Thanks Cory! Don
> -----Original Message----- > From: Cory Papenfuss [mailto:[EMAIL PROTECTED] > Sent: Tuesday, May 24, 2005 10:07 AM > To: PDML > Subject: Re: F stop question > > > On Tue, 24 May 2005, Don Sanderson wrote: > > > How does one figure partial stop numbers? > > For instance what stop is half way between 4 and 5.6? > > And where does 4.76 fall? This is a 2.8 lens with the > > SMCP-F 1.7x converter. > > I'm guessing there is a simple multiplier for this but > > with my limited knowledge of math I have no clue > > what it is. > > This is more out of curiosity than necessity. > > Someone posted a link to this info but I can't find > > it again. > > > > TIA > > Don > > IIRC, f-stops are defined by the *diameter* of the aperture, but > light transmission goes as the *area* of the aperture. Thus, > doubling the > diamter (i.e. f/8->f/16) *quadruples* the light transmission. A > "stop" is > defined as a doubling/halving of the light, so f-stops at a ratio of > sqrt(2) \approx 1.4 are one "stop" apart. > > Fratio = (sqrt(2))^N where N is the number of stops. Solving for N > yields: > > N = (2 log(Fratio))/(log(2)) > > e.g.... your question: > (2 log(4.76/4))/(log(2)) = 0.5, or 1/2 stop > > The 1/2 stop ratio is 2^(1/4) = 1.189 \approx 1.2 > The 1/3 stop ratio is 2^(1/6) = 1.122 > > So these sizes are "1/2 stop" apart: > 1.4 -> 1.7 -> 2 -> 2.4 -> 2.8 -> 3.4 -> 4 ... > > and these are "1/3 stop" apart: > 1.4 -> 1.6 -> 1.8 -> 2 -> 2.2 -> 2.5 -> 2.8 ... > > -Cory > > ************************************************************************* > * Cory Papenfuss * > * Electrical Engineering candidate Ph.D. graduate student * > * Virginia Polytechnic Institute and State University * > ************************************************************************* >
