Originally Posted by rt7snap
Front DOF = d • F • a2 / (f2 + d • F • a)f: focal length
Rear DOF = d • F • a2 / (f2 – d • F • a)
F: F number
d: circle of confusion diameter
a: subject distance (distance from 1st principal point to subject)
So DOF depends not only on the aperture size but also focal length, the COC and the distances.
Source : http://www.photozone.de/3Technology/depth2.htm
Try calculating the DOF here by imputing different values for the above in the table below : http://www.photozone.de/3Technology/demos/depth.htm
So your F/4.5-5.6 may not be able to get the desired DOF in certain situations.
General purpose of getting a lens with big aperture : DOF and also enable the use of faster shutter speed and/or lower ISO, especially in dim light shooting situations such as concerts/indoor performances/fashion shows etc. In some situations, if you don't have a fast lens, you may need to up the ISO and this introduce more noise in the pictures. At the extreme low light conditions, with a slow lens, your shutter speed to get a proper exposure may be too slow to freeze movement.
Yes, constant aperture means that the camera is capable of having a maximum F/2.8 aperture in all its focal length of the lens.
Keeping in mind that the F number represents the diameter of the aperture relative to the focal length, the actual aperture size at F/2.8 at 200mm is actually very much (4x) bigger than F/2.8 at 50mm. The theoretical aperture diameter are :
F/2.8 at 200mm = 71.4mm
F/2.8 at 50mm = 17.8mm
So for some zoom lenses, the maximum actual aperture size is not large enough to have F/2.8 when fully zoomed in but can still easily have F/2.8 at 50mm. So they have a larger F number at longer focal length.
So constant aperture has nothing to do with the light condition. It's just stating the maximum aperture size the lens is capable of relative to focal lengths.
Fast lens. As explained by others, it's one with a small F number (i.e. large aperture relative to focal length) and it simply means it allows you to use bigger aperture to shoot a picture than one with a bigger F number. With a larger aperture used, it enables you to use faster shutter speed and/or lower ISO to achieve the same exposure when compared to one with a smaller maximum aperture lens.
A fast lens is especially needed when the light condition is so dim that you're unable to get the picture you want with a slower lens which either requires your ISO to be too high for your liking and/or shutter speed to slow to freeze movement or prevent handshake blur just to get a proper exposure.
BTW, I only have a prosumer Nikon Coolpix 5700 to my name and don't own any SLR or DSLR cameras.