Educate me on aperture

[Z-man]

it seems most people prefer to use lens with the lowest possible aperture. i am a beginner and just starting to get out of the point and shoot mode. Why is a 50mm lens with a f2.8 better than a 3.5? or a 70-200 lens with a 3.5 better than a 4? Does it really make that much difference? This probably sounds like a stupid question to the more advanced users, but you have to start somewhere when it comes to learning these things.

[xerubus]

it's not a stupid question at all.

rather than me rabbiting on, take a look at the following link... it's gives an overview regarding aperture. i'll leave it up to the more learned gents to put their two bob in.

http://209.196.177.41/03/03-03.htm

[MCWB]

It all depends on what you want to do with your photos. If you want incredibly short depth of field (DOF), then you need small apertures. If you're photographing things in suboptimal lighting conditions and you want a decent shutter speed then you will often want to use a smaller aperture.

An example of which is close to my heart, my Tamron 28-300 has a maximm aperture of f/5.6 at 200 mm. Say under the prevalent lighting conditions and the ISO setting I've chosen, correct exposure is 1/500 s at f/5.6. The Nikkor 80-200 f/2.8 is two stops faster at 200 mm (f/2.8 vs f/5.6), i.e. it lets in four times the amount of light. The correct exposure at f/2.8 is therefore 1/2000 s. The difference between 1/500 and 1/2000 s may be the difference between a motion-blurred Formula 1 car and a 'stationary' one. The difference in lens cost is also considerable though.

[digitor]

it seems most people prefer to use lens with the lowest possible aperture.

A "lower" aperture (actually a bigger aperture) lets more light in. The f-number is the ratio of the diameter to the focal length of the lens, so a 50mm f2 lens would have an aperture of 25mm, and an f1 lens (very uncommon, although Canon do make one - about 4k in US$) would have an aperture of 50mm. You will notice that f-numbers have a progression related to the square root of 2, so 1, 1.4, 2, 2.8 etc. this is because increasing the diameter by the square root of 2 doubles the area, therefore letting twice the amount of light in. Pi r squared, and all that!

All this means that, for a given film (or ISO setting on your dig) speed, you can use a faster shutter speed, which can help with camera shake or motion blur. Anyway, I'm sure that Google will help you a lot more than I have!

Cheers

[phillipb]

Also, as it was stated on another thread, a lens rarely performs best at its widest aperture so a f2.8 lens should give you better results at 5.6 then a lens with f5.6 max. aperture at f5.6