Digital vs. Film in Astrophotography

Chris,

You have it backwards. Because film is nonlinear, it has a greater dynamic
range than a CCD. Don't confuse range with sensitivity. CCDs are more
prone to oversatuation than film. For example, a single unprocessed
exposure of the Orion Nebula will turn out better on film that on a CCD with
regard to dynamic range.

Also, the original poster meant to say that film has more definition rather
than more resolution. Definition is the pixel count (like HDTV); resolution
is the pixel size coupled with optical performance.

Del Johnson



"Chris L Peterson" wrote in message
...
On Sun, 28 Dec 2003 21:11:27 -0800, "Jason Donahue"
wrote:

OK, so that's the basic argument as to the superiority of digital over
film
in astrophotography, and it makes sense. However, is CCD imaging really
that much better? For example, the CCD has to be cooled to cut down on
noise, an issue you don't see with film.

Does that matter? A digital camera designed for long exposures _is_
cooled, so
it is that cooled camera you are comparing to film.


Also, the majority of CCDs in use
are smaller than 35mm film format - wouldn't that generally mean poorer
maximum resolution?

It depends on your ability to match your optics to your imager/film. CCDs
(with
pixels in the 5-10um range) are higher resolution than most films at the
focal
plane.


I mean, some of the better 35mm films give incredible
resolutions, and, combined with 40 megapixel film scanners, you get
better
resolution than digital.

There are very few films that (in 35mm format) deliver anything close to
40
megapixels. At best, typical color films used by most astrophotographers
can
yield spatial data at around 5-10 megapixels, and that varies with
contrast. The
MTF for a digital sensor is flat, so you get uniform response regardless
of
contrast. Since film is non-linear and doesn't have much dynamic range,
you have
to deal with much lower intensity resolution.

On Mon, 29 Dec 2003 11:22:01 -0800, "Del Johnson" delastro@{right star in
Orion's belt}.sdsu.edu wrote:

Chris,

You have it backwards. Because film is nonlinear, it has a greater dynamic
range than a CCD. Don't confuse range with sensitivity. CCDs are more
prone to oversatuation than film. For example, a single unprocessed
exposure of the Orion Nebula will turn out better on film that on a CCD with
regard to dynamic range.

I don't have it backwards at all. At the ends of the "S" curve you get with film
the range is compressed down to the noise level. A decent CCD detector will have
between 10 and 50 times the dynamic range of a good film.

Yes, a single exposure on the film may turn out "better" because the film is
compressing the range- that is, you are losing information at the top and
bottom. IMO that is not an advantage; I see how some might see it that way,
though.


Also, the original poster meant to say that film has more definition rather
than more resolution. Definition is the pixel count (like HDTV); resolution
is the pixel size coupled with optical performance.

Yes, the terminology is a problem here. Different disciplines use the same words
in quite different ways. In terms of pixel count, I'd say that a typical 35mm
color film is somewhere between 2 MP and 10 MP, depending heavily on the
characteristics of the image. A typical CCD these days is between 1 MP and 4 MP,
but without the dependence on the image. So the spatial information content
isn't all that different between the two, and CCDs are rapidly becoming higher
density devices.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...

You have it backwards. Because film is nonlinear, it has a greater
dynamic
range than a CCD. Don't confuse range with sensitivity. CCDs are more
prone to oversatuation than film. For example, a single unprocessed
exposure of the Orion Nebula will turn out better on film that on a CCD
with
regard to dynamic range.

I don't have it backwards at all. At the ends of the "S" curve you get
with film
the range is compressed down to the noise level. A decent CCD detector
will have
between 10 and 50 times the dynamic range of a good film.

The real problem, as you point out, is that with film, the toe is
compressed. We would rather have the toe be perfectly linear and do our
compression, if any, at the shoulder, because the faintest objects in the
picture are usually the most important. Also, the compressed toe works
against you if you want to stack images or subtract out the sky background.

