Nikon D300 Complete Manual page 365

Second we have a statement of maximum aperture (f/3.5-5.6).
A lens stated this way has a variable maximum aperture,
meaning that it has one aperture at one focal length (e.g. f/3.5
at 18mm) and a different at another focal length (e.g. f/5.6 at
200mm). Good lenses for low light have maximum apertures
of f/2.8 or lower (e.g. f/2 or f/1.4)
focusing more slowly when the maximum aperture of a lens
102
gets near f/5.6
a maximum aperture of f/8 because not enough light is getting
through the main mirror to let the AF sensors do their job).
The view through the camera (remember we're looking
through the lens) also darkens as maximum apertures get
higher in number. The 18-200mm I'm using as an example
here ranges from the middle to the low end of brightness
transfer. An f/2.8 lens would provide a brighter image in the
viewfinder, as all autofocus lenses are always viewed at their
maximum aperture, and f/2.8 would also allow more light to
get to the autofocus sensors than f/3.5 or f/5.6 (the range of
the maximum aperture of the 18-200mm).
Which brings us to those abbreviations:
G—The letter following the maximum aperture value tells
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us that this is a lens that provides distance information to
the camera, but has no aperture ring (a lens with an
aperture ring would have a D in this location; it, too,
provides distance information to the camera). D and G
101 A lot of confusing things come up in photography. One of them is that lens
apertures get physically bigger (larger in diameter) as the numbers get smaller. Thus,
a 50mm f/1.8 lens would have a larger maximum diameter lens opening than a
50mm f/2.8 lens. Almost all lenses allow us to choose smaller-than-maximum
aperture openings, so that f/1.8 lens would allow f/1.8, f/2, f/2.8, f/4, f/5.6, f/8, f/11,
and so on, while the f/2.8 lens would allow f/2.8, f/4, f/5.6, f/8, f/11, and so on.
102
The D300 has a sophisticated, fast, and accurate autofocus system, but with the
18-200mm lens mounted on it and set to 200mm, the camera's autofocus system isn't
as responsive.
Thom Hogan's Complete Guide to the Nikon D300
(and stops completely if you use a lens with
101
. The camera starts
V1.02
Page 365