> > lens you're talking about is really too slow for taking pictures of the moon,
> though.
> > You need a stupid expensive fast tele for doing that.
>
> This part in particular. Why a fast lens? I would think you need a slower one for
> more exposure or more light or something. I clearly need to read more about this
> stuff...
Fast lens (e.g. f/1.8) = lets in more light at a time
Slow lens (e.g. f/5.6) = lets in less light at a time
The dimmer the subject or the slower the lens, the longer the exposure time you need. The longer the exposure the more you get affected by heat shimmer, poor seeing conditions, vibration, etc. Also, longer lenses like that super-tele suffer more from these effects than shorter lenses.
It's probably OK for shooting scenery from a distance, but be prepared to have poor detail. The atmosphere is a bitch. The further you are from the subject, the more water haze, particulate pollution, dust, and heat shimmer affect you.
Double the f/ratio = one quarter the light gathering ability
i.e. an f/5.6 lens needs four times the exposure time an f/2.8 lens needs for the same subject in the same light.
Higher f/ratios increase diffraction, too. Above f/8 you can definitely see diffraction blur on a good digital camera. This is worse on smaller sensors, so going to a 35mm sensor lets you push the f/ratio higher if you have wicked bright light or you need massive depth of field.
Basically you can't win. You end up spending a fortune on lenses, and you're unhappy with something about every single one of them. Deciding which three to carry with you is agonising, too.
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