[reeftool]

I suspect it's actually 4mm. F/2.8 is actually. It is F/(sqrt(2)^3) (squareroot of 2 all to the 3rd power). F/11 is derived similarly. I may be talking out of my @$$.

For those of us who who went to summer school every year and still didn't pass math, 2.8 is almost useless unless your subject is flat, f/11 will probably get a whole flower.

[newarts]

I suspect it's actually 4mm. F/2.8 is actually. It is F/(sqrt(2)^3) (squareroot of 2 all to the 3rd power). F/11 is derived similarly. I may be talking out of my @$$.

Good point; the error you point out is in our sloppy use of assigning the name "2.8" to the number 2*SquareRoot(2) = 2.828427.....

Using the names of the f-stops in calculations is usually good enough; what the actually aperture opening a manufacturer calls F:2.8 on a particular 100mm lens is highly unlikely to be precisely 28.28427mm.

I suppose a reasonable thing to do for quick estimates is to use f-stop names in calculations then round up or down to the next common f-stop name; in this case f:3.9 rounds to f:4 (as would f:4.1).

Best wishes,
Dave