F/stops and lens advice

[ab1385]

I get most of the stuff about apertures etc, but I wanted to check a point - is it such that F/2.8 allows in exactly twice the amount of light as F/5.6, or is it not as linear as that?

The reason I ask is that I'm thiking of getting some prime lenses - I want them to be sharp - probably a 50mm ('cause I seem to be going to quite a few weddings at the moment!) and and another in the region of 75-100mm for sports. I like the idea of having a lens with a wider aperture so I can manage faster shutter speeds, but how much real world difference does having f/1.8 or f/2.8 make over f/5.6?

Any reccomendations for either of the above? I'm assuming the Canon 50mm f/1.8 is going to be quite popular? I'm looking for eBay (or anywhere else!) bargains really - maybe £50ish for the 50mm, and maybe £100 for something around 85mm f/1.8 or f/2.8...

[petrolhead]

[FONT=Arial]Found some info on three aspects of exposure which me be of use to you

Apature (F number) - [/FONT][FONT=Arial]http://en.wikipedia.org/wiki/F-number[/FONT]

[FONT=Arial]Shutter Speed - [/FONT][FONT=Arial]http://en.wikipedia.org/wiki/Shutter_speed[/FONT]

[FONT=Arial]Film Speed (ISO) - [/FONT][FONT=Arial]http://en.wikipedia.org/wiki/Film_speed[/FONT]

[FONT=Arial]There is also a usefull test - [/FONT][FONT=Arial]http://www.silverlight.co.uk/tests/f_stopc.htm[/FONT]

[FONT=Arial]Also this is quite good and is where the test came from - [/FONT][FONT=Arial]http://www.silverlight.co.uk/tutorials/toc.html[/FONT]

[ab1385]

Ah, excellent, thanks. So an increase of the aperture by the power of the root of two (1.4) allows half the light to enter, so f/1.4 allows in 3 times as much light as f/2.8?

[petrolhead]

I meant no

[Cadire]

There's a little tool here that allows you to play with all that sort of stuff (if you haven't already).

http://www.photonhead.com/simcam/

[onefivenine]

Originally Posted by ab1385

Ah, excellent, thanks. So an increase of the aperture by the power of the root of two (1.4) allows half the light to enter, so f/1.4 allows in 3 times as much light as f/2.8?

Is it not 4 times as much light?

f/2.8 to f/1.4 is two stops.

A stop is double the amount of light. Double something twice = x 4.

Yours pedantically!

[petrolhead]

You are so right

Silly me looked at the 1/2 stop chart - doooooh