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Interesting fact about pixel mapping

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David Ellsworth

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I posted a message a while back about how I detected exactly how many dead pixels my E-20N has by analyzing RAW files. But I've discovered a very interesting fact: the number of pixels it maps around depends on your exposure time! On my camera, the shortest exposure (1/640 sec) has only 17 dead pixels mapped, while anything longer than a certain length (I haven't probed yet, but I'm guessing 1-5 sec) has 672 dead pixels mapped out. (I also haven't probed in-between yet, but I'll bet it has some extra points between 17 and 672.)

I think it's very nice that the pixel mapping works this way. It's as it should be! Semi-long exposures come out looking very good even without noise reduction enabled.
 
I said:
(I also haven't probed in-between yet, but I'll bet it has some extra
points between 17 and 672.)
Click to expand...
I've done some extra probing, and it turns out there are just the two states. 1 sec and below = 17 pixels, 1.3 sec and above = 672 pixels.
 
David Ellsworth said:
I posted a message a while back about how I detected exactly how
many dead pixels my E-20N has by analyzing RAW files. But I've
discovered a very interesting fact: the number of pixels it maps
around depends on your exposure time! On my camera, the shortest
exposure (1/640 sec) has only 17 dead pixels mapped, while anything
longer than a certain length (I haven't probed yet, but I'm
guessing 1-5 sec) has 672 dead pixels mapped out. (I also haven't
probed in-between yet, but I'll bet it has some extra points
between 17 and 672.)

I think it's very nice that the pixel mapping works this way. It's
as it should be! Semi-long exposures come out looking very good
even without noise reduction enabled.
Click to expand...
Not to be a wiseguy, but those'd be an additional 655 HOT pixels, not DEAD pixels or there'd always be the same number mapped out. Er, wouldn't there? Still pretty cool. I'm just going to have to break down and part with mine for a few weeks someday and get it put right. :-)
 
Gerald M. Payne said:
Not to be a wiseguy, but those'd be an additional 655 HOT pixels,
not DEAD pixels or there'd always be the same number mapped out.
Er, wouldn't there? Still pretty cool. I'm just going to have to
break down and part with mine for a few weeks someday and get it
put right. :-)
Click to expand...
Er, good point. It's just that my mindset was on dead pixels: I thought originally that it just had 672 dead pixels mapped. But indeed, most of the 655 can be seen as hot pixels in a 1 second dark frame (and right now I'm taking a bunch of 1 second dark frames to average them out and reveal as many hot pixels as I can).

I suppose most of the 17 are probably just hot, too, but VERY hot. ;)
 
David Ellsworth said:
Er, good point. It's just that my mindset was on dead pixels: I
thought originally that it just had 672 dead pixels mapped. But
indeed, most of the 655 can be seen as hot pixels in a 1 second
dark frame (and right now I'm taking a bunch of 1 second dark
frames to average them out and reveal as many hot pixels as I can).

I suppose most of the 17 are probably just hot, too, but VERY hot. ;)
Click to expand...
LOL, very hot.

Well, I said I didn't want to be a wiseguy. I just wanted to point out that they were NOT dead pixels as people tend to freak out about dead pixels, but most have learned to deal with or accept hot pixels as normal. I think most people would be a LOT more upset about 672 DEAD pixels than the same number of hot pixels in longer timed exposures.

My guess is the 1 second figure is an arbitrary decision based on available storage requirements within the camera and the idea that anything longer than a certain period is a long exposure so they simply compensate for the worse case situation for any time exposure. Perhaps not ideal, but likely to cause little enough difficulty for applications that don't try to push the ability of the camera to capture detail. My guess is the improvement gained by multiple in-camera dark frame compensation maps would be minimal enough for most apps.
 
Guys I'm lost in your tech talk.
So what is the conclusion is there any dead pixels or not?
How can you find them if the pixelmaping cloned surrounding pixels?

That's raising another question is pixelmapping necessary for everytime you exchange batteries?
Gerald M. Payne said:
David Ellsworth said:
Er, good point. It's just that my mindset was on dead pixels: I
thought originally that it just had 672 dead pixels mapped. But
indeed, most of the 655 can be seen as hot pixels in a 1 second
dark frame (and right now I'm taking a bunch of 1 second dark
frames to average them out and reveal as many hot pixels as I can).

I suppose most of the 17 are probably just hot, too, but VERY hot. ;)
Click to expand...
LOL, very hot.

Well, I said I didn't want to be a wiseguy. I just wanted to point
out that they were NOT dead pixels as people tend to freak out
about dead pixels, but most have learned to deal with or accept hot
pixels as normal. I think most people would be a LOT more upset
about 672 DEAD pixels than the same number of hot pixels in longer
timed exposures.

