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Forum: Pentax Full Frame 03-27-2019, 10:01 AM  
Is this night photography coloured dot distortion from my sensor?
Posted By stevebrot
Replies: 16
Views: 2,012
Yes, it applies to both RAW and JPEG.



Well, it is must have been the Samyang!. :D



Congratulations! I hope it goes well.


Steve
Forum: Pentax Full Frame 03-26-2019, 08:16 AM  
Is this night photography coloured dot distortion from my sensor?
Posted By stevebrot
Replies: 16
Views: 2,012
If you do a lot of long exposures and the wait is tiresome, another option is to make a single "dark" frame for use later NR using manual dark frame subtraction as part of PP. That approach is a real time saver when doing long exposures in the field. Google "dark frame subtraction" for more information.


Steve
Forum: Pentax Full Frame 03-22-2019, 03:23 PM  
Is this night photography coloured dot distortion from my sensor?
Posted By stevebrot
Replies: 16
Views: 2,012
...Continued from above. Playing with things a bit, I did the following in LR:
  • Assigned a fixed color temperature of 3800K and adjusted the tint to +34. Auto WB is not recommended for astro.

  • Used the CA removal tool with the result that most of the bright color disappeared

  • Applied a little more than the default color NR (--> 34) to clear up the black

  • Reduced NR smoothness to 33

  • Added some Luminance NR (18)

I put the results (full resolution TIFF and JPEG) in my DropBox

stevebrot | Southern_Sky

Enjoy!


Steve
Forum: Pentax Full Frame 03-22-2019, 01:54 PM  
Is this night photography coloured dot distortion from my sensor?
Posted By stevebrot
Replies: 16
Views: 2,012
...or perhaps not.
That is a lot of hot pixels for a 30s exposure, even at ISO 1600.

I took another look at this file in something other than PDCU's image browser* and also the extracted JPEG and noticed (I feel real dumb) that the colored dots (far fewer in Lightroom) are streaks and are part of the capture, not sensor artifact. I also noticed that the color is much less pronounced on the extracted JPEG. Hmmmmm.

May I ask what lens was being used? I know the original post says Pentax 24-70 f/2.8, but the EXIF indicates a non-AF lens having mount contacts (KA mount) and a manually entered focal length of 15mm.


Steve

* PDCU's image browser is sort of strange in that much depends on whether the program is set to display the full resolution JPEG or a preview processed from the DNG/PEF. If the latter, the default is rendered at 1/4 resolution with whatever artifact that might carry with it. Yes, I am configured that way and the artifact took the form of lots of colorful dots. :o
Forum: Pentax Full Frame 03-22-2019, 11:17 AM  
Is this night photography coloured dot distortion from my sensor?
Posted By stevebrot
Replies: 16
Views: 2,012
I don't know what "dot distortion" is, but I do see lots of red and blue dots along with the stars. As noted above, these are likely hot pixels. The conventional way to avoid them is to enable in-camera background subtraction...
menu --> Record Menu 2 --> Noise Reduction --> Slow Shutter Speed NR
Default is "Auto", but EXIF says it was turned off for this shot.

Another option is to make a separate dark frame exposure that might be used with appropriate software to do the same thing. This might be easier than waiting 30s or more for the in-camera dark-frame subtraction if doing multiple exposures with the same exposure time.


Steve
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