Ok. I'm just going to babble a bit by myself about why a faster aperture lens is better almost 100% of the time.
If you have the money, one of the commonly debated choices is between the Canon 85mm f/1.2L, Sigma 85mm f/1.4, and Canon 85mm f/1.8. Don't get me wrong, they're all fantastic, but the 85 is optically better.
I hear reviewers say "oh, if you stop down an 85/1.2L, you are [wasting money/an idiot/killing innocent puppies]. No, they're wrong.
Take a shot from any lens wide-open, and it'll probably be the least saturated, least sharp, most heavily vignetted as it could be. Stopping down increases sharpness, saturation, and typically cuts down on vignetting.
So, in the 85/1.2L\1.4\1.8 debate, stop looking at each lens' maximum aperture. If you stop all of them down to 1.8 the differences will become more apparent. No, my 85/1.2L isn't the sharpest of the bunch wide-open, but at f/1.8 it IS the sharpest. (Call me biased.)
looking at the opposite end, the 50/1.8's reviews all sing praise and dodge responsibility for problems with clichés such as "it gets better stopped down," or "it's so cheap; why complain?"
Look at 85/1.2L reviews and tell me how many say "it's not as sharp as the 1.8." No shit. Stop it down to 1.8 to compare, or rip the aperture out of the 1.8, tear it apart, and see which is better at 1.2.
Faster apertures mean more expensive. Period. Take note of the apertures of lenses from other brands. Where's Nikon's 85mm f/1.2? 50mm f/1.2? 200mm f/1.8? You pay a premium to have the "fastest 85mm available". (Please don't realize the Fuji 56mm f/1.2 that just came out is 85mm 35mm equiv)
Canon 50mm f/1.0L. Discontinued. All or almost all are used. Start your bids at about $4,000.
But that $4k doesn't just gets you a lens, it gets you one of the fastest. It gives you exclusivity. It's an investment.
Done babbling now. Feel free to call me out on Nikon hate, bash me for sweeping generalizations, or make fun of me for thinking about cameras all the time. Do as you will; Carry on, nothing to see here.