My guess is the 1 second figure is an arbitrary decision based on
available storage requirements within the camera and the idea that
anything longer than a certain period is a long exposure so they
simply compensate for the worse case situation for any time
exposure. Perhaps not ideal, but likely to cause little enough
difficulty for applications that don't try to push the ability of
the camera to capture detail. My guess is the improvement gained
by multiple in-camera dark frame compensation maps would be minimal
enough for most apps.
Click to expand...
--
E-10 for now.........Gladly!!!
Take a Prayer!
http://www.pbase.com/image/5706283
 
Sunny Krpan said:
Guys I'm lost in your tech talk.
So what is the conclusion is there any dead pixels or not?
Click to expand...
Yep, there are 17 of them. (Though I don't know how many of those are 100% dead and how many are just very hot.) At least one of those was mapped my me, because one dead pixel developed after I bought the camera.
Sunny Krpan said:
How can you find them if the pixelmaping cloned surrounding pixels?
Click to expand...
RAW files have 16 bits per pixel, but the bottom 6 bits are just padding because the camera has a 10 bit ADC. But when it clones/interpolates the surrounding pixels around a mapped pixel, the average sometimes sets one of the padded bits -- and that's how I can detect bad pixels.

Hey, you asked. ;)
Sunny Krpan said:
That's raising another question is pixelmapping necessary for
everytime you exchange batteries?
Click to expand...
Nope! Only when you start noticing dead pixels showing up in exposures that aren't very long.
 
I've just found out that the E-20N reads the top interlaced field 50 ms after clearing the CCD, and then reads the bottom field 100 ms after the top field.

I know this because I have compared a composite of 256 averaged 1/640 sec dark frames with a composite of 256 averaged 1 sec dark frames. Even in the 1/640 sec frame a small amount of dark current has built up -- and more so in the bottom field than in the top! :)

And now I will be using that data to further refine my astrophotography images.
 
David Ellsworth said:
I've just found out that the E-20N reads the top interlaced field
50 ms after clearing the CCD, and then reads the bottom field 100
ms after the top field.

I know this because I have compared a composite of 256 averaged
1/640 sec dark frames with a composite of 256 averaged 1 sec dark
frames. Even in the 1/640 sec frame a small amount of dark current
has built up -- and more so in the bottom field than in the top! :)

And now I will be using that data to further refine my
astrophotography images.
Click to expand...
--
http://www.peakphotography.co.uk

Oly E20-P, WCON-08B, TCON-14B, 2 X Bowens 500DX studio flash.
 
Thanks David for explaining now I got it.
David Ellsworth said:
Sunny Krpan said:
Guys I'm lost in your tech talk.
So what is the conclusion is there any dead pixels or not?
Click to expand...
Yep, there are 17 of them. (Though I don't know how many of those
are 100% dead and how many are just very hot.) At least one of
those was mapped my me, because one dead pixel developed after I
bought the camera.
David Ellsworth said:
How can you find them if the pixelmaping cloned surrounding pixels?
Click to expand...
RAW files have 16 bits per pixel, but the bottom 6 bits are just
padding because the camera has a 10 bit ADC. But when it
clones/interpolates the surrounding pixels around a mapped pixel,
the average sometimes sets one of the padded bits -- and that's how
I can detect bad pixels.

Hey, you asked. ;)
David Ellsworth said:
That's raising another question is pixelmapping necessary for
everytime you exchange batteries?
Click to expand...
Nope! Only when you start noticing dead pixels showing up in
exposures that aren't very long.
Click to expand...
--
E-10 for now.........Gladly!!!
Take a Prayer!
http://www.pbase.com/image/5706283
 
markE said:
...pixelmapping too much or too often? What I mean, can frequent
pixelmapping cause any damage?
Thanks.
Click to expand...
Mark, I'm pretty certain that the answer to that is a definite No. I've taken almost a thousand calibration frames so far with no ill effects, and the in-camera pixel mapping takes MUCH less than that.

The only remotely possible wear and tear I can see it causing is through writing the on-camera Flash ROM, but this is pretty moot, because Flash-card ROM is written whenever you take a picture and that happens much more than pixel mapping. :) (Flash ROMs are supposed to have a capacity for only a limited number of writes, but the capacity is so high that I've learned not to worry about it.)
 
Pixel mapping is not stored indefinitely. Pulling the batteries and waiting about 10 minutes will lose the mapping information and your dead pixels will return.
D.
David Ellsworth said:
markE said:
...pixelmapping too much or too often? What I mean, can frequent
pixelmapping cause any damage?
Thanks.
Click to expand...
Mark, I'm pretty certain that the answer to that is a definite No.
I've taken almost a thousand calibration frames so far with no ill
effects, and the in-camera pixel mapping takes MUCH less than that.

The only remotely possible wear and tear I can see it causing is
through writing the on-camera Flash ROM, but this is pretty moot,
because Flash-card ROM is written whenever you take a picture and
that happens much more than pixel mapping. :) (Flash ROMs are
supposed to have a capacity for only a limited number of writes,
but the capacity is so high that I've learned not to worry about
it.)
Click to expand...
--
Oly E20, Kodak DC4800's, Canon GL1, ZR25, Minolta HTsi+
 
David M. said:
Pixel mapping is not stored indefinitely. Pulling the batteries
and waiting about 10 minutes will lose the mapping information and
your dead pixels will return.
D.
Click to expand...
I've left the batteries out for hours and all my settings are intact when I put them back in. I've only had one reset in all the time I've owned it, so far. I had 25 resets with my old C-2020.

But thanks for the information -- I'll be sure to watch out for it if I ever have another reset on my E-20! Then I can do a before-and-after mapped pixel count.
 
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