Very inexperienced person here when it comes to photography and raw processing here. I'm using a Canon SX 60HS camera. No attachments — filter/hood/etc. — of any sort were attached to lens.
After switching to raw+jpeg mode, I started noticing black round corners for some of the images. I had never experienced this while using camera in automatic mode.
After researching, I found out this is called as vignetting and is a limitation or function of lens and some other parameters at the time of taking the snap.
Why does this not happen when taking the picture in auto mode? Even the jpeg images generated in raw+jpeg mode do not show vignetting. Only raw images show it.
If it matters, I'm using Darktable 2.4.1 on Ubuntu. Since I don't have access to Windows PC, I can not test on Canon software.
I understand vignetting can happen. However camera image preview does not show vignetting. Is there any way to detect vignetting in the camera while I'm at scene so that I can adjust parameters and take the picture again. What sort of information I should check for possible vignetting. I tried to find a common pattern, but so far no luck.
Answer
Many modern digital cameras are designed with the expectation that lens compromises will be... de-compromised... in RAW processing. That appears to be the case here.
The preview image is based on the JPEG rendering, which includes this processing. The RAW file, however, does not.
The amount of apparent vignetting will not be dependent on the scene, but on the lens zoom and aperture at which the picture was taken — that's why you're seeing differences in different situations.
The answer here is to use the lens correction module in darktable to apply this correction to your images as you process them. There's no other way around it — it is simply how the camera was designed.
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