You're right, it's not linear it's based on with the inverse square law. So GN increases by ~1.41 times when ISO doubles. so a change from iso 100 to 200 give a new gn of 22 gives ~31 hey it was late and I was tired. (Hell just look at my first explanation), and I haven't seriously used manual flash in years.
Peter Fairweather wrote: > I might be wrong but doesn't the guide number go up ny the square root of two. > > If the GN is 30 at 100 !SO it is 40 at 200 ISO and 60 at 400 ISO > > > Peter > > > > > On 28/02/2008, P. J. Alling <[EMAIL PROTECTED]> wrote: > >> Sorry that was a bit convoluted. Just remember the GN (in this case 22 >> in meters) depends on the ISO. GN doubles as the ISO doubles. >> >> The formula for determining the proper f stop to use is: >> >> f = GN (in meters)/distance (in meters) >> >> You just plug the numbers you have in. >> >> If your lens just has distances marked in feet then you have to convert >> the GN in meters to one in feet by multiplying it by 3.28. You can >> simply convert feet into meters, but if you convert the GN it becomes a >> constant an you only have to do it once. >> >> Pretty simple unless you have one of those new fangled lenses with no >> feet/meter markings at all. Also remember you don't have to be exact. >> Being off by a 1/3rd to a 1/2 stop will still give reasonable results. >> >> P. J. Alling wrote: >> >>> Yes, I expect since TTL isn't supported on the K10D, so it does fire at >>> full. If you can find out the GN at ISO 100 you should be able to work >>> out your own flash table from there. The ISO 100 GN in Meters as >>> published by Pentax for the AF220T is 22 so to get the correct aperture >>> you need to do a little math.the formula is simple if you have a >>> calculator. (f number) = GN/distance(in meters). If you double the ISO >>> you have to double the GN by the way, so that parts easy enough at >>> least. So your distance of 1.5m at ISO 200 would give this >>> >>> f = 44/1.5 = 29 1/3 so yes f 8 was way too wide at that distance. I'd >>> go with ISO 100 since you have a K10d. That should give you an f stop >>> value of 14.66_ which is much more manageable. >>> >>> wendy beard wrote: >>> >>> >>>> I admit, I'm useless when it comes to using flash. I use it as little >>>> as possible. When I do, I just stick the flash on the Canon and it all >>>> magically works - even on the G9 (tho the flash is bigger than the >>>> camera!) >>>> I don't remember having any problems with the MZ-S and 360FGZ >>>> That's the magic of E-TTL/P-TTL, I suppose >>>> Anyway, I no longer have the 360FGZ and both the Canons are off for >>>> repair and sticking a flash on the G9 is ridiculous - so I'm trying to >>>> get the K10D working with an AF220T >>>> Here comes the stupid question. >>>> How do I know what settings to use? >>>> My old, old agfatronic used to have a table of aperture vs asa vs >>>> distance. All that's on the back of this flash is "control range >>>> 0.7-5.5m" >>>> It seems mighty powerful at close range. ISO 200 f8 1/180s and a >>>> distance of approx 1.5m and it's way over-exposed >>>> >>>> Is it because TTL isn't supported on the K10D so the flash just fires >>>> at full power all the time? Will it just be pure luck if I get a well >>>> exposed shot with this combination? >>>> >>>> Wendy >>>> >>>> >>>> >>>> >>> >>> >> -- >> Vote for Cthulhu. Why settle for a lesser evil... >> -- Dr. Jerry Pournelle >> >> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> to UNSUBSCRIBE from the PDML, please visit the link directly above and >> follow the directions. >> >> > > -- Vote for Cthulhu. Why settle for a lesser evil... -- Dr. Jerry Pournelle -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
