I think that the *istD is better at math than the PZ-1p. According to my calculations, half way between 2 and 2.8 is 2.378, but it depends on you define "half way".
The relationship between f-stops is not linear. f-stop is defined as the ratio of the focal length to the aperture diameter, but the difference between f-stop values is independent of focal length and diameter, as follows: Nf = f/d where Nf is the f-stop number (e.g. 2.8), f is the lens focal length and d is the aperture apparent diameter, as viewed through the front element. Also, Nf = sqrt(2)^(n), which reads as "the square root of 2 raised to the power of n", where n is an integer number. when n = 0, you have Nf = 1, for which the aperture diameter is equal to the focal length. Increasing 'n' by 1 corresponds to doubling the area of the aperture opening, which is why the shutter speed doubles when you open the aperture one full stop - the amount of light has doubled. Here is my F-stop table, where "f/" is the number commonly used to represent Nf. n Nf f/ 0 1 1 0.5 1.189 1.2 1 1.414 1.4 1.5 1.682 1.7 2 2 2 2.5 2.378 2.4 3 2.828 2.8 3.5 3.364 3.5 4 4 4 5 5.657 5.6 6 8 8 7 11.31 11 8 16 16 9 22.627 22 10 32 32 I figure that half way between Nf = 2 and Nf = 2.8 is for n = 2.5. Note that the normal way of "rounding up" decimal numbers is ignored for f/ = 3.5, 5.6 and 22, they "should be" f/ = 3.4, 5.7 and 23. Jim www.jcolwell.ca
