RE: [APML] Couple of Wide-Angle shots on Konica Centuria 400(andscanner noise suggestion)

["Lane, Jason R" <jason.lane@navy.mil>]

Hi,

I agree with Ray that the concepts of signal and noise apply to film as well...IMO it makes it much easier to visualize what's going on and what you can do with it.  

The caffeine's kicking in and my memory's working a bit better now...sorry about that.  I recall now that I did the smoothing in SGBNR after stacking the two frames, and then brought the image back into Photoshop for final touch-up.  So I'm a-ok on that part.

One question I found myself asking was what happens when you're trying to eliminate scanner noise and your color balance or black/white points from scan-to-scan are off.  That is, some scans may be slightly redder (for example) or slightly darker.  I would think that combining them would not be nearly as efficient at reducing scanner noise.   I guess the same would be true for stacking two (or more) different frames.

So I ended up being very careful about locking the exposure and color reference to achieve the most consistent scans possible (btw my scanner is an HP Photosmart s20xi).  I was able to confirm that the control points I selected for the scans of one frame (after scanning and opening in Photoshop before registering and combining) had levels that were close.  For the next combining step where I stack the two frames, I adjusted levels and color balance to get the control points approximately equal to each other.

As for a scanner "dark frame"...I know the calibration routine sets the white point, but don't know about setting the dark point.  I assume there's some correction that goes on because otherwise, as a CCD array you would get lines down your scan corresponding to the hot pixels I would think.  I'm speculating...someone with more knowledge about the inner workings of scanners would know better.

Cheers,
Jason


-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Ray Butler
Sent: Tuesday, August 31, 2004 9:56 AM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Couple of Wide-Angle shots on Konica Centuria 400
(andscanner noise suggestion)



Steve.Holak@bcitpa.com wrote:

> Jason,
>      With digital images, averaging multiple exposures is the best way to
> bring signal above the noise (median combines are noisier than averages,
> but you get rid of abberant pixcels by median combining), so scanning the 7
> multiples and average combining them is the correct first pass at reducing
> scanner noise.  

I would agree with this. If the noise source is "random" and follows a 
Gaussian distribution, then averaging or medianing will reduce it. If 
your scanner is based on a CCD, then its readout noise follows a classic 
gaussian bell-curve distribution. Combining 7 scans would reduce the 
scanner readout noise contribution by the square-root of 7, which is 
2.6x better.

That doesn't make the whole image 2.6x better overall of course - it may 
still be dominated by the noise contributions from film grain, source 
faintness, and background brightness. Source faintness is bad (as 
everyone intuitively knows), because low light levels translate to 
higher Poisson photon-counting noise as a _percentage_ of the signal. A 
bright background is bad, because high light levels in the background 
translate to higher Poisson photon-counting noise in _absolute_ terms, 
which is additive to the source's own Poissonian noise.

Median combination is just averaging with more robustness against skew 
by an outlying data point - a pixel which may be much brighter or 
fainter than the rest of the pixels in its little stack.

> I would next do final combining of the two images, any
> histogram / curve adjustments, and save smoothing in SGBNR for last;
> smoothing will cause some loss of the original data so this should go last.

I agree with this too. Avoid (or keep to the very end) anything which 
messes with the spatial resolution or spatial frequency content of the 
image (deconvolution, smoothing, unsharp masking, etc.). That's because 
these operations all have one thing in common - they correlate the noise 
across adjacent pixels. Once that is done, it can't be undone by further 
processing. "Don't f**k with the pixels", as one of my heroes memorably 
said.

> This is the approach with CCD imaging, so after scanning film you're
> essentially starting at the same point.
> 
>      While composing this, I was wondering if the CCD imaging concept of
> "dark frames" would apply to help reduce scanner noise with film images.
> Has anyone here considered or played with this?  I imagine a series of
> blank or unexposed scans could be median combined, just like CCD darks, and
> subtracted from the raw scans to remove any consistent noise induced by the
> scanner from scan to scan.  If it did have some positive effect, faint
> detail could be brought out more effectively.  The step order would then be
> to scan the negatives, create the darks, subtract the darks from the scans,
> and then proceed to average the scans, etc.

When people talk about CCD "dark frames" they are usually really talking 
about bias+dark frames. Some even confuse plain bias frames (zero second 
exposures) with bias+dark frames (long exposures with no external light 
source). Anyway, it is a convenient short cut to perform bias 
subtraction and dark subtraction in one pass, which is what I think 
Steve is referring to. This works fine, if the bias level is not 
changing over time, and if the dark frame is of the same effective 
exposure time as the science frame. Strictly speaking, you should 
subtract the individual bias levels from the dark frame, and call the 
result the "dark frame"; or if you have several dark frames sum or 
average them after bias subtraction, and call that the "dark frame". 
Separately you would subtract the bias from the science frame. Then if 
necessary scale the debiased "dark frame"to the same exposure time as 
the science frame; and subtract it from the debiased science frame. 
Doing this in a one-er pass might seem to be the obvious thing to do, 
but then you can't average multiple bias frames for superior debiasing.

True (debiased) dark frames consist _only_ of dark current (thermally 
generated noise), and in any given pixel this grows linearly with time. 
But since the time it takes a scanner CCD to scan any given pixel in the 
image is only a fraction of a second, there will be essentially zero 
dark current in that pixel - insufficient time has elapsed for a 
thermally generated electron to pop up in that pixel. The rate of dark 
events is higher than in an astronomical imaging CCD, since scanner CCDs 
are not cooled, but the extremely short exposure time per pixel makes up 
for that.

So, basically, there is no point in worrying about subtracting true dark 
noise. Having said that, there may be a point in investigating the 
subtraction of scanner _bias_. I have not had a chance to properly test 
this, but I am not entirely sure whether bias subtraction is done 
internally in a scanner or not. The operating manual will never go into 
such details. Bias being equally present whether the exposure is long or 
short, it is something that might be hiding in all our scanned images!

>      I don't have a film scanner other than a flatbed, but maybe it's worth
> a try for someone to check it out.

Even with a flatbed (which is all I have too) all of these things apply.
So I think it is worth investigating alright.

Where I completely like the way Steve is thinking is that he is taking 
universal concepts of signal and noise contributions (which most people 
only come to grips with in digital formats) and applying them to 
photographic imaging. This is something that I myself really believe in. 
It is the best way to really understand what you can capture with any 
image - including a photograph - and how best to process it from 
scanning onwards. The experienced photographers and darkroom whizzes who 
pioneered combining negatives in various ways, unsharp masking, 
manipulation of film's characteristic curves, etc. were all getting at 
this via the photographic idiom, but I'm not sure they all fully 
understood the mixture of physics, statistics and information theory 
which underpins it. I haven't mastered it yet either, but I keep working 
at it.

Ray "sorry for the long lecture" Butler.

> Steve Holak
> Senior Software Architect
> 
> Brokerage Concepts IS Dept.
> 610-491-4879

-- 
Dr. Ray Butler (ray.butler@nuigalway.ie || ray@physics.nuigalway.ie)
Lecturer, Dept. of Physics || Computational Astrophysics Laboratory
National University of Ireland, Galway, University Road, Galway, Ireland.
Tel: +353-91-524411 ext. 3788   FAX: +353-91-525700

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