Re: [APML] Fw: images

["Rick Kellogg" <rkellog1@twcny.rr.com>]

Chuck,

I objected to you being surprised at peoples confusion, when I've seen many
an expert get confused when talking to another expert, mainly because they
use different definitions or misunderstand each other.  S/N is a very
confusing subject, mostly because one persons S is anothers N, and so many
noise sources are lumped together. Some use full scale signal to RMS noise,
others use RMS Signal to RMS noise.  I see nothing but confusion when this
topic comes up.


----- Original Message -----
From: "Chuck Vaughn" <aa6g@aa6g.org>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Wednesday, June 23, 2004 5:25 PM
Subject: Re: [APML] Fw: images


> Rick,
>
> > You are "confusing" the innate recording bit level of film with its
> > useful
> > range needed for recording what you are interested in.
> >
> > The bit level of film, refers to its ability to record.
>
> You are confusing bit depth with dynamic range. The two have absolutely
> nothing to do with each other.
>
Says who?

Again, definitions,  (I generally work with linear systems), when I refer to
how many bits are needed to record a signal,  I generally mean recording the
signal (without cliping), down to the noise floor (in a linear space). Max
code large enough to hold the signal, LSB significantly smaller than RMS
noise level (lower level somewhat vague, depends on the nature of the
signals, and how linear you A/D is).

Your definition may vary. Using less bits than this will add noise to the
signal in the A/D conversion process.

> > What you are
> > showing here is how many bits of detail there are in the "desired"
> > part of
> > the image.
>
> I'm not sure what you mean by "desired."
>
When taking an astro picture, a significant fraction (especially at low S/N)
of the photons came from skyglow, or LP, and are (presumably) eliminated
from your shots by setting the lower slider in photoshop (levels), as you
are not interested in filming LP, but are interested in the celestial
target.  Take a wide field shot that has both trees and sky, setting the
correct levels for "nice looking m'way", will loose the detail in the trees.
Set for nice detail in the grass and trees and the M'way is pale, because
most of the light in the sky is skyglow and LP, not the Mway.

> > A quantitative measure of
> > films bit recording capability would be to measure a gray scale from
> > dmin
> > to dmax, and do a similar kind of test.
>
> How many steps do you propose to be in the grayscale?
>
No steps, continuous from black to white, using a linear scale.  If you
think of the A/D code as representing a fraction from 0 to 1, then make
black 0 and white 1.  Now increase the number of bits from 1 to N, stop
increasing N when the lsb is random for like samples, at the same point on
the ramp.  That is how many bits I would say the film has.  You can argue
how correlated the LSB needs  to be considered repeatable.

>
> It seems like what you're saying here is that dynamic range implies
> some bit depth. It does not. One bit can represent Dmax and Dmin,
> 1=white, 0= black.

It does if you don't want to add  noise by the digitizing process. The
classic noise added by digitization is LSB level, squared/12 (again linear
signals, also assumes signal is random).  So you want there to be enough
bits in the digitization such that the noise level of the film is digitized.
Moreover, you want this noise level to come from the photons themselves, not
from your recording/digitizing of them.   (Note:  since the film density is
already a non-linear process, it is probably a bit silly to talk in such
terms.)

Now if your negative is exposed as far as you can without foging the film,
the system noise is comming from the random variations of skyglow, LP and
the signal itself, and should reduce the number of bits needed to digitize
the film, but that does not mean that the inherent film can't record more
bits than this.

Besides, Chuck, don't you use D19 on your negatives, which is almost a 1/0
developer anyway <g>.

> I'm not sure I can say this correctly but I'll take a stab at it. What
> you want to determine is the number of bits it takes to just resolve
> noise in the strongest unsaturated signal the media can record. This
> would be the bit depth required to record the strongest signal to the
> noise.

Yes this is what I mean.


> > Also don't forget that 16 bit files of CCD record linear counts.  The
> > 8 bit
> > files of photoshop use a gamma of 2.2, so it can store a large dynamic
> > range.
>
> The implication of what you said is that the S/N is different depending
> on whether the data is stored as linear or log. Of course this is not
> correct. The only difference between linear and log is that the
> absolute amplitudes are compressed, i.e. the steps are smaller. The S/N
> remains unchanged and so does the bit depth required to represent that
> S/N.
>

S/N is not independant of bit depth unless you have enough bits that the
noise added by digitization is insignificant (compared to some other system
noise).  Thus without enough bits, the noise added by digitization it will
be noticed.


Rick

_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo