Hi Chuck,
Nice work. I did have one question/observation. From your first post
it sounds like you focused on a wall 10 feet away, then measured the
flash output along the wall, as opposed to keeping the flash meter a
constant 10 feet away from the flash. If you did the first, then a
lot of the variance in flash output can be explained by that. From 10
feet away, the T32 looks a lot like a point source, so the irradiance
falls off as 1/(distance)^2. So, if I got my math all right, at the
left center edge of a 24mm lens's field of view, the flat wall is 12.5
feet away, rather than 10 feet away, and the light irradiance is only
64% of what it is at 10 feet. This, I think, is a reduction of almost
2/3 stop, leaving the actual light falloff as -1/3 stop.
If you measured the output at a constant distance from the flash, then
you can disregard everything above, and Oly is very optimistic indeed
:)
I don't know for sure how Oly (and other flash makers) measure the
angle of coverage, but I would guess it's using a constant distance,
since it's the most favorable, even though the flat-field method would
probably be closer to how people take pictures.
Mark
On Mon, 10 Jan 2005 16:16:53 -0500, Chuck Norcutt
<chucknorcutt@xxxxxxxxxxx> wrote:
> Fall-off from the center as 0 point: T-32 @ 24mm
>
> Top center -2/3 stop
> Left center -1 stop
> Top left corner -1-1/3 stops
>
[snip]
>
> Comments anyone?
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