by Bob Atkins
It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness..
.Oh, sorry, that's A Tale of Two CTs...
Often we hear the plea "Which TC should I buy?". The answer isn't easy (though it's certainly not "buy the Cambron 3x"). There's more to a TC than just price and physical compatibility (i.e whether or not it will physically fit on the lens). The complexity lies in the fact that TCs can work differently with different lenses. In fact Nikon have different TCs of the same power for different lenses. The TC200/201 are for lenses 200mm and shorter and the TC300/301 are for lenses 300mm and longer. Don't ask which is best for lenses between 200mm and 300mm! Expecting one TC to work well with every lens is probably hoping for too much. Canon just make one 1.4x and one 2x TC for their EOS line of lenses, but it's only designed to work with their "L" series telephotos and a few "L" zooms. You can't attach it to a 17-35 zoom and even if you could, results probably wouldn't be spectacular. Bearing that caveat in mind, let's look at the test.
Based on the philosophy of "keep it simple" and "use what you've got", here is a comparison of the Canon 1.4x TC (Mark I, but the Mark II has exactly the same optics) and Tamron-F 1.4x C-AF MC4 TC. I chose the Tamron because I had one, not because it's the best or cheapest or because Tamron are paying me. Note this Tamron TC is several years old. It's not the new Tamron SP 1.4x Pro. I keep it around just in case I need a TC for a "consumer" lens, since it will physically fit on any EOS lens.
The lens I used in this test was a Canon 500/4.5L. This is a lens that I use quite often with a TC and TCs tend to work better on longer lenses, so it's a reasonable choice. I don't often use TCs on shorter lenses, because I have longer lenses, and a lens without a TC is usually better than one with one! So for me TC performance with long lenses is important, hence the choice of the 500/4.5L for this test.
First a general test shot and an example taken from the center of the frame.
Sharpness and contrast are very similar. Not much to choose between the two TCs on that basis, but look at the color. The Canon 1.4x maintains the color balance of the prime 500/4.5L lens. The tamron TC produces a distinctly cooler image. In some shots it's quite evident, in some it's not so obvious, but it's really there. I'd probably have to add a warming filter to the Tamron to get back to the original color of the Canon shot.
So here we have the first difference. Color Balance. Is this a big deal - not really. A slight tweak in PhotoShop will correct it - if you shoot digital or scan. If you have an original slide, you're stuck with it, and that may or may not matter, depending on your use for the slide.
The EOS 10D won't AF with a Canon 1.4x TC because the maximum aperture drops below the f5.6 limit. (It will AF with an EOS-3, 1v, 1D and 1Ds. It won't AF with any other body). This is a shame because you're only 1/3 stop too slow for AF. Now you can "trick" the body into AF by pretending the Canon TC isn't attached. You do this by masking off 3 pins on the lens. The body now doesn't know the TC is attached and tries to AF but it doesn't do very well most of the time. With the 500/4.5L AF hunts, then oscillates about the focus point. Not good.
However, with the Tamron 1.4x TC, the body doesn't know it's there anyway because it doesn't have contacts for those extra pins. The lens still thinks it's an f4.5 so it tries to AF. This time AF works though! It's actually pretty good and pretty accurate. The downside is that the AE is off. A 500/4.5L + Tamron 1.4x overexposes by about 1 stop so you need to dial in -1 stop EC for the right exposure. The aperture still reads f4.5 even with the TC attached when the true aperture is f6.3. The reason the metering is in error is not due to the presence of a TC which slows the lens by 1 stop - in fact the body meters two stops slower - but because of a change in certain lens properties (probably the position of the rear nodal point). It sometimes happens with EOS bodies when you do odd optical things, like using old manual focus lenses, telescopes and the like and it even happens on certain EOS bodies (notably the A2) with certain lenses and Canon TCs.
Now please don't ask me if this TC AFs properly with other lenses slower then f5.6 or on other bodies because odds are I don't own the other lenses or other bodies so I can't run those tests. What you see is what you get here. It works with this particular Tamron TC on a 500/4.5L and EOS 10D. Any other combination and you're on your own!
Well, a long time ago, back in the days when I used film (does anybody remember film?) I tested the Canon against the Tamron 1.4x TCs and found that they were similar in the center of the field, but the Canon was better at the edges and the Tamron vignetted slightly. With cameras like the 10D which use a smaller sensor these effects may be diminished - but are they?
Well, here's a corner, the Canon on the left and the Tamron on the right. Note the clear color balance difference as described above. The Canon does show more detail if you look closely, but the difference is fairly small. This shot is a crop at 100% from an EOS 10D image. With the smaller sensor of the 10D there is no visible vignetting with either TC (as expected).
Though it's a little difficult to see in the above (100%) images, there is color fringing in both of them. In the Canon it's red/green and in the Tamron it's yellow/purple. This is much more obvious in the shots below which are 400% blowups of the corners of a different shot (again with an EOS 10D) using a subject designed to maximize chromatic effects (high contrast black/white transitions).
The magnitude of the chromatic effects is pretty similar, but I find the Tamron's purple/yellow fringing more objectionable than the Canon's green/red. This is shown again below, this time in a section of an image taken from the edge rather than the corner. The subject is a white rope against a darker background.
Again the Canon image looks better, even though the magnitude of the aberration is similar (Maybe the Canon has slightly less). I suppose if the background were blue or purple the Tamron might look better, but most of my work is nature and my backgrounds tend to be brown and green!
Yes, or I wouldn't have mentioned it. There's Bokeh.....what's Bokeh? Bokeh is a Japanese word with several meanings some of which are close to "fuzzy" or "unfocused", but it has been used photographically to describe the nature of the out of focus parts of an image. Different lenses (and different TCs) can render out of focus objects differently. Below is a random example using the Canon and Tamron TCs. As you can see there is quite a difference. I'm not suggesting one is better than the other, just that they are different. At different degrees of defocus, they could be different again. My only point here is that different TCs are capable of giving different "Bokeh". Again note the difference in color balance.
Though the the Canon image may at first sight appear to be more in focus than the Tamron, this IS NOT because of any difference in depth of field. First there is no difference in depth of field. Second depth of field refers to the part of the image which appears to be IN FOCUS. Depth of field and DOF equations don't tell you anything about the nature of the image which is out of focus. The difference is almost certainly due to the different TCs giving rise to different amounts of different aberrations in the out of focus areas. It's possible, though unlikely, that some of the difference may be due to a slightly different focus point with the two TCs. Sometimes a very small shift in focus can give rise to a significant change in the appearance of out-of-focus areas.
TCs are tricky things. It's not possible to say which is "best" for any particular operation. In this case the Canon has a slight performance edge, but it's more expensive. On the other hand the Tamron will give you AF with a 500/4.5L - but the exposure needs -1 stop EC! The Canon preserves the color balance of the Canon prime lens, the Tamron renders a cooler image. The Bokehs are different. In the end it's up to the user to decide which one best meets their needs and, if cost is an issue, which provides the best price/performance ratio.
So next time someone asks "Is the TC 'A' as good as TC 'B'", you can point them to this article and ask them what they mean by "as good as"!
If you decide to purchase a TC, you can help keep photo.net running by ordering on-line from one of the vendors who support photo.net by giving us a small commission on sales made via the links which you see here.
You can read more about the Canon 1.4x II TC and the Tamron 1.4x TC in photo.net's "EZ-Shop" section and you can find vendor links and prices there too! You can also find info on the Canon 2x TC and 2x TC II as well as the Tamron 2x TC and 2x TC Pro.
©Copyright 2003 Robert M. Atkins All Rights Reserved