From: Roger N. Clark (change username to rnclark) on
Doug McDonald wrote:

> I have the Canon 30D. I took a bunch of very underexposed shots
> recently (no tripod at critical time) and found that background
> subtraction didn't help much. The annoying noise is some sort
> of horizontal banding or streaking (these are landscape shots).
> Looks sort of like they scan the image TV-wise and this is 1/f noise
> in the amplifiers.
>
> Comments?

Doug,
Did you record the raw data, or just jpegs?
You need to record the dark frames under as close to the
same temperature as you can. With the lens cap on
(a dark or dimly lit room is fine too) set the
exposure time to the same as the exposures with the problem.
Record ten to twenty of the. If raw, convert them
with the same settings as your landscape image.
Average all the darks into one master dark, then
subtract the master dark from the landscape frames.
The closer the environmental conditions are to the
landscape images, the better the correction will be.

Roger
From: John Sheehy on
Doug McDonald <mcdonald(a)SnPoAM_scs.uiuc.edu> wrote in
news:et3rqs$m2h$1(a)news.ks.uiuc.edu:

> I have the Canon 30D. I took a bunch of very underexposed shots
> recently (no tripod at critical time) and found that background
> subtraction didn't help much. The annoying noise is some sort
> of horizontal banding or streaking (these are landscape shots).
> Looks sort of like they scan the image TV-wise and this is 1/f noise
> in the amplifiers.
>
> Comments?

That's pretty typical of digital cameras in general; it is simply more
visible in cameras with a certain ratio of banding noise to total noise.
For the 30D it should be the same as the 20D (ignoring the 30D's fake,
extra ISOs):

http://www.pbase.com/jps_photo/image/65737967/original

The yellow line represents standard deviation of a blackframe, divided by
10 to fit in with the horizontal and vertical banding noises (they'd be
flat if the entire chart scaled for the the yellow line).

A few things become very clear here; the banding is generally only about
1/10 the strength of the total noise, and yet it is highly visible. With
more read noise, the banding would be less obvious (although it may still
contribute somewhat to visible noise, just without the obvious pattern).
The higher ISOs are all normalized for ISO 100; IOW the values for ISO
200 are divided by two, ISO 400 values are divided by 4, etc, so these
are proportional to electrons as units of noise. All noises decrease as
you get to the higher ISOs, and the total noise looks like it is leveling
off a bit from 800 to 1600, but still had room to improve a little at
3200, but 3200 is "fake" and is really ISO 1600 amplification, multiplied
by two, so it is exactly the same as ISO 1600. The horizontal banding is
still dropping dramatically from 800 to 1600, and seems to have the
capability of dropping even further if the amplification went to 3200 or
even 6400.


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John P Sheehy <JPS(a)no.komm>
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From: C J Campbell on
On 2007-03-07 04:03:00 -0800, "ipy2006" <ipyasaswi(a)gmail.com> said:

> I have to shoot action photos in low light conditions. What is the
> best DSLR for this purpose?
> Thanks,
> Yip

For your budget, any of the cameras that handle well -- a D40 or Rebel
will work nicely, especially if you get a fast 50mm lens.

If you want to use flash, get a real flash unit.
--
Waddling Eagle
World Famous Flight Instructor

From: John Sheehy on
"Roger N. Clark (change username to rnclark)" <username(a)qwest.net> wrote
in news:45F577C0.9020706(a)qwest.net:

> I and other astrophotographers tend to ignore fixed pattern noise
> because we can calibrate most of it out of our images.

I'm not sure where "fixed pattern noise" came into play here; the issue
was read noise and one of it's components, 1-D noise. There is, for all
intents and purposes, zero fixed pattern noise in my 20D. Subtracting a
stack of black frames from a short exposure results in nothing but
slightly higher noise.

> If that is an
> issue for other people, then I suggest they learn how to take
> dark frames, average them, and subtract them from their images.

What about the read noise in short exposures?

> It is really pretty easy, but for best results, it needs to be
> done on linear data.

And in the case of Canons which have "negative noise" at the blackpoint,
it needs to be done without any clipping at the black level.

> Another calibration that can improve images is
> flat field calibration, which not only corrects for pixel to pixel
> variations, but corrects for light fall-off from lenses.

> But if someone wants to pay me to run more tests......

I don't feel like financing anything right now, but I might suggest that
when you have the time, you do a "gap" test of large vs small pixels.
Your 1DmkII vs S70 page seems to be about pixel size, but it is really
about sensor size. Do a test with a small-pixel camera, and the 1DmkII,
both using the same real focal length, the same Av value, the same Tv
value, the same ISO setting, of the same detailed subject from the same
distance. I guarantee that your big pixels will fall to the ground like
Goliath, when viewing the subject at any magnification, from both
downsampled to both upsampled, or printed large. This is the real test
of pixel size. What you seem to overlook in your analyses is the fact
that standard deviation is only *one* factor in the noise equation;
magnification is another, and the low noise of big pixels is visually
magnified when the pixels are magnified along with the subject.

I am quite certain that the only benefits of big pixels are:

1) quicker readout time and less storage requirements, and

2) slight benefit in photons collection rate per unit of sensor area due
to less wasted space on the sensor (not always realized, however; my
1.97u FZ50, for example, collects about the same number of photons per
unit of area as the 1DmkII, at RAW saturation for the same ISO).

Here is one of my tests; it needs to be redone, because I realized after
doing it that ISO 1600 on the FZ50 is crippled by a very bad amplifier,
that is worse than pushing 100 to 1600. Here is the original, however:

http://www.pbase.com/jps_photo/image/74020772

Don't forget that the 10D images would need to be sharpened more,
sharpening the noise as well.


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John P Sheehy <JPS(a)no.komm>
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From: acl on
On Mar 15, 2:31 pm, John Sheehy <J...(a)no.komm> wrote:
> that standard deviation is only *one* factor in the noise equation;
> magnification is another, and the low noise of big pixels is visually
> magnified when the pixels are magnified along with the subject.

Exactly, and if you don't need the extra pixels you can bin.

>
> I am quite certain that the only benefits of big pixels are:
>
> 1) quicker readout time and less storage requirements, and
>
> 2) slight benefit in photons collection rate per unit of sensor area due
> to less wasted space on the sensor (not always realized, however; my
> 1.97u FZ50, for example, collects about the same number of photons per
> unit of area as the 1DmkII, at RAW saturation for the same ISO).

Well, as long as there are no constant noise sources (eg 10 electrons/
pixel independent of the area). I have no idea if there are or not.