by Bob Atkins
TCs (teleconverters, multipliers) seem to be a source of some confusion. There are questions about light loss, aperture change, depth of field and close focus distance that seem to be raised over and over again, as well as more subjective questions of how much sharpness is likely to be lost when a TC is used.
Lets deal with the technical stuff first:
The effect of TC use on mage quality is a bit more difficult to define because it depends on a number of factors, such as the exact prime lens being used, the exact TC being used and whether you're measuring using fine grain B&W film (e.g. Technical Pan), regular color film (e.g. Provia 100) or a digital sensor (and if so, which one and what the pixel spacing is). The images shown below were taken using a Canon EF 500/4.5L lens, Canon EF 1.4x and 2x TCs (Mark I TCs) and a Canon EOS 10D body.
It's not difficult to show that even a perfect TC on a perfect lens will decrease image quality. Since the maximum image quality (MTF) of a perfect lens depends on diffraction, which in turn depends on the relative aperture (f-stop), and adding a TC always decreases the maximum aperture, it clearly follows that even a perfect TC on a perfect lens will lower the MTF. For example, at 50m lp/mm the maximum possible MTF of an f2.8 lens is around 0.9. If you add a 2x TC (making it an f5.6 lens), the maximum possible MTF at 50 lp/mm drops to 0.8. If the TC also adds some optical aberrations (which is fairly likely, especially off axis), the MTF will drop even lower.
So here are some examples of the effect of TC use on image quality. The target magnification for each shot was maintained constant by moving to the appropriate distance from the target (i.e. with a 2x it was shot from twice the distance). The lens was mounted on a Bogen 3051 tripod with an Arca-Swiss B1 ballhead. Mirror lock up (MLU) was used.
Above: Full frame shot of test chart used
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This is a 100% crop from the center of the original digital
image. 500/4.5L with no TC. 500mm at f4.5 (800mm effective focal length if 1.6x crop
factor of 10D is taken into account.
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This is a 100% crop from the center of the original digital
image. EF 500/4.5L plus EF 1.4x TC. 700mm at f6.3 (1120mm effective focal length if 1.6x
crop factor of 10D is taken into account). Image quality is very close to the prime lens with no TC added. Resolution and contrast have not noticeably dropped. |
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This is a 100% crop from the center of the original digital
image. EF 500/4.5L plus EF 2x TC. 1000mm at f9.0 (1600mm effective focal length if 1.6x
crop factor of 10D is taken into account). Image quality has noticeably dropped. Contrast and resolution have dropped and the image is now slightly soft. |
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This is a 100% crop from the center of the original digital
image. EF 500/4.5L plus EF 1.4x TC, 12mm extension tube and EF 2x TC. 1400mm at f13.0
(2240mm effective focal length if 1.6x crop factor of 10D is taken into account). Large drop in contrast and resolution. Clearly not a combination you'd want to use on any subject with fine detail. |
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This is a 100% crop from the center of the original digital
image. EF 500/4.5L plus EF 2x TC, 12mm extension tube and EF 1.4x TC. 1400mm at f13.0
(2240mm effective focal length if 1.6x crop factor of 10D is taken into account). Same as above, but with order of TCs reversed. Seems slightly worse with 2x next to lens. |
These results pretty much reflect my experience shooting with both film and digital bodies, and with various "L" series prime lenses including the EF 600/4L, EF 300/2.8L. EF 500/4.5L and EF 300/4L. My experience has been that using a Canon EF 1.4x TC results in excellent image quality. Often it's hard to see a difference between the prime lens and prime lens + 1.4x TC, though in detailed tests using fine grain, high resolution film a small resolution drop can be seen.
However with the 2x I have typically seen the image go slightly soft. This isn't a focus error (I focus bracketed these tests), nor is it camera shake (I used a 12.5lb Bogen 3051 tripod with an Arca Swiss B1 head. plus mirror lockup). Images made with the 2x are usable, in fact I have made some excellent 5x7 prints from images shot with the 500/4.5L + 2x TC and an EOS 10D. Sharpness and contrast can be boosted in an image editing program to partially compensate for the degradation induced by the 2x TC and pretty good image quality is possible with care and a little effort. An example is shown at the end of this article.
