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Re: [OM] Re: T20 flash and red-eye

Subject: Re: [OM] Re: T20 flash and red-eye
From: "Paul Farrar" <farrar@xxxxxxxxxxxx>
Date: Fri, 18 Dec 1998 12:58:23 -0600 (CST)
> 
> Soenke;
> 
> Yes, I think you have a point here.  Just thinkng out loud: to banish
> red-eye the photographer needs to cause the flash-to-eyes vector to be
> non-parallel by some **reasonable** angle to the lens-to-eyes vector. ie
> flash **some** distance away from lens.  So if the subject is farther
> away (and you're using a longer , eg 150mm) lens then the distance from
> the flash to the lens need to be greater. 
> 
> 
> George

Yes, and my understanding is that the angle you need to exceed is 3 degrees.
The flash needs to be 3 degrees off the eye-lens line. The approximate
tangent of 3 degrees is 1/20, so the flash-lens distance should be
greater than 1/20 the eye-lens distance. (I'm the one who posted that.)
That's why people are saying they get redeye with longer lenses. But it's
not the focal length that does it, it's that they are taking from farther
away when they are using the longer lens.

I have not rigorously tested this rule, but it does seem to square with
my observations.

Paul Farrar

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