Originally posted by MikesChevelle I see alot of people recommending a fast 50 and was wondering what fast relates to.
Mike,
As others have mentioned already, "fast", when applied to lenses, simply means that the lens has a large maximum aperture. When somebody says a lens is fast, I usually understand that to mean that the lens can reach at least to f/2.8 and possibly larger (f/2, f/1.8 etc). Photographers like fast lenses, for two reasons. The most important reason is that larger apertures mean shallower depth of field (other things being equal) and photographers like to have the option of shallow depth of field. Also, faster lenses make it possible to shoot in lower light without a flash.
Now, to the "fast 50."
Back in the 35mm film days, 50mm focal length was deemed to provide a "normal" focal length. (More on this below.) And back in those days, just about every lens maker had a "fast 50", that is, a 50mm prime that went to f/1.4. Because 50mm lenses produced results that were pretty close to what our eyes normally see, 50mm was a popular focal length, meaning that manufacturers sold a lot of 50mm lenses. And because sales were good, makers were able to make these lenses high quality yet keep the prices low, which just meant that they sold more. So just about every serious photographer had a fast 50 in his kit bag.
These days, you can find an f/1.4 lens at 50mm from just about every lens manufacturer, and the lenses are usually relatively affordable - and usually high quality, too. My Pentax FA 50 f/1.4 is one of my best lenses - and one of the cheapest.
Interestingly, you don't see a lot of lenses at other focal lengths that go to f/1.4. Sigma has a 30mm f/1.4 for the Pentax K-mount that appears to be a very nice lens. Perhaps there are other f/1.4 lenses out there but I'm not aware of 'em. I have a Sigma 28 f/1.8, a Pentax 35 f/2 and a Pentax 70 f/2.4. Zoom lenses seldom go faster than f/2.8, apparently for engineering reasons that I don't begin to understand.
Anyway, we Pentax users no longer have a lens for the "normal" field of view that is both really fast AND affordable. Normal field of view for an APS-C sensor like the ones in the Pentax DSLRs would be a focal length in the low 30s. Actual "normal" human field of view is represented by a lens a little wider than 50mm (on a 135-format film SLR), closer to about 45mm or even a tad less. (Experts are not unanimous about this.) Given the 1.5x "crop factor" of the Pentax DSLR cameras, a lens with a normal field of view - that is, a field of view that shows things pretty much as they appear to your unaided eyes - would be around 30mm, give or take 1-2mm. My normal lenses are my Sigma 28 f/1.8 and my Pentax 35 f/2.
If I win the Texas lottery, one of the first things I'll do is buy the Pentax 31 f/1.8. But right now at Amazon, the Pentax 31 costs almost three times as much as the Sigma 30 f/1.4 or my Sigma 28 f/1.8. I'm posting a photo
Will