Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-24-2017, 07:19 PM   #16
Pentaxian
photoptimist's Avatar

Join Date: Jul 2016
Photos: Albums
Posts: 2,578
QuoteOriginally posted by wombat2go Quote
Yes,
The Nyquist for monochrome is then about 46 arcsec and for Bayer cell is about 1.6 times that = 73 arc sec.

I used the diagonal out to the corner to get the 40mm AOV so my numbers were more conservative, using 80 arcsec for the Bayer.
Then FWHM of 3 pairs = 240 arc secs for smallest extended sources.
The Nyquist numbers don't actually matter and certainly not the Bayer ones because were not talking about trying to resolve two closely-spaced stars on a Bayer filter camera.

We're talking about how light from a single isolated star (point source) on a dark background might pass through a high-quality lens to produce a circle of light on the sensor that is so small it has some chance of falling on only a single pixel (either a red, green, or a blue one). In turn, the RAW-cooking hot-pixel destroyer sees a portion of the image that has only one of the four Bayer pixels lit and everything else dark and it resets the bright pixel with a dark value -- the star is gone.


Note: this problem is exacerbated by a design flaw in the A7Riii. It has no optical low-pass filter which would have prevented this by slightly blurring the point of light of the star into a multi-pixel blob that would survive the RAW-cooker.

11-24-2017, 07:47 PM   #17
Pentaxian
Digitalis's Avatar

Join Date: Mar 2009
Location: Adelaide.
Posts: 9,348
QuoteOriginally posted by photoptimist Quote
this problem is exacerbated by a design flaw in the A7Riii. It has no optical low-pass filter which would have prevented this by slightly blurring the point of light of the star into a multi-pixel blob that would survive the RAW-cooker.
Purposefully de-focusing the lens could simulate an AA filter and effectively circumvent the star eating. Though if the lens has an inconsistent PSF* recovering sharpness lost from de-focusing could be problematic.

* most lenses do unfortunately.
11-24-2017, 08:36 PM   #18
Site Supporter
Site Supporter




Join Date: Jan 2010
Location: Michigan
Photos: Gallery
Posts: 2,206
QuoteOriginally posted by photoptimist Quote
The Nyquist numbers don't actually matter and certainly not the Bayer ones because were not talking about trying to resolve two closely-spaced stars on a Bayer filter camera.

We're talking about how light from a single isolated star (point source) on a dark background might pass through a high-quality lens to produce a circle of light on the sensor that is so small it has some chance of falling on only a single pixel (either a red, green, or a blue one). In turn, the RAW-cooking hot-pixel destroyer sees a portion of the image that has only one of the four Bayer pixels lit and everything else dark and it resets the bright pixel with a dark value -- the star is gone.


Note: this problem is exacerbated by a design flaw in the A7Riii. It has no optical low-pass filter which would have prevented this by slightly blurring the point of light of the star into a multi-pixel blob that would survive the RAW-cooker.
Hi P/O
I appreciate your comments and look forward learning about it and going out and taking some astro photos with my humble gear here.
My counterpoint
Nyquist & Shannon might disagree.
It is like saying you are going to hear your friend playing the drums in another state by phoning him every 2 seconds and hanging up after every second.

If it were possible that one pixel in a Bayer cell would be illuminated by a point source through a sharp 40mm lens,, with dark levels on all other pixels:
Firstly the level in that pixel will be attenuated by the Bayer filter, while other colors remain dark except for cross-talk.
Secondly the de mosaic will load the surrounding Bayer cells by ( let us say) linear interpolation.
Thirdly, the illuminated pixel will show little true color except its primary.
So we get a "smearing" which reduces the resolution to a level between the monochrome Nyquist and the Nyquist of the Bayer.
( My ref: Holst & Lomheim, who give detailed explanations about the Nyquist frequencies of the various arrays)

Today I looked at some wide angle astro images on inet . Most are blurred by being very low resolution jpgs.
Some (including Nasa) showed over exposed bright stars with aperture artifacts etc. I think they do this to show the point source stars.
Regards
11-25-2017, 07:45 AM   #19
Pentaxian
photoptimist's Avatar

Join Date: Jul 2016
Photos: Albums
Posts: 2,578
QuoteOriginally posted by Digitalis Quote
Purposefully de-focusing the lens could simulate an AA filter and effectively circumvent the star eating. Though if the lens has an inconsistent PSF* recovering sharpness lost from de-focusing could be problematic.

