SLR cameras let you attach different lenses so you can see the scene at various magnifications. The magnification power of a lens is described by its focal length (mm).
In cameras, the focal length is defined as the distance between the lens and the real image of an object in the far distance. Understand this as the distance between the flim plane / CCD and the optical center of the lens
(NOT THE SAME AS THE PHYSCAL CENTER OF LENS) it really depends on how the lens gets made... hence diffent sizes, the sigma 50-500 can be smaller than a 300mm for example.
Just note higher focal length number indicates a greater image magnification.
EF 50mm 1.8 Mark II
OKay,
EF - Electrofocus a name canon devised to brand their new line of lenses that suceeded the older range, compatable with EOS cameras. Nikon uses like AF or AF-S or AF-D, all having their own meanings like -S being silent wave motor, and D being the distance chip for 3D metering applications
50mm - as discussed it is the focal length, this affects how a picture looks and how far away u have to stand from an object of a certian fixed size to fill the frame (magnifcation). This also affects persepective, u know, when u look at a picture and it looks like woah!! heck alot of space... and the things in the foreground are like HUGE! thats probally taken with a wide angle lens 24mm perhaps. but if it looks like what ur eyes is used to seeing, it cld be a 50mm lens.
1.8 - Maxium Aperture value. This denotes the maxium amount of light that can pass throught the lens. glass is not all that clear, u lose light when u stack glass infront of it. Smaller numbers make faster lenses. This also affects whats called depth of Feild (DOF) at 1.8, the area in focus is shallow compared to say F8. Remeber the expriments in school? smaller pin hole sharper image? same goes. the aperture is a mechnical iris that closes in to form a smaller opening to get you a shaper image.
Mark II - denotes its a second edition of the lens. thats all. u may find other words here like USM, meaning ultra sonic motor a kind of motor used by canon in some lenses, in Sigma they are called HSMs. You may find the word Macro there, with denotes that the lens is made to focus closer than that of those with the same focal lenght, giving u high magnification at the same focal lenght.
Here a link thats good in describing focal lenght and crop factors:
http://digilander.libero.it/fotoreportage/tecnica/sk_05_e.htm
basically crop factor happens due to a physically smaller size of the sensor, like when we were kids and held cardboard tubes to our eyes, its not that we saw further, its jsut a cropped view.
) to print out at a certain size, then the photo will be scale accordingly, and the dot density (dpi) will be dependent on the number of pixels in the photo you're printing; as well as how large you want to print. my 10D's 3072x2048 photos can hit 512 dpi (logically speaking) if i want to print in 4R (6x4 inch) because 3072/6 and 2048/4 gives 512.
