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Re: [APML] Re: (OT) IC1396 CCD with Schmidt-like FOV
Philip,
I agree that there are limits to enlarging smaller files.
My point is that for a given FL a well exposed CCD
image will produce a higher resolution (more recorded information)
image which will under close observation have crisper detail and
subsequently can be enlarged to a greater degree without breaking down.
In daylight photography medium format is said to have greater resolution
then lets say 35mm because one is projecting the same image scale
onto the medium format film by increasing the FL of the lens system.
Lets compare an image of say NGC7331 taken with the same scope,
a 12" reflector at the "same" FL say 3000mm. One imager uses an ST10
and other uses a medium format camera. Yes the medium format image
can be enlraged to 30 x 40 but ngc 7331 will look like a little dot in the
middle
of the larger field the medium format film has recorded. If scanned and
enlarged
to the same image scale as the ST10 image the film image of ngc7331 will be
a
collection of featureless grain compared to the ST10 image at the same image
scale.
Of course the ST10 image can't go to 30 x 40". If you printed both images at
say
11 x 14" and made ngc 7331 fill up the entire print the difference would be
dramatically in favor of the CCD. I have actually made a similar comparison
in real life <G>.
Rob Gendler
Email: robgendler@att.net
Web site: http://www.robgendlerastropics.com/
----- Original Message -----
From: "Philip Perkins" <philip@astrocruise.com>
To: <astro-photo@seds.org>
Sent: Tuesday, August 06, 2002 8:38 AM
Subject: Re: [APML] Re: (OT) IC1396 CCD with Schmidt-like FOV
> Rob,
>
> Thanks for your comments. Perhaps the criterion you have in mind the is
> ability of an image to sustain enlargement while still appearing
reasonably
> smooth. If so, then I agree that a CCD image can definitely sustain a
> greater degree of enlargement. But smoothness is not the only thing that
> counts. If it was then we could take a well executed ST-7E image that is
> extremely smooth and enlarge it to a 30x40" print and, by sole virtue of
> the fact that it remains visibly smooth, claim that the enlargement is a
> complete success. I would say that it is not. Why not? Because those
> field stars, which with a correct degree of enlargement appear as
miniscule
> pin-pricks, instead appear the size of peas. It doesn't matter how smooth
> the image is, it has simply been over-enlarged. The limit of enlargement
> is the point beyond which further detail ceases to become visible. Go
much
> beyond that, and the image structure starts to break down and to overwhelm
> the structure of the actual object we're trying to portray. I have seen
> this happen with a few of my own 13x18" CCD prints. This is the fallacy
of
> the extravagent enlargement claims we sometimes see for CCD images, where
> apparently the only thing that counts is whether the image remains smooth,
> regardless of whether the actual object structure and star sizes retain
> their original integrity.
>
> Also I wonder whether all such comparisons are like for like... It is
> accepted by default that a CCD image will consist of a large number of
> sub-exposures to reduce the noise - so much so that the sub-exposures are
> routinely forgotten, and we frequently see references to "2 hour
exposures"
> and suchlike. Of course these are not 2 hour exposures at all, and the
> results are nothing like what would truly result from 2 hour
> exposures. But that's beside the point - what sort of film images are you
> using for comparison? In my case I will use an accumulation of not less
> than 4 images for similar noise reduction purposes. The result, with the
> correct compositing technique, is an image that is very smooth. I have a
> set of medium format images produced in this way that appear grain-free
> when enlarged to 30x40". Four of these 30x40" prints which have been on
> public display at various Astrofests, seminars, etc. Even on very close
> inspection these prints appear grain-free and field stars appear as
> pin-pricks with a size small enough to be indeterminate without the aid of
> a magnifying lens. That would be impossible in the case of an ST-10 image
> enlarged to 30x40" purely because of the physics involved in such a huge
> degree of enlargement.
>
> To take this further... I produce such images with a 6x7 camera attached
to
> the back of an Astro Physics 155 EDF, which has a focal length of 1144mm
> (with field flattener). The resulting images are around 112Mb. To
> properly discern all of the detail in such images I need to zoom in to
100%
> or 200% in PhotoShop. There is detail there very close to the pixel
level.
>
> I can produce a very similar looking image (at web resolution) by
attaching
> a 200mm lens to an ST-10E. The image will cover a very similar area of
> sky. The resulting image will be 9.6Mb. Now, I don't think anyone is
> claiming that the 9.6Mb image will contain the same level of detail as the
> 112Mb image produced with the 155 EDF, are they? At least I hope not!
> :-) Again, bear in mind that the 112Mb image has very little noise and
has
> detail down to nearly the pixel level.
>
> The CCD image will enlarge proportionately much more than the film image,
> but it will still not come anywhere near the quality of the film image
when
> enlarged to 30x40". Some of the almost "infinite enlargement" claims that
> we hear are claims too far. There is the physics of detector size to
> consider - enlargement is much more than a matter of image smoothness.
>
> --Philip
>
>
> At 01:01 05/08/02 -0400, you wrote:
> >Hi Philip,
> >Resolution in the context of image size is really a meaningless concept.
> >The ability of an image to hold up to enlargement and in particular
printing
> >is much more than a function of actual pixel count. The overall
smoothness
> >and
> >in particular the S/N is much more important. I know Tony will back me
> >up on this since he has done alot of printing for me. I have given him
files
> >that were barely 6MB but were very well exposed with excellent S/N
> >and he has made quality 40 x 20" prints. CCD images if well exposed are
> >very smooth in quality and the actual detail in the image can be much
> >greater
> >than comparable film images scanned at much higher resolution. Loke even
> >pointed out that VDB142 in the CCD image had more detail than the film
> >image despite identical focal lengths and much smaller aperture for
> >the CCD image (8" vs 3"). Ultimately its not the pixel count but the
amount
> >of real information recorded by the medium that counts.
> >Rob Gendler
> >Email: robgendler@att.net
> >Web site: http://www.robgendlerastropics.com/
> > > The detail and FOV may compare favourably at *web resolution* with a
> >medium
> > > format image. But as you know (from your own excellent work with
medium
> > > format) the medium format image has to be hugely downscaled for web
> > > resolution. The raison d'etre for medium format (with you also I
assume)
> > > is to produce large exhibition-quality prints while preserving the
full
> > > integrity of the image (little or no upscaling). By dowscaling for
web
> > > resolution, the detail of a medium format image is compromised
> > > severely. When comparing like for like, a full resolution medium
format
> > > image (when scanned on your UMAX or equivalent) will yield a
resolution of
> > > around 8,100 x 6,900 pixels. I am not sure that an ST-10 image at
2,184 x
> > > 1,472 pixels "compares favourably". Much more detail will be apparent
in
> > > the full resolution medium format image. The ST-10 image has nearly 18
> > > times less area. And that is only the resolution aspect - in terms of
> > > field coverage the difference is even more stark.
> > >
> > > --Philip
> > >
> > > Philip Perkins - philip@astrocruise.com
> > > Wiltshire UK & Luberon France
> > > Astrocruise - http://www.astrocruise.com
> > >
> > >
> > > -- APML Archives at <http://astro.umsystem.edu/apml/> ---
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> >
> >
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>
> Philip Perkins - philip@astrocruise.com
> Wiltshire UK & Luberon France
> Astrocruise - http://www.astrocruise.com
>
>
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