Feedback on vignetting model
Nov 10, 2010 4:56 PM
After going through the exercise of creating lens profiles to corect the vignetting inherent in wide-angle pinhole photos, I thought I'd offer some feedback on the data model. Not a complaint, just an observation.
The model requires the vignette to be represented as the coefficients for 2nd, 4th, and 6th degree terms in a polynomial. This representation offers a very close approximation of real falloff for moderate ƒ/Dmax ratios, but cannot represent the *actual* falloff and the approximation gets increasingly poor as the ratio shrinks.
Here's a graph of the actual falloff as expressed by the function y = 1/(((d/ƒ)^2)+1) graphed against the representations created by the Lens Profile Creator, which offers a handy tool for fitting the curve (click it to see it bigger):
The first curve is a hypothetical 6x6cm frame with a "normal" 80mm focal length, as if one converted an old 6x6 film camera to pinhole. The approximation here is fantastic.
The second is my 6x12cm pinhole camera with a 40mm focal length. This is fairly common; both my $480 Zero 612F and the $50 Holga 120WPC use this combination. The fit here is pretty good, but it's a good thing the frame isn't any wider.
The last is a relatively common large-format pinhole ratio, with a 25mm focal length on 4x5" film. The Zero 25B uses this combination.. As you can see, the model is quite poor at representing this curve.
Since the actual falloff due to distance can be expressed by the simple formula above, it's too bad it can't be represented precisely. Maybe if someday there's a v2 of this specification...
