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07-20-2009, 11:30 AM   #16
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I checked the read-out noise distribution during my K-7 alpha test as well.

[IMGWIDELEFT]http://www.falklumo.com/downloads/pub/2009.06/IMGP0657-73.jpg[/IMGWIDELEFT]
This is an image at the equivalent of ISO25600 (ISO1600 and LV pushed +4). To single out the read-out noise (from the amplifiers), I superimposed 16 images which minimizes the shot noise which otherwise completely hides read-out noise!

Note that there is a subtle magenta glow across the image. This is the read-out noise main component. It is free of banding or hot areas. And it is much smoother than the same for a K20D which has the magenta glow concentrated towards the outer areas (which provides proof that we are seeing read-out noise indeed rather than some odd color cast ...).

I would say that magenta glow as in the images by the TO is a camera defect. The glow is ok, but not so isolated at a few spots.


Last edited by falconeye; 07-20-2009 at 11:38 AM.
07-20-2009, 06:53 PM   #17
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just a guess

Your camera probably came with a little dealy that fits over the eyepiece to block out light coming through the viewfinder during long exposures. Light entering the camera through the eyepiece can give errant lights as well as trick your meter.
It also stands to reason that different photos and different people experience different reults. the first images in the thread look as if intense light was spilling around the mirror in specific places, where as the one above this post looks very diffuse. Every camera is built different, set different, and equipped different.

Try doing the same shots with and without your eyepiece obscured and see if it has any effect.
07-20-2009, 10:20 PM   #18
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I would not say this is a defect. I got this with all my digital cameras more or less.

But to provocate this you have to exposure really long.
With my Nikon CP8800 ist started at about 2 minutes and with my K10D I'm beginning to see this at abaut 5-8min I would guess.
But in my cases I never went above 100iso for this kind of images so the effect is rather small and just in the upper left part of the image.
07-21-2009, 02:58 AM   #19
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QuoteOriginally posted by petercrane Quote
It's disapointing to hear that there are problems with your Pentax. Aside from the purple streaks the photos look great. Good job.
All digital cameras have this problem it not just a Pentax thing.

QuoteOriginally posted by Darky Quote
That noise is called amplifier-noise...
It comes from the amplifier sitting next to the sensor. I got that quite strong on my Nikon CP8800 but on the K10d it is much less of a problem.
I just saw this yesterday after a 1300 seconds exposure at iso 100.

If you increase iso the purple noise will also increase.
It is defiantly amp glow, do a dark frame subtraction to cure it.

07-21-2009, 09:46 AM   #20
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QuoteOriginally posted by gremlindesign Quote
Your camera probably came with a little dealy that fits over the eyepiece to block out light coming through the viewfinder during long exposures. Light entering the camera through the eyepiece can give errant lights as well as trick your meter.
It also stands to reason that different photos and different people experience different reults. the first images in the thread look as if intense light was spilling around the mirror in specific places, where as the one above this post looks very diffuse. Every camera is built different, set different, and equipped different.

Try doing the same shots with and without your eyepiece obscured and see if it has any effect.
If I was anywhere but where I was I would agree. But I was in the middle of a national forest on a moonless night with the camera viewfinder firmly planted against a wooden bridge (for the star pics).

I am beginning to understand the issue however, as amplifier noise. It seems that long exposure photography is a small niche that doesn't get the engineering resources from the manufacturers. Looks like I will have to learn how to remove it in post. That and lower ISO.

Thanks for all the input.
08-16-2009, 06:40 PM   #21
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QuoteOriginally posted by falconeye Quote
I checked the read-out noise distribution during my K-7 alpha test as well.

