Lenses are an integral part of the photographer's stock in trade. Yet many photographers hold to at least some common belief about lenses that isn't quite true. Myths abound when it comes to lenses.
Here are some of the more prevalent.
Focal length measures how long the lens is
When you see a photographer with a really big lens it's natural to assume that a long lens means a long focal length. After all, what else does a lens do except focus the image, so what else could that length be for? It is true that, with the exception of such oddities as mirror lenses, a lens is rarely much shorter than its focal length, but it may indeed be significantly longer. Focusing the image to a single point isn't easy. Different wavelengths of light focus differently and in order to get them to converge properly to create a sharp image, a lens has its work cut out for it. Different manufacturers, too, may have different ways to solve this problem, resulting in different sized lenses for the same focal length. A standard Nikon 50mm f/1.4 lens is 1.7 inches long. The Sigma ART 50mm f/1.4 lens for Nikon is twice that long due to the way Sigma chose to bend the light around to get the best image they could.
By the way, it's worth noting that this 1.7 inch length is actually slightly less than the stated 50mm focal length (50mm would be about a quarter inch more than 1.7 inches). At first glance, you might think I've gotten something wrong here, but you have to remember that the focal point of a lens is the sensor within the camera body, not the mount where the lens connects to the camera. The focal length includes this extra distance.
Lower aperture numbers (faster f/stops) mean bigger lens diameters
Every wildlife photographer who can only afford a modest telephoto lens plus a teleconverter longs for one of those really big lenses needed to get the best shots from a distance. The big lens they dream of may have focal lengths of 400mm just like their current 200mm telephoto plus 2x teleconverter does, but beyond that there's no comparison. Those really big lenses are big in every respect, including their diameter. That 200mm telephoto may even have an f/2.8 aperture which, together with a 2x teleconverter, yields an effective aperture of f/5.6. The dream 400mm monster is probably f/4. But unlike the 77mm filter threads of the 200mm telephoto, the 400mm f/4 has a front diameter of over 6 inches (over 150mm). That's a lot bigger.
All other things being equal, this particular myth is in fact reasonably true. If you have two lenses of the same focal length but different widest aperture, the one with the fastest aperture (lowest aperture number) will indeed have a larger diameter. But all other things aren't necessarily equal. The f/stop number may bear a relationship to diameter, but f/stop also relates to focal length. Technically, the f/stop number is defined as the ratio of a lens's focal length to the diameter of its entrance pupil. Without veering off into a tangent here too much, the entrance pupil is how big the lens opening looks when wide open, as viewed from the front of the lens. If you look at the lens from the back, the opening diameter you would see would be referred to as the exit pupil. So as you increase focal length, it takes an ever larger lens diameter (and entrance pupil) to maintain the same f/stop.
Prime lenses are better than zoom lenses
Back when zoom lenses were newfangled inventions, they provided flexibility, but necessarily suffered from somewhat diminished optimal quality. Purists stuck with a carefully chosen selection of prime, fixed focal length lenses. But that was long, long ago now, and these days computer designed zoom lenses compare quite favorably to prime lenses. There may still be a slight edge in terms of sharpness to the best fixed focal length lenses, but it's probably worth considering whether this is the only metric that matters. The best of both lens types are so good these days few could possibly see the difference except under controlled laboratory test conditions.
Meanwhile, the flexibility advantage of zoom lenses remains as true as it ever was. A single zoom lens can substitute for several primes. Pound for pound, and dollar for dollar, zoom lenses make great sense for most of us. I'd rather invest in one really great zoom lens than a pile of moderately good fixed focal length lenses. And I think the resulting images would back up that choice. And I'd definitely prefer to carry around just that one zoom lens instead of its equivalent in prime lenses.
"Fast" f/2.8 lenses are sharper than lenses with smaller maximum apertures
It's commonly understood that "professional" lenses all have a widest (fastest) aperture of at least f/2.8. "Consumer" lenses typically aren't faster than f/4 or perhaps even f/5.6. Faster lenses cost more, so they must be sharper. But is it in fact the other way around: do faster lenses cost more, or do more expensive lenses have faster apertures? Which comes first, the chicken or the egg?
The truth is that professional lenses naturally do cost more, but their fastest aperture is only one of the reasons why. They tend to have better build quality and often use exotic ED or Fluorite glass elements. They almost certainly have a lot of man hours invested in their design to optimize every possible aspect. It's no wonder they cost more, and it's no wonder they tend to be sharper than consumer lenses. But their f/2.8 aperture isn't a cause for this greatness; it's just one of many consequences of it.
As a side note, a curious aspect of this myth is that even when shooting with an f/2.8 aperture lens, few shots are actually made at that wide aperture. Yes, some are. When shallow depth of field is called for, you simply can't get the same effect with a lens that won't go that wide. But most shots are taken with the lens — any lens — stopped down at least somewhat. So if you're going to stop a lens down to f/5.6 anyway, why do people pay more for a lens that goes wider than that? There are three reasons I can think of. First, they buy fast lenses because those same professional lenses do generally have better designs in other regards as I've already discussed. Second, most lenses are sharpest at apertures in the middle of their range and may fall off somewhat at either end. So by buying a lens that goes faster than what they generally actually need, they're able stop down somewhat from what may not be the lens's sharpest aperture. And third, you have to be able to see your subject to accurately focus on it. Even if don't ever shoot at f/2.8, your focusing will likely be better when you can do it with a brighter viewfinder image. Autofocus too works better with the brighter image possible from wider maximum apertures for the same reason that it works better when you focus manually. And if you (or your camera) can focus better, you may indeed end up with sharper images as a result.
Changing focal length changes perspective and wide angle lenses create distorted images
I've written about this before, but it's worth including it here while we're on the subject of lens myths. Indeed, this may well be the most pervasive of all lens myths. Common wisdom is that wide angle lenses cause exaggerated perspective and that telephoto lenses compress perspective. Neither of these is actually true.
The truth is that subject distance creates both of these effects, not focal length. Images shot from right on top of your subject will appear to have exaggerated perspective. From such close range, relative distances will proportionately be huge. If your subject is only one foot away, something another foot behind that will be twice as far as your subject. Distance ratios become exaggerated, and thus perspective will appear exaggerated. At the other extreme, images shot from far away will appear to have compressed perspective since everything in the frame will be far away in order to be in focus. Distance determines the appearance of perspective, not focal length.
To put this in context though, an image shot from far away will likely be taken with a telephoto, and an image shot from right on top of your subject would need to be made with a wide angle lens (or a macro lens of course) since telephotos generally have minimum focus distances that make their use from that distance impossible. But if you did shoot a faraway mountain with a wide angle lens and later crop all the irrelevant surroundings out to create a result with the same framing as you would have gotten with a telephoto shot from the same distance, you'd end up with an image with the same perspective as that telephoto shot. Focal length determines framing. Distance determines perspective.