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03-03-2019, 05:02 PM   #1
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Is this night photography coloured dot distortion from my sensor?

Hi all,

I had my first go at night photography at Halls Gap, Victoria, Australia on the weekend. Pentax K-1 Mk1 with Pentax 24-70 f/2.8 as released for K-1. I had the camera & lens callibrated in Melbourne tech lab of distributor; it made a big difference.

I'm not sure what the small blue and red dots are on my RAW file. Has anyone experienced similar? How did you deal with it? The file is in my Google Drive due to upload limitations here:

K1SD7509.DNG - Google Drive

Regards from Australia.

03-03-2019, 05:23 PM   #2
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The only dots I'm seeing are stars, and I'm pretty sure they're supposed to be there.

Do you have bad pixels on your monitor? What happens if you move your photo on your screen?
03-03-2019, 05:30 PM   #3
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I think that the background color is excellent. On my images, I can get the background to a somewhat dark grey. I've been told by a professional astronomer that the background color (Milky Way landscapes) that I have been getting is as good as you can get.

In terms of the small blue and red dots, they are hot pixels. You can remove them within the camera - check page 112 in the K1 manual. It's the Wrench 4 menu page.You can also address them in post-processing with varying degrees of success.
___________________________

I had to download the image and then bring it up in a viewer in order to zoom in to see the red/blue pixels.


Last edited by interested_observer; 03-03-2019 at 05:51 PM.
03-04-2019, 01:12 AM   #4
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I do not see red and blue pixels, but I see white dots (plenty of them at 100 percent magnification). So my advice is to use long exposure noise reduction, especially at higher ISOs (more than 800).

03-22-2019, 08:29 AM   #5
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QuoteOriginally posted by stevebarnes Quote
Hi all,

I had my first go at night photography at Halls Gap, Victoria, Australia on the weekend. Pentax K-1 Mk1 with Pentax 24-70 f/2.8 as released for K-1. I had the camera & lens callibrated in Melbourne tech lab of distributor; it made a big difference.

I'm not sure what the small blue and red dots are on my RAW file. Has anyone experienced similar? How did you deal with it? The file is in my Google Drive due to upload limitations here:

K1SD7509.DNG - Google Drive

Regards from Australia.
If you are getting red and blue dots, they are probably hot pixels. Definitely do a pixel mapping, and turn on long shutter speed noise reduction. That should take care of most of it.
03-22-2019, 09:39 AM   #6
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Blue and red dots?? Can't see any on my computer
03-22-2019, 10:53 AM   #7
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QuoteOriginally posted by interested_observer Quote
I think that the background color is excellent. On my images, I can get the background to a somewhat dark grey. I've been told by a professional astronomer that the background color (Milky Way landscapes) that I have been getting is as good as you can get.

In terms of the small blue and red dots, they are hot pixels. You can remove them within the camera - check page 112 in the K1 manual. It's the Wrench 4 menu page.You can also address them in post-processing with varying degrees of success.
___________________________

I had to download the image and then bring it up in a viewer in order to zoom in to see the red/blue pixels.

I get a couple on my K-3 and used to get a tonne on my K-7...post takes care of them
03-22-2019, 11:17 AM   #8
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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


Last edited by stevebrot; 03-22-2019 at 11:47 AM.
03-22-2019, 01:54 PM   #9
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QuoteOriginally posted by stevebrot Quote
As noted above, these are likely hot pixels.
...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.

Last edited by stevebrot; 03-26-2019 at 08:10 AM. Reason: completeness
03-22-2019, 03:23 PM   #10
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...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

Last edited by stevebrot; 03-22-2019 at 03:30 PM.
03-26-2019, 04:50 AM   #11
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QuoteOriginally posted by stevebrot Quote
...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
Hey Steve, That is really good info...I think I will try that on a couple of my night shots. Thanks for publishing this!

On a side-note, I am a little sick of waiting for the in-camera NR to do it's thing; especially when it is minus stupid outside and I am shivering in the middle of a field. I never used NR with my K-7 and Dxo cleans up those images quite nicely of all the red-blue pixels. I think it is time to try disabling NR on my K-3 and see what happens.
03-26-2019, 08:16 AM - 1 Like   #12
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QuoteOriginally posted by CharLac Quote
I think it is time to try disabling NR on my K-3 and see what happens.
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
03-26-2019, 02:08 PM   #13
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Looks pretty good to me at 1:1 on a 27" monitor in Lightroom. There's trailing, but I don't see much in the way of hot pixels. The K-1 sensor is pretty good on that front, you have to be doing pretty long exposures or a lot of them in warmer temps.

As others have said, if you don't want to use the LENR on the camera, stack primary frames with dark frames. I use a program called Sequator to do that.
03-27-2019, 01:50 AM   #14
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Wow guys! What a lot of input. Thank you. Here's a bit of clarity:

A. I wasn't using long exposure noise reduction. Does it affect RAW files? If so, I must reconsider.

B. I've heard of using a dark frame subtraction. I'll have to look up how to do this on my K-1. Any references appreciated.

C. I used two lenses in this shoot. I believe I started with my Samyang flat 14mm f/2.8. It's this lens that the camera always thinks is 15mm. I should post another topic about this to see what people have to say. (advice, opinions?)

D. The other lens is my (made for K-1) Pentax D-FA 24-70 f/2.8. I must confess I thought the image from the prime lens was much superior. I use the 24-70 a lot for dog photography (fourfootfotos.com) and for portraits. It is wonderful on auto-focus compared to my other lenses, and produces a consistently nice image.

E. I'm a Ubuntu Linux devotee (please don't run away). After an injury that left me unable to work and took my house, I learned to week with open-source software. I use Darktable and GIMP. I also have RAW Therapee, but it's very technical, and I'm still learning it when I have time (yes, and a million other things too).
Whilst Darktable is quite amazing, offering drawn masks, parametric masks and other goodies Lightroom doesn't, I still haven't got Chromatic Aberration working properly! I need to get some expert advice I think.

I must download the images stevebrot produced! Thank you so much Steve.

Lastly, as a small VICTIMS fur my K-1 photography, I've been asked today to be a featured photographer at the Ballast International Foto Biennalli here in Australia. I'll be doing two (weekend) days during the 6 week festival of dog photography! It's a paid gig (only a few hundred each time at my own determination), but the amazing thing is the exposure. We have international exhibitors here, and people come from overseas and all round Australia to this amazing festival. I'll have my 4.5x3m black gazebo set up as a studio and my Pentax equipment will be proudly on show. I'll be advertised, and the organisers will take bookings for 15 minute time spots. My dog photography site is https://fourfootfotos.com. The Biennalli site is BALLARAT INTERNATIONAL FOTO BIENNALE |.
03-27-2019, 10:01 AM   #15
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QuoteOriginally posted by stevebarnes Quote
A. I wasn't using long exposure noise reduction. Does it affect RAW files? If so, I must reconsider.
Yes, it applies to both RAW and JPEG.

QuoteOriginally posted by stevebarnes Quote
C. I used two lenses in this shoot. I believe I started with my Samyang flat 14mm f/2.8.
Well, it is must have been the Samyang!.

QuoteOriginally posted by stevebarnes Quote
Lastly, as a small VICTIMS fur my K-1 photography, I've been asked today to be a featured photographer at the Ballast International Foto Biennalli here in Australia.
Congratulations! I hope it goes well.


Steve
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