Thanks Steve! Got it! Don
> -----Original Message----- > From: Steve Jolly [mailto:[EMAIL PROTECTED] > Sent: Tuesday, May 24, 2005 10:27 AM > To: [email protected] > Subject: Re: F stop question > > > Aaaaaaaaaaaaaaarrrrgh! > > I got some numbers wrong. :-( > > See below. > > Steve Jolly wrote: > > Don Sanderson wrote: > > > >> How does one figure partial stop numbers? > >> For instance what stop is half way between 4 and 5.6? > > > > > > 4.76 :-) > > > >> And where does 4.76 fall? > > > > > > Half way between 4 and 5.6 :-) > > > > Or in other words, f4.76 is 1/4 of a stop slower than f4 and 1/4 of a > > stop faster than f5.6 > > Sorry, 1/2 a stop in both cases, not 1/4 > > >> This is a 2.8 lens with the > >> SMCP-F 1.7x converter. > > > > > > Therefore the 1.7x converter introduces a 3/4 stop decrease in the > > effective aperture size. > > And that should read "a 1.5 stop decrease". > > The rest is correct. > > >> I'm guessing there is a simple multiplier for this but > >> with my limited knowledge of math I have no clue > >> what it is. > > > > > > It's "easy" - multiply by sqrt(2), or by sqrt(sqrt(2)), etc. Here's an > > example: > > > > Halfway between f4 and f8 is f5.6 > > 4 * sqrt(2) = 5.6 > > 5.6 * sqrt(2) = 8 > > > > Halfway between f4 and f5.6 is f4.76 > > 4 * sqrt(sqrt(2)) = 4.76 > > 4.76 * sqrt(sqrt(2)) = 5.6 > > > > Spot the pattern? :-) > > > >> This is more out of curiosity than necessity. > >> Someone posted a link to this info but I can't find > >> it again. > > > > > > Well, I haven't tried to explain it exhaustively, 'cos people complain > > when I do that... but I've answered your questions at least. ;-) > > S >
