can someone explain to me whats focal length?
and Because of basic optical
principles, the absolute aperture sizes and diameters depend on the
focal length. For instance, a 25mm aperture diameter on a 100mm
lens has the same effect as a 50mm aperture diameter on a 200mm
lens. If you divide the aperture diameter by the focal length, you
will arrive at 1/4 in both cases, independent of the focal length.
Expressing apertures as fractions of the focal length is more
practical for photographers than using absolute aperture sizes.
These "relative apertures" are called f-numbers or f-stops. On the
lens barrel, the above 1/4 is written as f/4 or F4 or
1:4.
from
http://www.dpreview.com/learn/?/key=aperture
i catch no ball leh
