Re: [APML] a short note on film flat fielding

[Wei-Hao Wang <wang@ifa.hawaii.edu>]

On Wed, 25 Feb 2004, Jerry Lodriguss wrote:
> 
> Hi Wei-Hao,
> 
> Didn't you say you were using daylight shots to model the flat 
> fielding?  This isn't the same level of illumination as when the film would 
> experience low-intensity reciprocity failure. With a short daylight 
> exposure, you might not experience any LIRF at all anywhere in the frame. 
> With an optimum exposure for a low intensity scene, the RF would change 
> directly with the vignetting, wouldn't it?
> 
> Different films also have different HD curves. Wouldn't this be a reason to 
> use the same film?
> 

Jerry,

You are perfectly right that there is no RF at all in the daylight shot 
of flat fields.  The RF only exists in the real images taken at night.
The RF and the nonlinear HD curve, alter the shape of the illumination 
pattern.  That is all why I cannot use the daylight shot directly as 
flat fields, unlike CCD flat fielding.  I have to use selected background 
regions from the real night-sky shot to modify the shape of the daylight
shot.  So the modified flat field has the same values as the HD+RF altered
illumination pattern.  This modification absorbs all the differences in 
HD curves and RF of different films at once.

Hope this answers the question.

Cheers,

Wei-Hao

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