When to use a SLOWER glass

hi all,
Iv been bitten by the optics bug. (Letus on the way) and now need more glass. Iv read lots of recommendations of good primes to get, so I'm squared away for a while.. (see, I even learned the lingo!)

But Im curious if its always advisable to use the fastest glass you have.. is there ever a situation where you would choose (for example) a 50mm 1.8 OVER a 50mm 1.4 lens?

Thanks, noob wants to know :)
 
The more light you have, the slower the lens you can use.... so broad daylight is a good example.

The Letus takes at least a stop to a stop and a half of light away from you, so that's why they recommend the faster lenses. Light is traveling down that barrel and into this lens extension you are adding to the front of your camcorder - that means it won't be as bright when it hits your camcorder's lens and eventually the CCD or CMOS sensor that creates the image.
 
I could be wrong but I think I remember reading that most films are shot on some kind of f/3-5 range. So it sounds like, as sonnyboo said, if well lit, the slower lenses are not all bad.
 
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