Hallöchen!
Owen Mays writes:
> [...]
>
> I'm a bit suspicious of the calibration values I get. When I apply
> the correction to the original test image, the corrected version
> has a "halo" in it, a brighter circle a little ways in from the
> corners.
The correction function can never model the original data perfectly.
Some lenses have a steep light fall-off towards the corner (a kink
in the brightness curve), which can not be really models with a
polynomial. Small deviations should not be visible in real-world
photos though.
> [...] But if I'm going to submit the calibration data for
> inclusion in the LensFun database I feel I should make sure it is
> as good as possible.
It sound fine to me.
Really, don't be perfectionists. Lensfun (and Adobe LR, for that
matter) does never correct perfectly. It's about the art of
photography and not about scientific measurements after all. :-)
> Am I correct to think that a good calibration image should have
> brightness values that are symmetric?
Yes.
> In other words, if I compare my image to a copy that has been
> flipped over the horizontal axis, they should look the same?
Ideally, but see above.
> I checked this using matlab, and the largest difference between
> the original and flipped image (R,G,B) is (18,12,11) on a 0-255
> scale.
You can also check the Gnuplot files. They are generated in the
same directory as lensfun.xml and have the extension .gp. Just say
gnuplot foobar.gp
If the red dots follow a clear stripe, everything is okay. If the
stripe is almost as wide as its falloff from left to right,
something is very wrong.
Note that the y axis does not starts at zero necessarily. Thus
sometimes, the stripe may look bad because it is blown-up too much.
> The sky was completely clear and the sun was going down. I aimed
> at a "plain" blue section, but it's possible there was a gradient.
The diffuser should take care of that.
The problem with photos at daylight is the light from the ground.
Take care that you don't do this on a white patio, for example.
Tschö,
Torsten.
--
Torsten Bronger Jabber ID: torsten.bron...@jabber.rwth-aachen.de
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