35mm and 50mm primes make perfect sense to me: sharper images, simple operation, and framing can usually be handled by taking a few steps forwards and backwards.
I'm struggling to see how one would find use in a long focal length prime, 300mm and above for example: without zoom, isn't your shot composition always at the mercy of how close or far away your subject is, meaning heavy cropping is almost always necessary in post?
Can somebody please enlighten me?
Who in the world buys large primes?
Wildlife and sports photographers, mostly.
I'm struggling to see how one would find use in a long focal length prime, 300mm and above for example: without zoom, isn't your shot composition always at the mercy of how close or far away your subject is, meaning heavy cropping is almost always necessary in post?
It's the difference between cropping and heavy cropping. Would you prefer ending up with a 6MP image? Or a 12-15MP image?
When shooting distant subjects, where you can't get closer, either from issues of personal safety, regulations, or stressing a bird to the point of abandoning a nest of young, if you had a zoom, you'd be zoomed in all the time, anyway.
I own the Canon 400mm f/5.6L USM prime lens. It's lighter than the 100-400L, about half the price (or less) is sharper than the 100-400L Mk I, just as sharp as the Mk II, and autofocuses more quickly. It's also a bit better at taking a teleconverter. And delivers images to me that I simply couldn't get with a 70-300mm zoom lens--especially with cropping. For a birder, feather detail is something you really really like. For a bird-in-flight or sports shooter, responsiveness is huge.
And if you have the big bucks, Canon also makes a 400/2.8L. They don't make 400mm zooms that fast.
You might buy one in order to get a balance of high quality, reasonable size and weight, fast aperture, and a lower price. As you say, people often get shorter primes because they like a particular field of view, and I don't think that's usually the case beyond, say, 90mm (in 35mm terms).
That list of compromises can't be avoided — but giving up zoom lets the designer (and therefore the buyer) compromise less in the other areas. In fact, the size and weight required to do a high-quality wide aperture zoom at higher focal length basically exceeds what the market would bear (see for example Is it physically possible to build a long zoom (17-300 mm for example) at an aperture value of around 1.4 or 1.2?), so in that segment, primes are really your only option.
Might also have to do with speed. Some zooms are limited in their range, being slower at longer focal lengths.
Take a look at photographers on sidelines of football matches; they might have one long fast zoom and then a shorter lens on a second body in case the player gets close.
They are, however, specialized lenses. Usually for situations where you are planning shots that make use of them, not something you carry around while sightseeing (unless maybe on the deck of a sightseeing boat, or in the vehicle on a safari...).
People like me who still shoot old, manual, film cameras loaded with B&W film. Primes are cheaper, have basically nothing that will fail, and you can get faster primes for the same price. I also don't need AF (my bodies are all older Nikons that are MF only), and most MF lenses are primes. Having access to a bigger aperture when using film can be a big deal.
