Hi there,
I am pretty new to photography (or rather very new to raw-processing)
though aware of the lensfun library for quiet some time now. Anyway, I
recently bought a Sony Alpha 6300 (also named ILCE-6300) with a Kit-Lens
which is found in lensfun as: Sony, E 16-50mm f/3.5-5.6 OSS PZ. Today
when comparing the JPEG produced by my camera with the lensfun-corrected
RAW result I noticed quite different results and I'm wondering if you're
interested in my observations and if the lens-correction is doing
something wrong.
Vignetting
At 16mm and 17mm the Lens has pretty strong vignetting. You can see
that here: https://jeanbruenn.info/lf_original.jpg and here:
At 18mm the "strong" vignetting disappears.
However, as soon as I do apply lens-correction (I do use darktable
for that) I believe (it's difficult to spot/see on my screen and
it's mostly noticable on thumbnails) that there is some sort of
"light" vignetting as soon as I do apply the lens correction. Take
the following downsized photo for example
(https://jeanbruenn.info/thumb.jpg), all four corners show a white
dust/mist just like in a vignette-effect. You'll get it more visible
if you open that photo and play around with the contrast slider so
that the white from the borders is increased: The same in the
following two Pictures (uncorrected:
https://jeanbruenn.info/lf_original2.jpg, corrected:
https://jeanbruenn.info/light_vignette.jpg)
Cropfactor
This made me check the JPEG of the camera and compare it with the
lens-corrected RAW one. At first I've been pretty much impressed:
JPEG from my Camera: 6000x3376
RAW lensfun corrected: 6048x4024
Which means the camera (is probably aware of the issues with the
lens and) crops a massive amount of 648px in width and 48px in
height. However. This might as well be some sort of misconfiguration
of me (since I told my camera to take photos in 16:9 when I took
those photos - Going to check with the 3:2 setting later / tomorrow).
Further correction
Now it's getting interesting and difficult as well. With the above
in mind, I did the following experiment: I opened the cameras JPEG
in gimp, I applied lens-correction to the RAW variant of this
Picture, exported it with darktable and opened it in GIMP as well -
Now I took that, overlayed it as new layer on the cameras JPEG and I
did set opacity to 50%. It seems the camera crops with different
values from left and right - happily I used a picture which has a
small duck in the middle. So I moved the layer until it exactly
matched the duck.
In theory everything else in the photo should match as well now.
However, the more you slide to the right, left, bottom or top, the
more you can see ghosting while it is fine in the middle. Which
pretty much means (assuming that the camera is correct) that the
lensfun corrected image is not correctly distorting/de-distorting
(whats the term I search for?)/correcting the photo. You can see
that, if you zoom in and move from the middle to the borders:
https://jeanbruenn.info/overlayed.jpg
https://jeanbruenn.info/camera.jpg
https://jeanbruenn.info/lensfun-export.jpg
This is even better to see with portrait-like photos of persons.
So, are my findings correct, that the parameters for the lens in lensfun
are incorrect? Or is it a combination of the Alpha 6300 with that
specific Kit lens? I already tried another lens which does not have the
strong vignetting (though it's 50mm-210mm) and is also not so much
"corrected" by lensfun. So I assume it is not a problem with my cam. Not
sure though. Assuming that my findings are correct, is there anything I
can provide to get a more accurate correction for that specific lens (or
camera/lens-combo)? I do not own any calibration equipment, though :(
By the way, I believe the same lens is used for previous camera models
like the Alpha 6000. Would be interesting to know if those show the same
behavior (with the same lense)
Jean
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