There are a few things I keep coming across on the Internet. Some are truisms, some should be and some... well, some make me wonder exactly what I am missing, because they seem to be completely at odds with my own experiences.
One such oft-repeated pearl of wisdom is: teleconverters degrade the image a lot. A 1.4x TC may be acceptable, but a 2x TC has an excessively detrimental impact on image quality, and the only people who use 2x TCs are Those Other Sorts, The Ones We Have Nothing To Do With.
On virtually every forum I participate in, I regularly come across people steadfastly repeat this mantra, to a chorus of approving cyber-nods.
I, on the other hand, have rarely used my 500/4 without a TC, usually a Canon 2x. Those images have done fairly well in terms of sales and in terms of positive comments from viewers. So, are my clients idiots (Must. Resist. Urge) for buying these photos, or do I happen to have a wonderfully high-quality sample or is there something amiss here?
Well, as it turns out, while processing some images from a shoot, I happened to come across an image which might put this discussion to rest.
The shot below was taken with my 500/4 and a 2x TC mounted on a 1DMk2 at ISO 400. It was taken wide open - f8 and at a shutter speed of 1/90, handheld but braced. I haven't done much post-processing, barring a little contrast and saturation, and a moderate bit of sharpening,,, the same stuff I would do for any normal image. The one thing I did NOT do is noise removal, as that can fudge the results a fair bit, depending on how aggressively one reduces noice.
And here is a 100% crop of its eye:
Let me reiterate that this is at ISO 400, and off a 1300mm lens shot handheld (but braced) at a shutter speed of 1/90. This is what you get shooting a 2X TC on a lens wide open under sub-optimal conditions. Good enough for ya?
You will note that I am not comparing it to what the eye would look like without the TC. TCs do degrade the image - I bet if you compare the result above to a similarly-framed shot taken with just the 500/4, you'd probably see a little more detail. However, as Metallica would say: so what (well, they'd say So Expletive What, but nevermind that)? Phoography isn't about trying to competing to see which image is the sharpest. If an image depends on this extra bit of resolution and sharpness to be successful, it is probably not a very good image to start with, or it really should have been shot on a 4x5 view camera.
The main question - is the image sharp enough? Start with a compelling image and you'll find that as long as it is sharp enough to convey the message and not distract, it will satisfy everyone except the most hardened of measurbators and graphpaper photographers.
Another thing to keep in mind is that you need to start with a high-quality lens. So don't run off and start putting multiple TCs on 70-300 consumer zoom yet (or if you do, don't send me hatemail afterwards). TCs work well mainly with primes and high-quality (read: fixed aperture) zooms, such as 70-200/2.8. With high quality optics, the degradation introduced by the TC will not matter significantly.
So, can we put this bit of common wisdom away for good now?