I seem to recall that CCDs typically have a 12- to 16-bit dynamic range. 12
bits is 12 stops, photographically speaking; film is 9 or 10 stops maximum
when developed normally, much less when processed for high contrast in
astronomy.

On Mon, 29 Dec 2003 17:19:40 -0500, "Michael A. Covington"
wrote:

I seem to recall that CCDs typically have a 12- to 16-bit dynamic range. 12
bits is 12 stops, photographically speaking; film is 9 or 10 stops maximum
when developed normally, much less when processed for high contrast in
astronomy.

The sensors in most commonly used cameras have dynamic ranges of 76-78dB, or
about 13 bits. A couple of cameras are as high as 85dB (14 bits); that's as high
as any sensors I'm aware of amateurs using. There are some tradeoffs. If you
really want a lot of pixels (11M) for not too much money, you could go with the
SBIG STL-11000M, but then you get a sensor with rather poor performance: only 11
bits of dynamic range, shallow pixels, poor linearity, and microlenses. Of
course, with a linear detector, you can just take more pictures- every time you
double the number it is like you added another 6dB, or one stop. This kind of
stacking is very imperfect when applied to film, because you've already lost the
information down at the low intensity end of the range, and because adding
non-linear frames just makes things less linear.

It is important to remember that lots of pixels (or the film equivalent) is
really only important for wide field imaging. For the vast majority of DSOs, one
or two million pixels is more than enough to reach the point where the seeing
and optics are the limiting factor, not the sensor pixel count.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

It is this "compression" that gives film greater range. The CCD may
distinguish more levels of brightness, and is certainly more sensitive, but
film prevails when there is a large difference between bright and dark
objects in the same image.

Why would this not be an advantage? The alternative is a complete loss of
detail in the oversaturated areas.

Del Johnson



"Chris L Peterson" wrote in message
...

Yes, a single exposure on the film may turn out "better" because the film
is
compressing the range- that is, you are losing information at the top and
bottom. IMO that is not an advantage; I see how some might see it that
way,
though.

On Tue, 30 Dec 2003 15:21:14 -0800, "Del Johnson" delastro@{right star in
Orion's belt}.sdsu.edu wrote:

It is this "compression" that gives film greater range. The CCD may
distinguish more levels of brightness, and is certainly more sensitive, but
film prevails when there is a large difference between bright and dark
objects in the same image.

Why would this not be an advantage? The alternative is a complete loss of
detail in the oversaturated areas.

It's not an advantage because you pay for your lack of saturation at the bright
end with a huge loss of detail at the dim end (everything is compressed into a
very narrow brightness range). It is precisely in the ability to show detail at
the dim, low contrast end of things where CCDs shine, and that is the region
that is usually of the most interest (to me, anyway) when examining astronomical
objects.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Sounds like you are still confusing range with sensitivity. It is a given
that a CCD will pick up fainter objects for the same integration. If one
adjusts the integration time so that the faint range is about the same
between film and CCD, one will find that film will be less likely to
overexpose the brighter portions of the image.

Any single unprocessed CCD of the Orion Nebula that shows the fainter wisps
will completely burn out the bright core, whereas a film image will do a
better job of capturing the entire dynamics of this object (albeit with a
longer exposure). The only good images of the Orion Nebula with a CCD are
a result of multiple stacked images or mosaics that have been pumped up with
paint software.

Del Johnson


"Chris L Peterson" wrote in message
...

It's not an advantage because you pay for your lack of saturation at the
bright
end with a huge loss of detail at the dim end (everything is compressed
into a
very narrow brightness range). It is precisely in the ability to show
detail at
the dim, low contrast end of things where CCDs shine, and that is the
region
that is usually of the most interest (to me, anyway) when examining
astronomical
objects.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Wed, 31 Dec 2003 10:08:49 -0800, "Del Johnson" delastro@{right star in
Orion's belt}.sdsu.edu wrote:

Sounds like you are still confusing range with sensitivity.

Not at all. Sensitivity is a whole separate issue, and I'm not discussing it at
all.


