John (and Wayne);
Yes, of course I should have said something like 'not enough light' will
fall on the 64 meter object. My mistake. I can dig the math you
describe below, but something doesn't 'feel' right to me. I think the
inverse square law applies to a 'point source' of light, which a single
flash approximates. But when one starts placing more flashes in the
picture, I'm just not sure the effect is additive. To carry this a step
further, if 2 T32s will provide enuf light to properly expose an object
45 meters away, then it follows from the math that 4 T32s should have a
GN of appx 64. When I think about the physicality of this, it says that
4 T32s will properly expose an object which is farther than *** 2
football fields *** from the camera. I have a tough time believing
this.
As I think about it, perhaps the relative size of the object being
illuminated has something to do with it? Like, perhaps a child would be
properly exposed by 1 T32 at 32 meters, a basketball player by 2 at 45
and the Pillsbury Doughboy from the movie Ghosbusters at 64 meters?
Maybe I'll go find my college Physics text.
george
"John A. Lind" wrote:
>
> At 23:40 2/29/00 , George wrote:
> >Barry;
> >
> >I don't know what the solution is (besides possibly a T45) but I'd like to
> >just caution about this statement:
> >
> >"Two T32's=GN64"
> >
> >This is not true. Guide Number is a measure of distance the flash is capable
> >of lighting up, assuming some fixed ASA/aperture/shutter speed. 2 T32s will
> >be able to throw more light on a subject at a distance of 32 meters, but
> >none of that light will reach a subject at 64 meters.
>
> George is only partly right. It's not that _none_ of the light will get
> there, it's not nearly enough light will get there. Two T32's is not GN64
> (EI100, meters). It is about GN45 (EI100, meters). Zounds, that means it
> takes a pair of T-32's to equal a T-45. It all follows the inverse-square
> law. Twice the distance requires four times the light. Think in square
> roots of two (approx. 1.1415926). Two flash units of equal GN triggered
> together gives you approx. 1.4X the single unit GN. Twice the film speed
> gives you 1.4X the GN.
>
> Soooo, lesssseeee for 64 meter distance:
> 1. A T-45 or a pair of T-32's (BG2 plus hot shoe) gets you to GN45
> 2. Going from ISO100 to ISO200 film gets you to GN 64 (close, but _no_
> Zuikos open up to f/1 :-( hint, you're short by at least an f-stop).
> 3. Go from ISO200 to ISO400 and voila, you now have a GN90.
>
> Use your 50/1.4 lens wide open and just barely eek out enough light for a
> proper exposure at 64 meters. Just to be safe, make certain it's an MC
> with S/N > 1Meg; you wouldn't want to lose any of that precious light on
> the way through the glass.
>
> Seriously, I'm not trying to make fun of you Barry. You are pushing the
> edge of the envelope for flash though. By bumping up to ISO800 you could
> use f/2 and at ISO1600 f/2.8. Now you're in the range of one of the modest
> telephotos, say a 180mm (still expensive) or a 135mm. Remember you _will_
> get some ambient light on a playing field, so you're not trying to photo
> the "black bear in a cave." OTOH you don't have any reflectivity from
> walls or ceilings as you do indoors.
>
> -- John
>
> < This message was delivered via the Olympus Mailing List >
> < For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
> < Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|