Comparing Minox Lens Apertures And Required Shutter Speeds
There seems to be some confusion about the relationship between required exposures for the f3.5 Minox cameras and the f5.6 EC/X cameras. The main source of confusion arises from the way that f-stops are defined; the definition being "the focal length divided by the aperture diameter." Our problem is that exposure is proportional to aperture area and not diameter. From basic geometry (area=pi r^2) it can be shown that an increase in diameter by a factor of Sqrt[2] will produce an increase of light by a factor of 2, thus each full f-stop is Sqrt[2] times greater than the previous one (2, 2.8, 4, 5.6. . .). If we want to know how many f-stops separate a f5.6 lens from a f3.5 lens we must solve for x given:
3.5 Sqrt[2]^x = 5.6
which yields: x -> 1.356
Thus the EC lens is shown to be slower by 1.356 f-stops than an f3.5 lens, but this isn't of much practical use. To determine the difference in required shutter speed we must remember that each increase in f-stop equals an increase of exactly 2X exposure time. The f3.5 to EC exposure factor (fe) is then given by:
fe = 2^1.356 = 2.56
thus, when an exposure of 1/100 is required in a f3.5 Minox an EC/ECX will require an exposure of about 2.56/100 or approximately 1/40.
A second problem is that most standard light meters are marked for f4 and not at the Minox's f3.5 aperture. Using the same procedure as outlined above, it can be shown that f3.5 is .38 stops below f4 and that shutter speeds should be increased by 30% over the f4 light meter recommendations.