>>3784082Fast = more light = faster shutter speed. They are mostly used for portraits, when subject separation. Yes, you can use them for low light situations, despite the narrow depth of field. As long as you focus on your subject, then there's no problem.
>>3784090>You see this is what I dont get. According to the exposure triangle, to get more light you either increase the apperture or you slow the shutter speed.NO. You can't always just "slow the shutter speed." Motion blur becomes a problem, which can either be caused by the movement of the subject or movement of the camera. Modern cameras correct only camera movement with IS, but even then, IS is no substitute for a faster shutter speed or a tripod. So you still have the issue of freezing motion, and sometimes that's a real challenge.
I loved my Pentax K-5IIs, but the K-3 I replaced it with was SHIT at metering. Looking back on my old photos, I think the IBIS unit was actually defective. Lots and lots of blurry photos I could post, but here's one where the slow shutter speed was clearly operator error. A faster lens or higher ISO would have allowed a faster shutter speed to freeze the puppy's movement, and with ISO, you lose a lot of detail by setting it too high.
>If you're shooting at lets say f/1.8 then you're letting in a lot of light which means you can increase the shutter speed, but you have that nagging problem I was mentioning (ignoring that you're totally right by saying a lens is never the sharpest at the extremes) that the depth of field at f/1.8 is really really small and hence you're not getting anything in focus when if you shoot landscapes or street photography you usually start at f/8.If you're shooting landscapes in low light, you'll want to be using a tripod anyway, and street photography seems to look good all grainy from high ISO. Like I said above, if you can get the subject clear and in focus in low light with a fast lens, then a narrow depth of field will rarely ruin a photo.