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Teleconverters 101

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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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10 Responses to “Teleconverters 101”

Chris Hicklin said:

I have a 600mm Nikon Lens reserved with you for a trip to Yellowstone this summer. I was considering changing the order to the 300mm 2.8 with a 2x teleconverter due to weight conciderations. Your notes here remind me of the poor photographic experience I had shooting a running wolf 2 years ago with a 1.7 teleconverter on a 400mm prime lens. Soft images and slow focus issues cost me the shots of a lifetime. I'll stick with the larger prime lens. I'd rather deal with the weight than chance a missing opportunity... Chris Hicklin , Sarasota

Dickson Smtih said:

I found this information very usefull. I was not aware that you could not stick a converter on all lense and the effects they would have. I do most of my bird photography with my Sony 70-400mm f4.5-5.6 lens and wanted to try a converter on it for some longer shots. I have found when I'm zoomed out to a full 400mm most of my photos that have some greater distance to the subject are soft and disapointing.
Thanks for the education.
Dickson

Robert Kaplan said:

Not being a professional bird photographer, I unfortunatley cannot justify the expense of a 500-600mm Nikon lens (but that keeps you in business). I have been pleased with the images from my 300 f/4 and the 14TCII and, thanks to your extremely informative analysis, I will probably invest in a 17TC II. If I need to increase my ISO by another 1/3 to 1/2 stop, my D7000 can handle it.

Thanks for the info.

Bruce MacDermott said:

I was contemplating adding the Canon 1.4x III to my order to go with the Canon 24-70 f/2.8L until I read your article. Thanks for the insight. I will stick with the Canon 18-200 on our 50D and keep the 24-70 on the 7D. If conditions allow, I might put our Sigma 28-300 DG on the 7D just to see what happens.

Thanks for providing such a great service and advice.

Bruce

ray allen said:

I'm starting an insurance photography business on a budget, and I'd like to use a wide-angle teleconverter to change a mid range zoom to a short zoom for interior shots until I can afford the toys I want.

I also realize that a prosumer "bridge" camera would meet my needs, but since I'm doing this for a living, presentation counts, and that's why I'll be using a dSLR.

Any comments on the wide angle teleconverter?

LensRentals Employee

Roger Cicala said:

Ray,

Those are really wide-angle add-on lenses. They work and some aren't bad at all, but you will see some loss of image quality and some decrease light gathering. It's not a tool I'd want to use on a a paying job it the customer had high expectations.

jonty said:

You stated:
"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"
I may not be a photographic expert, but this is ilogical, the crop factor does not change the required shutter speed, by your logic a full frame shot would have motion blur in the centre, becoming less towards the edges. This is not the case, if I take a photo on 35mm film, using a 100mm lens at 1/100, but then only print the middle of the shot equivelent to a dx sensor size it doesn't suddenly get motion blur!

Petrus said:

Jonty, you better rethink the focal length versus shutter speed connection: It has nothing to do with sensor/film size and focal length, but only the angle of view of the camera/lens combination.

The ancient rule of minimum shutter speed = focal length is relevant only with 135 film cameras (35 mm) and now so called "full frame" digital cameras. With those cameras the angle of view of the lenses happens to fit quite nicely to this "rule". If we take a 1.5X crop camera, the angle of view with the same lens (lets say 500mm) is now 33% narrower, or there is a 1.5X magnification of the image compared to a FF camera. Same shake (angle) will show 1.5X larger in the image. So, to keep the camera shake blur the same, we need 1.5X higher shutter speed. The focal length = shutter speed rule does not apply to 1.5X crop cameras, whith those the rule is "minimum shutter speed = 1.5 x focal length". Conversely large format cameras follow the rule "minimum shutter speed = N x focal length", where N is the film or sensor size devided by the size of 135 or FF camera.

Summa summarum, what determines the minimum shutter speed rule are the angle of view and the angular velocities of the camera shake, not the sensor size or focal length alone. That old rule was handy for 135/FF cameras, for cameras of different film/sensor size it has to be adjusted accordingly.

jonty said:

Petrus, the angle of view is a product of the film or sensor size and the focal length of the lens, so it has everything to do with it.

My point still stands, if I use the same lens to take a shot with an F80 nikon, then take a shot with a fugi s2 pro,(built on the same body) the only thing that has changed is the size of the sensor/film. There is no 1.5x magnification, it is a crop of the image. For the logic to work the middle of the shot would have motion blur progresively geting less towards the outside.

jmt said:

Petrus takes a principled stance when stating "It has nothing to do with sensor/film size and focal length". He is right, the perceived sharpness has something to do with the angular movement, (caused by the shake). More angular movement blurs the image more until it is perceived as unsharp. For all practical purposes, we can express that in terms of film size and focal length.

Focal length and sensor/film size just come into play since the angular movement is defined by the angle when looking at the object (the focal length) and the resulting angle when looking at the film/sensor (film/sensor size).

For 135 film, this conveniently translates into the "1/shutter speed" "rule". For cropped sensors, this is not the case anymore, since the angle on the object side translates into a different angle on the image side. This is obvious even in the naming conventions, 28mm or 35mm lenses are not wide angle on APS-C, the same as 50mm lenses are wide angles on 6x6.

The angular movement is the same for the whole image projected, that is why jonty does not see differences between center and sides of the image. But Petrus never stated there would be.

The same angular movement of the lens amounts to a larger angular movement on the sensor for a cropped sensor than for a full frame, hence you need to adjust the rule (as was always the case for 120 or 110 film). But talking about sensor size and focal length is just more convenient than to talk about angular movement. Still, the angular movement on the film side is what actually makes a shot (un-) sharp.

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