the f number refers to the aperature, which is the hole that lets light in when the shutter releases, essentially the smaller numbers like f/2.8 let in more light. so the lower numbers are better when buying the lens. Though with lower f numbers u get smaller depth of field (area in focus) which mostly comes into effect when taking macro shots.
if the nikon 105mm f2.8 costs to much u might consider the 90mm tamron f2.8 lets, it doesn't have vr but is considerably cheaper
or maybe try a 50mm macro lens f2.8 those are cheaper than the longer ones 90-110mm range and with shorter lens length you get more depth of field. but you also get less working distance (distance from you to the subject).
just some general advice for taking shots
use a tripod for corals
turn off water flow when taking shots
shoot perpendicular to the glass as much as possible (angles blur shots)
as for settings (i have a nikon d50 older version of d40)
i like to use the manual modes (M,A,S,P - respectively manual, aperature priority, shutter priority and programmed)
each of these have certain advantages
i don't really use M because u have to adjust aperature and shutter, it's just too much of a hassle for me, i find the other modes more flexible
I use A for coral shots (still shots/slow moving) because you have time to adjust it for the depth of field you want and the slower shutter won't make the picture blurry as easily. also i like to use ISO 200 for lowest noise. on this mode the shutter adjusts to the aperature to let in the proper amount of light. also optimal for most lens for aperature is f/4 to f/11 speaking in terms of sharpness, though i still find it hard to tell a huge difference higher or lower but i try to stay in that range
I use S for fish shots (fast moving). I set the shutter to at least 1/250 of a second (displayed as 250) to get sharper pictures. i like to use ISO 400 to let in more light than ISO 200 and still little noise. on this mode the aperature adjusts to the shutter to properly expose the picture
as for P i really don't use this for my aquarium but for when i'm just going out to take pictures, it quickly adjusts aperature and shutter for you so you don't miss shots where as in my aquarium they're all staying in the box. I suppose if the situation arose that something require a quick shot came up i'd use this mode.
other quick notes
i don't use a flash, hard to work with with the glass and changes the whole color scheme though some people use remote flashes and put them pointing down into the tank, haven't tried that yet
white balance - either use auto or the setting cloudy, i think it'll probably default to cloudy anyways, using the custom preset is technically the best but takes a lot of time to get good at (i'm still not that great at getting what i want with it)
quality - i shoot in raw+jpeg cause i edit CS3 and like to have a quick preview with the jpegs on my comp
if you have any questions feel free to ask
o and
www.bhphotovideo.com is a good place to get stuff, reputable business with good prices
