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I'm baffled as to why we (yes i include myself) are drawn towards faster lenses.
It can only be very good advertizing hype or a case of "penile dysfunction" well for us guys anyway.
Ok this may sound crazy yet when you start reading into the reviews and technical stuff they invaribly mention something along the lines of "sharpest when stepped down to F4" or similar so why pay out F2/F2.8 prices.
As for low light step up the ISO a bit.
We have all seen cracking images on various web sites /magazines etc yet when you read the camera settings few if any are at full aperture so why bother with the mega expensive fast lens in the first place?
I add that the only time I have had the use of a fast lens is a relative that has loaned me a Nikon 200-400 to take to kenya next year and I took it out yesterday to my local woods took some shots with it and also similar shots with my Nikon 18-200 apart from the obvious reach advantage (overcome by cropping in) I fail to see any advantage.I will still take the 200-400 with me anyway
I have ordered the drawbridge raised ready to repel my attackers
Speaking from personal experience of owning a Canon EF 70-200 f4L IS USM, which I sold and purchased an EF 70-200mm f2.8L USM (non IS), shooting cycling with them both at f5.6, there was, in my opinion a difference with both sharpness, colour saturation and a smoother background blur.
Others may have a different experience, but that was mine. At that time I was shooting on a 40D and found an improvement again when I went to the 1D MkIII.
Oh and being a lady I don't suffer from "penile dysfunction" 
Quote: I have ordered the drawbridge raised ready to repel my attackers
LOL!!!
Cole makes a good point...
Quote: As for low light, step up the ISO a bit.
Fair enough - so long as the sensor involved has excellent low noise specifications.
However, ramping the ISO will always reduce the colour saturation.
So part of the answer as to 'Why fast lenses?' has to consider what the predominant choice of subject matter will be when arriving at a purchase decision involving 'fast lenses'
joking apart? if it is possible on a forum? ah well on to my current hobby horse
why not try some of the older DIY focussing lenses (cries of WOT I hear, me focus how do I do that ha!) that us old geezers used to use in the last millenium (ie. before AF etc. and cameras that take the thinking out of photography) it really makes you think (and in low light or close up makes sense) PLUS the glass seems better than the plasticky lightweight stuff around now,
If you've got plasticky lightweight lenses that's because you've probably bought plasticky lightweight lenses. Plenty of high quality lenses out that with prices to match
I use f/2.8 or f/3.2 on my 70-200 more than any other apertures on that lens to be honest - much the same on my 300mm too. In any event, as Cole says, the wider the maximum aperture, the more light enters the lens aiding the AF systems and viewfinder brightness. Remember at f/2.8 double the amount of light enters as does at f/4
perhaps I did not go into describing and comparing my 60/70's (and pre 60's) lenses, which I still use, on the origonal and newer bodies with adaptors, and whilst quality is still excellent they do weigh a bit more than todays lighter weight lenses which I feel have a rubber or plastic feel*, now bearing in mind these gems which were FAR more expensive in real terms than today.
(*so possibly dysfunction addicts may feel at home?)
Quote: (overcome by cropping in)
So basically you have a 12 mega pixel image, Then you throw X percent of the resolution away.......
Like your theory on f/2.8 lens models.....Not a whole lot of sense......
When you have studied the reasons and fully understand the why & where fore.....
Maybe it will make sense to you to.....Until that happy event takes place thank the person who is lending you a Nikon 200-400 " f/4 " for he/she is a truly trusting and generous person...!!!
Quote: when you start reading into the reviews and technical stuff they invariably mention something along the lines of "sharpest when stepped down to F4" or similar so why pay out F2/F2.8 prices.
Err.... because if you use an f/4 lens it is likely to be sharpest at f/6.3 or thereabouts; if you use an f/6.3 lens, it is likely to be sharpest at f/11 or thereabouts.
So, the faster the lens, the faster the aperture at which it is likely to give its sharpest results.
(Although that is a generalisation to which there will obviously be a few exceptions. For example, the Nikkor 1.8D is supposed to give sharper results at f/1.8 than the Nikkor 1.4G gives at f/1.8 )
.
Quote:
fill the frame waste nothing"
I remember being told that but my Mum also told me to always boil everything first before I put in my mouth and thankfully I don't do that either
Seriously though, with regard to cropping, I do think digital has made us lazy about framing/composing etc. "in camera" ... I know I am
...its all been said 
most lenses are sharpest stopped down a bit (from what I've read many many times, and a bit of what I've seen)
my old 50 F1.8 wasn't amazing at F1.8, but was lovely at F2.8
but like Barrie, I shoot F2.8 with my 70-200 a lot of the time - sometimes because I need to for the light conditions (e.g. dark churches) but mainly because I like the blurred background it gives.
its also long, hard and white.....
I've often wondered why macro lenses are F2.8, I very rarely use mine at anything less than F8 because of the DOF?......Although on very rare occasions I have used F2.8 when doing some portrait work with it, but that really is once in a blue moon.....
Ade
I've heard that a lot of lenses are sharpest around F8 - then tail off as you go beyond..... so I guess working out the "hyperfocal" distances for things like landscape could pay off
then again, shooting at f16 and focussing 1/3 into the shot usually works
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