[ptx] new snapshot

[Peter Suetterlin]

  Hi Pablo,

Pablo d'Angelo wrote:
>   - interface to enter physical focal length. crop factor is calculated
>     from the EXIF data.  Hmm strange, according to my exif data, I do not
>     have square pixels... different ways to calculate the equivalent 35 mm
>     focal length exist, mine is: 35mm diagonal / real diagonal.

As I tried to explain in an earlier Mail, this is quite sure a
consequence of lens errors.  The image scale will be different at
different distances from the center, and thus integrals along the two
axes give different values (i.e., different from b=x/y*a)...

  Pit