The Impact of Teleconverters on Image Quality: a comparison between Canon's Type II and III EF Extenders
October 24, 2011
by Dave Etchells and Andrew Alexander
A teleconverter, also known as an extender, acts as a magnifying optic for a lens, increasing the focal length that lens can provide. Using an extender exacts a small price: the increase in focal length is accompanied by a decrease in the light-gathering ability of the lens proportional to the square of the magnification ratio. Typical extenders offer a 1.4x increase, a 1.7x increase, and a 2x increase, but exact a toll of 1, 1.5 and 2 stops of light.
Many people might assume that a good enough extender wouldn't affect the quality of the image produced by the lens it's attached to. In fact, you should always expect to see a decrease in resolution at least proportional to the magnification. One way to think about it is you're basically employing a form of "optical cropping", akin to the digital cropping used in "digital zoom."
Consider a lens that offers 140 lp/mm resolution on its own, and you then add onto it a 1.4x teleconverter. Any given part of the image coming from the lens is going to be "stretched" across a 1.4x greater distance on the film or sensor surface than it was originally, so the 140 lp/mm would become 100 lp/mm. It's just a fact of life, and an unavoidable tradeoff.
Another thing to consider is that while a teleconverter effectively shifts all the apertures one stop higher, diffraction limiting is a function of the lens' aperture, so the f/11 that becomes f/16 will have diffraction limiting just as the unmodified f/16 of the lens by itself has.
Once these unavoidable losses are taken into account, the optical quality of a teleconverter comes into play simply in trying to avoid any more loss of sharpness due to the optics of the converter itself.
Canon's EF Extenders
|Canon's series of 1.4x and 2x EF Extenders.|
We have been looking at producing an article on the effect of teleconverters on image quality for some time now, but it wasn't until we arranged to get a whole series of teleconverters at one time that it became feasible. At this point we have to thank Roger at LensRentals.com, who provided us not only with both sets of 1.4x and 2x Extenders (types II and III) as well as the Canon 200mm /2.8L USM telephoto lens on which to test them.
This article will compare the quality of all four of the Extenders, as well as the un-extended lens itself.
We've hacked up our normal viewer program to facilitate viewing of the same lens across multiple converters: in order for it to see results from the the two pairs of converters as being different focal lengths, you'll note values of 281mm and 401mm. These are actually 280mm and 400mm.
Without an Extender attached, the Canon 200mm /2.8 II USM lens shows very good results at /2.8, becoming almost tack-sharp at /4 through to /11.
With the 1.4x series of Extender attached, the /2.8 aperture setting is no longer available and /4 becomes our maximum setting. There is some softness introduced at this setting, but not much - it's still sharp, just not quite as sharp as the un-Extended lens. There's no appreciable difference between the version II and III 1.4x Extenders. Once you get to /5.6, there's no further improvement in sharpness, and you never get to tack-sharp results. The 1.4x Extenders also produce one other side effect - the smallest aperture setting is moved up one stop, so you can stop down to /45. It's not advised, as it produces some quite soft images.
With the 2x series of Extender attached, the /2.8 and /4 aperture settings are no longer available, and /5.6 becomes our maximum setting. Image sharpness is impacted upon slightly more with the 2x Extender than the 1.4x Extender series at the maximum aperture; for maximum sharpness, you'll need to stop down to /11, which still produces sharp images - but again, not as tack-sharp as an un-Extended lens. Similary to the 1.4x series, there isn't an appreciable difference between the version II and III 2x Extender, at least as sharpness is concerned. Using a 2x Extender further reduces the smallest aperture setting to /64, which is also not advised, as it produces some very soft images indeed.
The 200mm /2.8 II USM lens produced very little chromatic aberration, making it an excellent base to test what happens when an Extender is added. This is a case where there is a notable difference between the version II and III Extenders. The version II 1.4x Extender produced substantial chromatic aberration both in the corners and throughout the image; the version III 1.4x Extender isn't much different than the lens itself.
The same can't be said for the 2x Extenders; interestingly, they did not introduce as much chromatic abberation as the 1.4x Extenders, presumably because they focus on the "better" central section of the lens elements. That said, there is some aggravation of chromatic aberration compared to the un-Extended lens, particularly in the corners, but only somewhat through the majority of the image. There is less overall chromatic abberation when using the version III Extender compared to the version II Extender, but the corners show slightly more.
As the lens suffers very little from corner shading, both with and without the Extender, there's not much to consider here.
The 200mm //2.8L offers light barrel distortion in the corners and overall, very light pincushion distortion. Adding a 1.4x Extender to achieve 280mm introduces barrel distortion throughout the image (to a maximum of +0.25% barrel distortion in the corners). Adding a 2x Extender actually alleviates any problems of distortion, producing almost no distortion throughout the image.
In our testing of the Extenders, we only notice a significant impact on the autofocus performance of the 200mm /2.8L II USM lens when using the 2x Extender. However, Canon warns that there is an impact on autofocus performance with type II and III Extenders. Specifically, Canon suggests that AF drive speed is reduced by 50% when using the 1.4x Extender, and speed is reduced by 75% when using the 2x Extender. Canon has indicated that autofocus speed and accuracy has been improved in the type III version of the 1.4x and 2x Extenders.
The quality of all of Canon's Extenders is excellent, as they are meant to pair with Canon's L series of pro lenses. In the version II series of Extender, the Extenders could be paired to produce a 240% increase in focal length at the cost of 3 stops of light-gathering ability (your /2.8 lens just got turned into a /8 lens). The version III 2x extender cannot be combined with a 1.4x Extender to produce a significant increase in focal length. This is due to a redesign of the location of the lens elements, with a rear element that protrudes much further out.
It's worth noting that Canon added some significant technology into the version III Extenders: flourine anti-smear coating, improvements to the lens elements used, and rubber seals to improve weather resistance.
Click on the links below to open new pages with sample images for each Extender:
As indicated in the introduction to this article, the design imperative for the Extender is to mitigate as much as possible the impact of reduced light-gathering ability. There is indeed a slight aggravation of image quality in terms of sharpness and chromatic aberration, but not so much as to override the obvious benefit as regards the increase in focal length.
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