by sftonkin » Mon Jan 28, 2008 5:13 pm
Rough way: Do this indoors; just focus a distant object (e.g. clouds, Moon) through a window onto the bit of wall adjacent to the window. The focal length is the distance from the mirror to the wall. Should be easily good enough to tell you whether it is f/6 or f/8.
Another rough way, but doesn't require distant object: Mount the mirror vertically (making sure it can't fall and damage itself or anything else) Hold a light (penlight, LED) to the side of your head near your eye so that you can see its reflection. Move the light. If its reflection moves in the same direction, move away from the mirror. If the reflection moves in the opposite direction, move closer to the mirror. There comes a point where the light appears to fill the mirror. Your eye/the light is now at the centre of curvature. The focal length is half this distance. Again, for speed and ease, if you just test it at a distance equivalent to f/7 (i.e. 112" from the mirror), if the reflection moves the same way as the light, it is f/8; if it moves the opposite way it is f/6.