Monday, June 30, 2008

Focal Length Calculation for a DSLR Camera

Canon 400D with a Vivitar 400mm Lens
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.


Anonymous said...

Wow, math! Goody!!!

slavino said...

not math... physics !!!

Tim said...

You're a lifesaver! I needed this exact formulae for my next school event. I also run on the Canon 400D and I can purly say it's the best ever made.

Thank you dear

Syed Kollol said...

Wow Great.
What would be the focal length to shot saturn ?

Just kidding!. great and thanks for this.

theowl84 said...

For shooting saturn, any focal lenght is suitable. In any case, it will be one pixel wide ;)