It seems that there’s just one question I can’t seem to shake, as it just keeps showing up over and over in my In Box (and on Internet fora, for that matter).
What’s the question?
Some form of the “is the X lens with a teleconverter (TC) a good substitute for the longer Y lens?”
The answer is invariably “not really.”
First, why does this question keep coming up? Well, it’s basically due to economics. In almost all cases, X+TC is cheaper than Y without TC. Some people do ask the question the proper way, which is “can I save money by buying the X+TC over the Y?” Yes, you can. But you also want to know what you might give up when you do that, and the answer to that is always “some optical quality.” While this is less of a problem today than it was with yesterday’s TCs, you'll still see a loss, and usually a clearly observable loss.
The more devious versions of the question are actually a statement that goes like this: “I’m using the X+TC and I can’t see any difference versus what I see others getting from the Y.”
Which brings us to the second reason why the answer is “not really.” Again, I’m an “optimal capture, optimal processing” kind of guy (my pronoun is optimum ;~). Pretty much always the X+TC is not optimal. Just how unoptimal it is varies, but generally you’re going to lose some edge acuity, which you’ll probably try to get back via sharpening. So: not optimal capture, not optimal processing (the act of sharpening starts moving those pixel values in ways that can be detected, sometimes even in just casual view).
The “I can’t see a difference” construct is one you need to be careful about trusting. If you don’t know the person making that statement and they haven’t established enough credibility so that you would trust their assertion, then you should immediately discount this kind of statement. Seeing a difference as opposed to testing to reveal a difference are very different things. Those of us who value our credibility do both. If we think we see something, we test to verify that what we thought we saw is true.
Underlying all this teleconverter discussion is another factor: we are all at a different stage of our photographic ability. When you just start out, you don’t notice much. The fact that smartphone camera quality is gushed about is a good example of that: at the lowest common denominator level—both viewer ability and the restricted resolution/size of social media—the results look better than a lot of alternatives (film, instant photography, smartphone, etc.). Particularly when you also consider that focus doesn’t have to be quite so precise given the often larger depth of field of some of those other captures.
As you get better, what you can see gets better and you start to see issues. Or at least you become affected by them (e.g. you might not know why you don’t like what you’re seeing, but the issue rose to the level where you’re aware of something being wrong, even if you don’t know what it was that caused it). So you come to the Internet asking questions. And immediately have to understand that some answers you’ll get are correct, some are misleading, and some are wrong ;~).
If you keep progressing in photographic ability, smaller and smaller things start to become important in your work. One of those will be edge acuity in telephoto use. And that’s where the teleconverter (TC) becomes important to understand. I’ve seen good TCs and bad ones, but I’ve not seen a teleconverter ever improve a lens’ performance, only weaken it when it comes to contrast, resolution, and chromatic aberration. The contrast loss comes from additional air/glass elements, the resolution loss and chromatic aberration increase come from taking an image “in air” and altering it.
Whether you can see the impacts a TC has on a lens or not is pretty much fully dependent upon the level of training and photography you’ve achieved. Virtually every professional has and sometimes uses a TC, but only because there’s no other answer to what they’re trying to achieve. Almost always, that same pro will simply use the right lens for the photographic problem if they have it available. The lenses that have built-in teleconverters—400mm f/2.8 TC VR S and 600mm f/4 TC VR S in the Z world—have us using the TC mostly as convenience (not having to change lenses), but their TC is also designed in conjunction with the lens itself so it does a bit better with understanding the “in air” image and changing it, as the designers know exactly what it is they’re changing. Generic TCs don’t have that knowledge, so don’t perform quite as well.
I’m not a fan of using a TC to solve a “reach” problem. But I understand why a TC is desirable: it costs far less than a bigger lens, and you can’t get closer to your subject. Just understand that neither of those things is optimal. You’re taking a shortcut. If you want to achieve the same results as those that don’t take those shortcuts, you’ll have to address both things at some point.
