RE: [Leica] digital quality

["B. D. Colen" <bdcolen@earthlink.net>]

But then, given that not a lot of people walk into an art gallery or my
house with a densitometer, we get back to the real work question...What does
the print look like, framed, under glass, from 18" to six feet away?
:-)
B. D.

> -----Original Message-----
> From: owner-leica-users@mejac.palo-alto.ca.us
> [mailto:owner-leica-users@mejac.palo-alto.ca.us]On Behalf Of imx
> Sent: Tuesday, July 25, 2000 3:14 PM
> To: LUG
> Subject: [Leica] digital quality
>
>
> An intriguing topic, this one about the image quality of a  digital print.
> I did some densitometric measurements on digital prints, made by Epson.
> These were promotional and of very high quality. Measuring white gave
> D=0.04, which is as good as the best BW papers I have tested. But
> measuring
> black gave a D=1.42, which is very good for a colour neg print, but far
> below what you can get with a BW print, which can handle easily
> D=2.30. This
> simply tells you that he brightness range of a digital print is 1:26,
> compared to 1:199 for a BW print and 1:1000 for a slide projected on a
> screen. As reference note that the average computer or TV screen
> has a range
> of 1:30.
> Whatever your opinion on digital prints, it is undeniable that the
> brightness range is much compressed in the shadow area and you will loose
> valuable information when printing from whatever scan you have.
> A second important aspect is the linearity of the CCD sensor
> versus the more
> natural logarithmic respone of the silver halide. A digital capture or
> recording then is always a discrete sampling and they need to use a linear
> or a logarithmic mapping to translate the analog nature of the film. A
> logarithmic mapping would be best, but the amount of data is too high so a
> compression algoritm has to be used, which always means reduction of
> information.
> If we scan a negative with 4000 dpi, it would be logical to print with the
> same amount of dpi as a pixel to pixel mapping is needed. This is not yet
> possible. Scanning with 2800 dpi has the same logic. Now 2800 dpi delivers
> 110 lines per mm, assuming that a dot represents a line. A typical ISO100
> film resolves 150 to 200 lines per mm, as does a Velvia slide
> film. Typical
> grain size is 2 micron, while the dot size of a 2800 dpi scan is about 10
> micron. So I cannot imagine that scanning a fine grain slide of negative
> will capture the grain size. The 200 lines of the Velvia would have a size
> of 5 micron,which cannot be capured by a scan with a minimum spot size of
> twice that area.
> So whatever parameter you use, brightness range, image size of a
> small spot,
> grain size, the analog process has a much higher capacity for recording
> spatial information. The several degrading steps in the digital
> process and
> the inherent limits of the technique itself, deliver a lower
> quality product
> than does the analog technique.
> To see grain with a ccd capture device, you need a lens with a
> resolution in
> the 1 micron area. This can be done of course and Crossfield in a paper
> about image quality note that they use high end scanners with this
> capability. One scan however would be a 28.000 dpi (!) scan. Or a 1 micron
> scan of a 35mm negative would generate  864 million pixels which has to be
> recorded 3 times for colour info too and so would have a 3
> Gigabyte file for
> one 35mm negative. Note this size is needed to capture the grain image.
> As long as we need image quality on a A4 size, on the threshold level of
> what the human eye can detect at normal looking distances, a digital print
> will suffice and even impress. But that is not the same as
> implying that the
> digital capture of information is in the same ball game as what silver
> halide can record.
>
> Erwin
>
>