Originally posted by C_Jones Fast primes can be useful, and handle some light situations that some zooms cannot, but there are also fast zooms, such as the Pentax 15-30, 24-70, and 70-200.
The 31m 43 and 77 ltd are 1.8. My FA 50 is 1.7. There are a lot of primes that will give you more than a stop advantage over the fastest zooms. The 55 1.4, 50 1.4 and coming 85 and 50 will give you a two stop advantage. I always say if you can't fake a stop, you aren't a photographer, but over one stop you need to change lenses. There simply are no 1.4 zooms. Lets not make it sound like a zoom can do anything a prime can do, They can't. Just to be clear, for low light, primes are clearly superior. Even ƒ4 zooms can be less than functional in low light, in terms of both enough light for a decent shutter speed, and AF speed. I've been there enough with the ƒ4 60-250 to know how real this is. And most zooms are considerably slower than f4 in the long end. Quite simply stated, ƒ4 is too slow for many golden hour situations. It's all question of cost.
I'm as big a fan of using zooms where possible and efficient as anyone. In low light, I switch to primes, not because I want to, because in many situations I have to. It's either that, or lose the shot. My moto has been cover your range with zooms, fill in the important gaps with fast primes. Unfortunately it's not about IQ, it's about your ability to acquire an image at all. The downside to the wrong take here is missed opportunites.
I would say the biggest factor in folks wondering why their camera doesn't work the way they think they should is slow lenses. Slow speed wise and slow in terms of too small an aperture to shoot in the available light.