I use Photolab as my main RAW converter and I’m happy with it.
But recently I’ve started with astrophotography and noticed that Photolab isn’t working properly. The raw conversion is causing weird concentric circles in the astro photo’s. Post processing astro photo’s is difficult because of the lack of light, the peak of the histogram is mostly around 1/4.
Knowing the difficulties in post processing astro photo’s I’ve searched the internet for possible reasons. I found a post on Cloudy Nights that using lossy compression with RAW for Nikon can cause this effect: Nikon Coloured Concentric Rings - DSLR, Mirrorless & General-Purpose Digital Camera DSO Imaging - Cloudy Nights . But I’m using a Nikon D7200 with 14 bit losless compression, so this is not the reason for these weird concentric circles in my images. And using other RAW converters (Nikon Capture NX-D, Rawtherapee and Affinty Photo) showed that this effect is almost absent.
But I think I’ve found the reason for these circles, it’s the vignette correction in Photolab. Without the auto vignette correction these circles don’t appear. To show the effect I’ve made a photo with the histogram peak around 1/4 from the left.
Converted the RAW file with Photolab using the preset ‘no correction’ and then only applied the camera and lens corrections (and vignette correction on) with the prime noise reduction. Saved this as a 16 bit TIF file and the processed this TIF file in Affinity Photo using the same techniques for astro, i.e. stretching the histogram with levels and/or curves. Doing this will show the artifacts introduced by the vignette correction of Photolab:
Not using the vignette correction of Photolab, the concentric circles don’t show up. And converting the RAW file with Nikon Capture NX-D, Affinity Photo or Rawtherapee with vignette correction on, there are no concentric circels. As a new user I’m not allowed to upload these photo’s.
Now I’ve found this topic and see that this problem is already reported in december 2019 for Photolab 3. Unfortunatly this problem hasn’t been solved and still exists in Photolab 4.
Any feedback from DXO would be appreciated.
Many thanks.