simple lenses

Has anybody tried using simple lenses like loupes or diopters or
reading glasses etc. as lenses also if I were to do so how would I
calculate aperture?

 

Ross,

Sure, it's a lot of fun.

As for calculating f-stops,

http://www.apug.org/forums/showthread.php?t=25071

Also, verify with a Polaroid before doing any "critical" work.

Steve

 

How would I calculate the focal length of the lens?

 

Don't calculate, measure.

 

How do I measure?

 

Use it as a burning glass. Focus it on the sun, measure lens to hot spot distance.