* most lenses do unfortunately.
Quite true and, in fact, the Kepler space telescope is intensionally set out-of-focus for this (and related) reasons.

11-25-2017, 11:21 AM - 1 Like   #20
Pentaxian
photoptimist's Avatar

Join Date: Jul 2016
Photos: Albums
Posts: 2,578
QuoteOriginally posted by wombat2go Quote
Hi P/O
I appreciate your comments and look forward learning about it and going out and taking some astro photos with my humble gear here.
My counterpoint
Nyquist & Shannon might disagree.
It is like saying you are going to hear your friend playing the drums in another state by phoning him every 2 seconds and hanging up after every second.

If it were possible that one pixel in a Bayer cell would be illuminated by a point source through a sharp 40mm lens,, with dark levels on all other pixels:
Firstly the level in that pixel will be attenuated by the Bayer filter, while other colors remain dark except for cross-talk.
Secondly the de mosaic will load the surrounding Bayer cells by ( let us say) linear interpolation.
Thirdly, the illuminated pixel will show little true color except its primary.
So we get a "smearing" which reduces the resolution to a level between the monochrome Nyquist and the Nyquist of the Bayer.
( My ref: Holst & Lomheim, who give detailed explanations about the Nyquist frequencies of the various arrays)

Today I looked at some wide angle astro images on inet . Most are blurred by being very low resolution jpgs.
Some (including Nasa) showed over exposed bright stars with aperture artifacts etc. I think they do this to show the point source stars.
Regards
Everything you have said is 100% correct but it applies to the issue of resolution.

The issue with star-eating is a matter of response, not resolution. How does the sensor + RAW-cooker respond to point sources of light?

As you correctly note, the star's pinpoint of light would be attenuated by the particular color filter (as would the light from any point, line, or diffuse source) with whatever crosstalk occurs in adjacent pixels.

But demosaicing doesn't enter into situation because this is strictly in the processing of the RAW sensor output to create the RAW image data file. It's at this point that the A7Riii seems to run the star-eating noise reduction algorithm that resets the levels of pixels that appear to be hot.

Kasson's originally referenced paper (The Sony a7RIII eats stars) proves this is true by finding a statistical anomaly in the power spectrum of the camera's dark-field images (lens-cap on). Dark-field images taken at short shutter times times show almost no roll-off in the power spectrum which is to be expected given that the image is just random noise. Dark-field images taken at long shutter time times (≥ 4 sec) show an unexpected roll-off which should not be there. The roll-off implies there's spatial filtering occurring that should not be because this is all in RAW image.

Now ordinary linear filtering to smooth an image would not eat stars - it would only blur them. But a non-linear filter (e.g., one that clips the value of each pixel to the maximum value found in the eight neighboring pixels) has the effect of eating hot pixels and stars (and creating the anomalous roll-off in the power spectrum of dark-field images).
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
astro, camera, dr, fps, heat, implications, lens, lenses, level, noise, photo industry, photography, pixel, reduction, sensor, sony, stars
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Sony a6000 or Sony a6300 or Sony a6500? LeDave Non-Pentax Cameras: Canon, Nikon, etc. 5 09-27-2018 03:45 AM
People If your pictures aren't good enough, you're not close enough, Robert Capa Kerrowdown Post Your Photos! 35 07-08-2018 12:46 AM
Pixel shift appears in the new Sony A7riii travelswsage Non-Pentax Cameras: Canon, Nikon, etc. 15 10-28-2017 07:18 AM
645Z owners: Will you switch to a Sony 75MP A7RIII? SeattleDucks Pentax Medium Format 79 04-09-2016 06:20 AM
FA Limited lens series won the 2010 Good Design Long Life Design Award Patriot Pentax SLR Lens Discussion 26 11-29-2010 06:16 AM



All times are GMT -7. The time now is 03:15 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top