Focal Length Calculation for a DSLR Camera

As I'm studying for a subject concerning cameras at university, I have to used a formula for calculating the required focal length to fill the sensor with an image. And I thought this formula could be quite practical for you as well.

That formula is

f = d * I / (I + O)

where f[m] denotes the required focal length to picture an image with the width of O[m] on the sensor with the width of I[m] when the object is d[m] away from the camera.

For my Canon EOS 400D camera with a sensor dimension of 22.2mm x 14.8mm a sample calculation could look like this:

I would like to picture an object that is 20cm wide and 1m away from my camera. So I set d = 1m, O = 0.02m and I = 0.0148m (sensor width).
Inserting into the formula from above this gives me a focal length of f=0.0689m or 69mm.

This formula is valid for all cameras (just find out your sensor dimensions) and all lenses.

Red Hot Chili Peppers

Red Hot Chili
Originally uploaded by theowl84

My home grown chilies start to grow bigger and bigger, the first are bright red already. Soon others will follow. I'm planning to sell some of them on eBay as I can't handle the huge amount of them on my own.
So in case you want to try some or grow them yourself, just mail me and I can send you some exemplars.