>> You can download a DNG sample at http://dl.free.fr/grImV4Y5M
>
> This seems to be mechanical vignetting.
Yes, it was the original view of the Pentax community, and is still often the
case, but I don't agree.
The viewfinder shows 100% of the picture, including the corners, but there is
almost no vignetting in the viewfinder.
Therefore this vignetting is mainly caused by elements that are not between the
subject and the main mirror… that's why my conclusion is that it is pixel
vignetting, caused by the depth of the sensor pixels and the angle of the light
reaching the sensor.
But whatever causes this vignetting, the thing is that the corners are sharp,
that's why I want to be able to compensate the vignetting!
> Lensfun should work for all lenses,
> but this is really unusual.
This is not so unusual: I have the same behaviour using the smc 55-300 fully
open at 300mm on the Pentax K-1.
>> The coefficients above work well on this picture (using darktable)
>> but not on the TIFF. [...]
>
> Is the colour space linear RGB? You might try
> "dcraw -g 1 1 -T -W <DNG>".
With the version of dcraw I am using, this command line fails to create a valid
TIFF file, because the tags are not sorted in ascending order, but ImageMagick
can manipulate it therefore it might not be a problem.
And the correction with
convert IMGP4694.tiff -fx "Xi=i-w/2; Yj=j-h/2;
r=hypot(Xi,Yj)/hypot(w/2,h/2); s/(1-0.156*r^4-0.711*r^6)"
IMGP4694-corrected.tiff
does not give the same result as having
<vignetting model="pa" focal="250" aperture="4" distance="1000"
k1="0.0" k2="-0.156" k3="-0.711" />
used by darktable
> Out of curiosity: What is the maximal value for sensor displacement?
It depends on the body.
For the K-1, the maximum lateral displacement is a value of +24, which I
measured to be 276 pixels, which is 3.75% of the image width.
Regards,
Louis
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