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[OM] Re: Flash coverage angles - Oly found to be optimistic

Subject: [OM] Re: Flash coverage angles - Oly found to be optimistic
From: Mark Marr-Lyon <mark.marrlyon@xxxxxxxxx>
Date: Mon, 10 Jan 2005 20:16:29 -0700
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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