Any single unprocessed CCD of the Orion Nebula that shows the fainter wisps
will completely burn out the bright core, whereas a film image will do a
better job of capturing the entire dynamics of this object (albeit with a
longer exposure).

You are fooling yourself here. Most film has an overall dynamic range of about
3A, or maybe 3.5A if you really push out into the extremely non-linear portion
of the response curve (that is, where a factor of 10 difference in source
intensity produces essentially the same density on the film). Most CCDs have an
overall dynamic range of about 78dB, or 3.9A, and that is nearly linear end to
end. The simple fact is that if you make a single CCD exposure of the Orion
nebula adjusted just to the point where the core is about to saturate, you will
have captured more detail at the wispy edges than the film image will give you
assuming that you expose it long enough to compensate for the difference in
sensitivity.

The trick with film is that you can expose it even longer than that in order to
bring some of the wispy detail up on the image, and still have a reasonable
looking core. But even so, that core has been pushed up onto the flat part of
the response curve, which means that much of the structural detail has been
obliterated, even if it doesn't have the characteristic blown-out appearance of
a saturated CCD image.


The only good images of the Orion Nebula with a CCD are
a result of multiple stacked images or mosaics that have been pumped up with
paint software.

I think this is a bit of an exaggeration, but if not, so what? In what way is
taking multiple CCD images a problem? There is virtually no difference in effort
between taking one CCD image or taking several, and the overall exposure time is
still shorter with the CCD. So if the normal CCD technique involves collecting
and stacking several images, so be it. This is neither an advantage nor a
disadvantage compared with film.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Talking about Orion - here's a film Orion - a stacked composite. Technique
are the same on software. Well exposed film has a very good range.

Take Care,

JAS


"Chris L Peterson" wrote in message
...
On Wed, 31 Dec 2003 10:08:49 -0800, "Del Johnson" delastro@{right star in
Orion's belt}.sdsu.edu wrote:

Sounds like you are still confusing range with sensitivity.

Not at all. Sensitivity is a whole separate issue, and I'm not discussing
it at
all.


Any single unprocessed CCD of the Orion Nebula that shows the fainter
wisps
will completely burn out the bright core, whereas a film image will do a
better job of capturing the entire dynamics of this object (albeit with a
longer exposure).

You are fooling yourself here. Most film has an overall dynamic range of
about
3A, or maybe 3.5A if you really push out into the extremely non-linear
portion
of the response curve (that is, where a factor of 10 difference in source
intensity produces essentially the same density on the film). Most CCDs
have an
overall dynamic range of about 78dB, or 3.9A, and that is nearly linear
end to
end. The simple fact is that if you make a single CCD exposure of the
Orion
nebula adjusted just to the point where the core is about to saturate, you
will
have captured more detail at the wispy edges than the film image will give
you
assuming that you expose it long enough to compensate for the difference
in
sensitivity.

The trick with film is that you can expose it even longer than that in
order to
bring some of the wispy detail up on the image, and still have a
reasonable
looking core. But even so, that core has been pushed up onto the flat part
of
the response curve, which means that much of the structural detail has
been
obliterated, even if it doesn't have the characteristic blown-out
appearance of
a saturated CCD image.


The only good images of the Orion Nebula with a CCD are
a result of multiple stacked images or mosaics that have been pumped up
with
paint software.

I think this is a bit of an exaggeration, but if not, so what? In what way
is
taking multiple CCD images a problem? There is virtually no difference in
effort
between taking one CCD image or taking several, and the overall exposure
time is
still shorter with the CCD. So if the normal CCD technique involves
collecting
and stacking several images, so be it. This is neither an advantage nor a
disadvantage compared with film.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

I think that is what I have been saying......

Del Johnson



"Chris L Peterson" wrote in message
...

The trick with film is that you can expose it even longer than that in
order to
bring some of the wispy detail up on the image, and still have a
reasonable
looking core. But even so, that core has been pushed up onto the flat part
of
the response curve, which means that much of the structural detail has
been
obliterated, even if it doesn't have the characteristic blown-out
appearance of
a saturated CCD image.