Awesome pictures, and amazing lenses!

Calculating the focal length would use a rearrangement of the formula I found

here
my (perhaps wrong, anyone want to check my maths?) answer is....

f = d / [2(1+m/p)tan{2alpha}]

where...

"f" = focal length

"d" = measured linear dimension for the angle your using (i.e. if you are measuring the horizontal angle fo view, this number is the horizontal length of the sensor = 17.3mm)

"m" = magnification (with 1 being eqiv to 1:1)

"p" = ratio of apparent exit pupil diameter : entry pupil diameter

"alpha" = angle of view (the angle of view for the distance "d")

I'm not sure how you measure the apparent pupil diameters though...

I like this game

Code:

```
incidentally my rearrangement went...
alpha = 2arctan {d / [2f(1+m/p)]}
2alpha = arctan {d / [2f(1+m/p)]}
tan{2alpha} = d / [2f(1+m/p)]
(1+m/p)tan{2aplha} = d / 2f
d / (1+m/p)tan{2aplha} = 2f
f = d / [2(1+m/p)tan{2aplha}]
```