Can anyone explain to me about the f/ no. for apertures such as f2.0, f4.5.
i really dun understand what it means and how to use it..
thanks
HI, im no expert - as a matter of fact im a slow learner but im goona go out on a limb here ( my shrink would be so happy with me ) and try to explain what i know or think i know.
the size (opening) of the aperture determines how much light, ie the quantity of light, that will reach the media ( cf card etc. ) you use to record an image. The wider the aperature, the more it is open and the more light that can reach the media. The smaller the aperature, the less wide it is open and therefore the less light that can reach the media. So what you dare say?? well the fact is that the quantity of light that reaches the media is not only a necessary ingredient for a properly exposured image to happen but the quantity of light also affects depth of field. You know those beautiful pictures you see of flowers or birds that are so sharp with a background that seems blurried, well that has to do with depth of field. do you like those images where the trees in the background are nearly as sharp as the people swimming in the lake, well that too has to do with depth of field. your choice of aperture can affect depth of field. Anyway we have a scale that denotes the size of the aperture opening, and we call it the f- stop scale. Commit these f-stops to memory. you can thank me later, f/2, f/2.8, f/4. f/5.6, f/8
lets talk about this "f-ing" scale a bit...

DONT WORRY ABOUT THE ACTUAL NUMBERS!!! DONT TRY TO FIGURE WHERE THEY COME FROM. THESE ARE THE F-STOPS AND THATS IT!
( umm, actually i lied, there are other f-stops, but for now the ones i listed are most important)
f/2 is a wider opening than f/4, f/4 is a wider opening than f/5.6, f/5.6 is a wider opening than f/8 yadda yadda ......
the smaller the number the wider the opening!!!! why do you care? if you need more light use a smaller f-stop. maybe your shutter is closing so fast ( another story for another time ) , the extra light is needed for that "properly exposed image.The math that goes into the formula for f-stops is complicated and it may not seem rational BUT IT IS! i gave you a series of f-stops above. As you go from one f-stop to another in sequence, one half the light becomes available. so the amount of light available at f/4 is half the amount available if you used f/2. and yea the amount of light hitting the media at f/5.6 is half the amount that hits it at f/4 and 1/4 the amount that hits it at f/2. This will all become understandable as you now read more on your own.
so in conclusion you need f-stops to know how much light to send to the media ( you'll really need this info when you learn about shutter speeds ).
you also need to know f-stops to control depth of field ( look that up ) and finally...
it is a given ( from what ive heard ) that a lens is sharper about two stops down from its widest opening! Yo!!! did you hear this? I just showed you how to get a sharper image. if your lens is a f/4 lens, try "stopping it down" to f/8 and see what happens! Look at us using photog lingo!! now we can "step down" which means closing the aperture more to let in less light. AND i might do this to get a narrower depth of field. More of my image is looking focused and sharp. Yippee!! I almost understand what Im talking about. I left out a lot of stuff and i dont even know if what i said is accurate, but ya know what? this incomprehensible rant helped me!!!!
troy