[Leica] Re: F stops

[lrzeitlin at optonline.net (Lawrence Zeitlin)]

F stops, f stops, f stops. The weather is so lousy in the US  
northeast. The only photographic subjects in my area are  
grandchildren and rain drops on the windowpane. We are back to  
discussing how many angels can dance on the point of a pin.

Anyway, in theory, opening a lens by one stop lets in twice as much  
light. Opening it by 1/2 stop lets in 1.414 times as much light (i.e.  
the square root of 2). A half stop increase from f3.5 would be f2.94.  
A f2.8 lens lets in 1.56 times as much light as an f3.5 lens, about  
10% more than a 1/2 stop increase. Bear in mind that these  
differences in lens opening are much smaller than the tolerance in  
mechanical Leica shutter speeds. I don't know about electronically  
controlled shutters but I felt lucky if my M3 shutter could be  
adjusted to plus or minus 20% of the nominal speed.

Movie makers have it much easier. The exposure is usually determined  
by an open sector in a shutter disc directly geared to the carefully  
controlled film transport mechanism. The lenses are calibrated in T  
stops which are based on the actual transmission of light through the  
lenses. Bell and Howell tried using T stops on its ambitious Foton  
camera. To no avail. Still camera makers insisted on using f stops  
because it made for numbers which implied faster lenses. Score one  
for the marketing departments.

Larry Z