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RE: [APML] Fw: images
Hi Chuck,
I'm not sure how useful it is to talk about bit depth with an analog media, since the amount of exposure is not recorded in digitized steps (or histogram bins) in the silver halide grain. Are you saying 5-6 bits of information after conversion to digital? Do your numbers account for scanner noise? 6 bits = 64 levels per channel, which is hard to believe for something like a color photograph. It is awfully similar to what practical tests have shown for S/N of film: A S/N ratio of 4-5 if the element is to be detected against its surround, 8-10 if it is to be recognized. I don't know if it's correct to talk about film directly using digital nomenclature...maybe for a scanned image of a film negative, though. I think this requires some clarification, because even after dealing with this stuff at work I must admit you have me confused.
On the other hand, there is an expression giving the information capacity of film..that is the bits per unit area that can be stored on a photographic layer. Altman and Zweig (Phot. Sci. Eng. 7:173, 1963) reported levels up to 160x10^6 bits/cm^2 for one set of fairly stringent conditions. I haven't read their paper to see the details, but I have seen it referenced. The expression is a nasty double integral, but probably doable in something like Matlab if you knew the values for all the variables involved for a given film. Information capacity depends on the frequncy response and noise of the system, as you would expect. If you know the grain density / unit^2 and information capacity / unit ^ 2, you could probably calculate an equivalent "bit depth" for each film grain. Naturally, an actual film would have a gaussian-type distribution of actual # of grains at a given "bit depth" (due to grain size, etc), but it probably is a good estimate.
-Jason
-----Original Message-----
From: astro-photo-bounces@seds.org
[mailto:astro-photo-bounces@seds.org]On Behalf Of Chuck Vaughn
Sent: Wednesday, June 23, 2004 9:49 AM
To: Discussion of Film Astrophotography
Subject: Re: [APML] Fw: images
It amazes me how much confusion there is over bit depth, dynamic range,
and S/N. In practicality, the finest grain films are limited to about 5
- 6 bits and most color films are about 4 - 5 bits.
Let me provide an example as to what I mean. The following image is a
section of a Tech Pan photo originally scanned at 3048 dpi and 16 bits
with no processing at the scanner.
http://www.aa6g.org/Images/m81.psd (2.5MB)
It was contrast stretched in Photoshop at 16 bits and converted to 8
bits. (No one I know of can view a 16 bit image directly.) I then
copied the image 5 times and in PS posterized them to 7, 6, 5, 4, and 3
bits. I added the labels after so they're not part of the posterizing
process. I left this as a .PSD file so you could click on each layer,
look at the histogram, and see the number of discrete levels. What
you'll see is that there is no improvement in the image beyond 6 bits.
There is some degradation at 5 bits. So this TP images has 5 - 6 bits
of real data. Beyond that it is just noise. What you're seeing here is
3 to 8 bits covering the full dynamic range of the image. As I've said
on here many times, any number of bits can cover any dynamic range.
The reason CCDs need more than 8 bits is that they're noise is much
lower than film so in order to cover the dynamic range AND reach the
noise floor, more than 8 bits are required. This also relates to why
film scanner noise is not an issue except on very dense negatives
because the scanner noise is totally masked by the film noise.
In order to find out what bit depth of noise-free data you really have
in a CCD image, you'd need to posterize the unprocessed 16 bit images
to 15, 14, 13, etc. bits and then process the images as you normally
would and check to see at what bit depth degradation is noticeable. I'd
be really surprised if it 16 bits. If you can get 10 - 12 bits
noise-free data, you really have something. I can show you weather
satellite images that started out as 10 bit noise-free data. They look
amazing at 8 bits even after extensive PS processing.
Note: I'm only taking about viewing these images on a monitor or print,
not scientific analysis of the raw data where you might squeeze out
another useful bit.
Chuck
--------------------------------------------
> >In fact, it takes a FILM person to trully appreciate what a CCD can do
> >.... downloading 16 bit data right out of the sky is more meaningful
> to a
> >film person than to any CCD newbie ... what do they know about being
> >stuck
> >in an 8 bit world?
>
> Hi Tony
>
> I have to desagree a little with you. Film isn't limited to the 8bits.
> Becouse it is a chemical medium, I think that the only limitation that
> we face with the bit deph is the scanner quality. Most modern scanners
> have a value higher than 10 bits (there are some that works with
> 16bits), and of course they generate 16bits files. Also, we have the
> possibility to integrate several film images to increase the SNR, and
> in some ways the bit deph is increased too. Of course we need a
> software that matches that potential. Before Photoshop CS we where in
> fact limited... but now we have other softwares as well that works
> with 16 and 32bits. I am convinced that film shots can be pushed to
> higher levels with the new digital tools, as well with the stacking
> procedure. There are a lot of techniques that most of film guys don't
> use, but it is possible to apply them. Just for example think of
> deconvolution or wavelet processing. Also we can make pseudo-flat
> images, in a better way than lin ear gradients or bluring. Just see
> the work that Wei-Hao is doing in that field.
> What am I trying to say with all of this? Simple. Film shots are
> highly subestimated, and there is a lot more to do than we have seen
> in the past years.
>
> PS: Just in any case, I am not an enemy of the CCDs. In fact I have
> used a ST7... and I follow very closely the work of guys like Daniel
> Vershatse, Volker Wender, Robert Gendler, and of course you. I wish
> I'd could have the equipment you use. <g>
>
>
> Regards,
>
> Carlos Milovic F.
> -------------------------
> Astro & Photo - CMF
> http://www.astrophoto.vze.com
> -------------------------
> Visita "AstroFoto", el foro de astrofotografía en español
> http://espanol.groups.yahoo.com/group/astrofoto
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