, a tripod is probably a really, really good idea. I can’t tell you the number of people who tell me a teleconverter made their images really soft and when I look at their shots they’ve tried to shoot a 750mm focal length hand held for at a 1/100 shutter speed. Its not going to work.

So What’s the Bottom Line:

• Teleconverters work very well with f/4 or f/2.8 telephoto prime lenses (300, 400, 500, 600), especially mounted on a tripod.
• Teleconverters work acceptably on high quality zoom lenses with maximum apertures of f/2.8 or f/4.
• Teleconverters don’t work well with consumer (f/5.6 zooms), medium range zooms, or wide angle lenses. Most manufacturer’s teleconverters won’t even mount to such lenses.
• Third party (Sigma, Kenko) teleconverters will mount to other lenses that the manufacturer’s teleconverters won’t mount to.
• Third party teleconverters don’t ‘report’ electronically to the camera body and may ‘fool’ the body into trying to autofocus when it shouldn’t. Results are pretty variable (often poor).

Roger Cicala
Lensrentals.com
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