[IMGWIDELEFT]http://www.falklumo.com/downloads/pub/2009.06/IMGP0657-73.jpg[/IMGWIDELEFT]
This is an image at the equivalent of ISO25600 (ISO1600 and LV pushed +4). To single out the read-out noise (from the amplifiers), I superimposed 16 images which minimizes the shot noise which otherwise completely hides read-out noise!
Shot noise, I believe, would rarely ever overwhelm read noise.
Re: Throw me in the high-ISO user group: Open Talk Forum: Digital Photography Review
The major difference is that read noise sits like a blanket or a cloud in the image; obscuring the hills and valleys, making them them all flat through a lack of contrast. Photon shot noise is more like a marbled texture in the subject, and when the subject approaches black, it actually is close to black, so you can see lots of contrast with pure shot noise that you can not see with a cloud of read noise hovering in the image.

Read noises generated by cameras after the photonic capture also have macro and micro patterns in them; one-dimenional components, etc. These components are much more visible than the truly random component of read noise, or photon shot noise (which is totally and purely random).

08-17-2009, 03:58 AM   #22
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QuoteOriginally posted by jeffkrol Quote
Shot noise, I believe, would rarely ever overwhelm read noise.
[...]
The major difference is that read noise sits like a blanket or a cloud in the image; obscuring the hills and valleys, making them them all flat through a lack of contrast.
[...]
Read noises generated by cameras after the photonic capture also have macro and micro patterns in them; one-dimenional components, etc.
Jeff, thanks for the quote.

First, wording ...
I meant the term "read-out noise" exactly in the way as your citation above did. However, one should note that it is more about dark currents and artifacts than noise, actually. I called it read-out noise though because this is a more commonly used term.

Second, shot vs. read noise ...
You didn't see the images before the 16x super imposition ... Totally useless. Actually, shot noise can be computed:

1 W (watt) = 93 lm (lumen = 1 cdsr = 1 lxm^2) (using the luminous efficiency of sunlight of 14% only);
1 green photon = 3.6E-19 J (joule).
18% graycard at 1000 lx (lux = 1 lm/m^2) = 9 EV (light value)
[source: f8 1/30 ISO400KODAK: Estimating Luminance and Illuminance: Pub A-105KIC]
Luminous flux onto sensor = 18% * Luminous flux onto greycard / (2 * pi * f-stop^2)
[source: own]

Example for EV 2 at 1s exposure at ISO 1600 (=> f/8):
- 18% graycard at 7.8 lx
- sensor at 0.0035 lm or 0.038 mW (milli watt)
- 1.0E14 photons hitting sensor within 1s.
- Imperfect bayer filter (15%), quantum efficiency (25%), 14.6 Mio. Pixels =>
- 260000 photons detected by a sensor pixel
- Shot noise: 500 (Poisson law) (affecting 9.0th bit) for mid gray (rgb 117)

Now for RGB value 10 (1/224 of 18% gray only; sRGB gamma correction!!):
- 1160 photons detected by a sensor pixel
- Shot noise: 34 (Poisson law) (affecting 5.0th bit) for dark gray (rgb 10)

And if you push 14 Bit RAW darks (RGB value 1, 1/30000 of 18% gray only):
- 9 photons detected by a sensor pixel
- Shot noise: 3 (Poisson law) (affecting 1.5th bit) for black (rgb 1)

And you loose 1/2 additional bit per one stop decrease of EV in scene or increase in ISO. So, at ISO25600, you have lost 2 more bits, leaving 3 bits for dark gray.

Just because there are a finite number of photons around only ...


DISCLAIMER:
I do not claim that my above calculation is correct. It is of the back-of-an-envelope kind


P.S.
You can invert all that fuzz to compute quantum efficiencies for the 3 colors from noise measurements. That would be useful numbers to classify sensors, as opposed to the DxO wizardry. I think, Gordon B Good tries this with some success and computes the full well capacities (a closely related measure) from the noise. Also, while noise measurements at high rgb values give you the quantum efficiency or full well capacity, any excess noise seen in low rgb values can be used to compute the read-out noise from.

Last edited by falconeye; 08-17-2009 at 04:17 AM.
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