Though the Mark I Canon TCs cannot be directly stacked (the Mark II versions can), on many of the long Canon prime lenses, TCs can be stacked with a 12mm extension tube between them and distant focus is still possible (you can't quite get to infinity, but you can focus up to a few hundred yards away). The last two images in the sequence above show the results of stacked TCs on a 500/4.5L. Clearly not very good. Now they might be usable in some cases where fine image detail was not required (maybe shots in fog or silhouettes or sunsets), but if fine detail is important, this isn't the way to go for large images. For small web images, or for shots without fine detail it could be OK as shown below:
 Above is a full frame shot with 1.4x and 2x TCs stacked on a 500/4.5L. Doesn't look too bad! On the right is a crop at 50%. Though soft, since there isn't much fine detail the image is acceptable. |
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Here are some 100% crops of a test pattern about 80% of the way to the corner of the image.
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500/4/5L at f4.5, no TC |
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500/4.5L plus 1.4x TC (f6.3). There's a little more degradation in image quality here at the edge than was seen at the center. Resolution is slightly down and there's a little extra chromatic aberration. |
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500/4.5L plus 2x TC (f9). Again image quality takes a bit of a hit with the 2x. Contrast is slightly down, resolution is down and there's more chromatic aberration visible. |
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500/4.5L plus 1.4x plus 12mm extension tube plus 2x TC. Not a pretty sight. Resolution and contrast are down. Chromatic aberration is up. |
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500/4.5L plus 2x plus 12mm extension tube plus 1.4x TC. As above, resolution and contrast is down, Chromatic aberration is up |
Edge quality suffers more from the effects of a TC than center quality. One reason is that chromatic aberration (at least lateral chromatic aberration) doesn't affect the center of the image. Another reason is that aberrations such as astigmatism and coma also don't affect the center of the image, so it's easier to keep center image quality high than edge image quality.
There are two differences here. The first is that film is capable of higher resolution than digital. I say capable because it depends on the film (which needs to be slow speed, high resolution) and how you measure the resolution (from a scan or using a microscope). So on film, under the right conditions, you may well see more difference between the prime lens and the prime lens + 1.4x TC. In the past I've done film tests which showed maybe a 10% drop in resolution, say from 80 lp/mm to 72 lp/mm. Digital can't resolve more than around 60-65 lp/mm, so this difference will be less obvious on digital than film (though the effect may still be seen since overall MTF drops along with "resolution").
The other difference is that most current digital sensors are smaller than 35mm film, so "edge" quality on digital may well look better than "edge" quality on film, since on film the "edge" is further from the optical axis and so off-axis aberrations will be stronger.
Since results using a 2x TC are always inferior to those from the prime lens with no TC added, the question is sometimes raised as to whether it's better to use a 2x TC or to simply enlarge the image from the prime lens by a factor of 2. Below is an example that gives a clear answer. The image made using the 2x TC is clearly much better then the 2x enlargement of the image from the prime lens.
The bottom line on TCs is that the lower the power, the better the image quality, so a 1.4x will generally yield better results than a 2x. Stacked TCs don't work well. There may be some situations in which they are usable, but not many.
Finally an image shot with a 2x TC on the 500/4.5L in the field (1/250s at f9, ISO 800 on EOS 10D) . It's an American Bittern. This is actually a crop of a larger image.This is just to show that although the strict test using a resolution chart makes the use of a 2x TC look undesirable (and it is if you have a better alternative), when you need the 2x, it's possible to get pretty decent images, especially if you have them in the digital domain and so can optimize them.
And here's a 100% crop of the Bittern's head. Not bad for a 2x, though both images shown here did have a little help from my image editor in boosting contrast and unsharp masking. A 4x6 print looks excellent.
TCs are very useful accessories but they work best when low power TCs are used with high quality prime lenses. If you're looking to preserve the maximum amount of image quality from your prime lens, use a 1.4x TC and preferably one made by the lens manufacturer. 3rd party TCs can be quite good, especially when used with digital cameras with small sensors since in that case the image quality at the edge of the full 43mm image circle used in full frame 35mm format isn't seen by the sensor. See my article here on photo.net comparing a Canon 1.4x with a Tamron 1.4x TC.
2x TCs will drop image quality but can yield good results when used with care an in conjunction with a quality prime lens. Putting a 2x TC on a 100-300 consumer zoom will probably drop you into the "unacceptable image" region so bear that in mind if you're looking for a really cheap way to get a long lens. "Cheap", "good quality" and "telephoto" aren't words that go together!
Stacking TCs is a last resort. Maybe you can get away with it sometimes on the right subject, but even with quality lenses and quality TCs your chance of getting usuable images isn't good.
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All text and images are © Copyright 2004 Bob Atkins. Visist www.bobatkins.com