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[filmscanners] Re: Scanning negs vs. slides



On Thu, 18 Apr 2002 12:44:16 +0100 (BST)  =?iso-8859-1?q?David=20Harris?=
(davidjharris_2000@yahoo.co.uk) wrote:

> As I understand it neg film has lower density so that
> it can be printed with short exposure times.

Its density range is matched to the sensitometric requirements of
photographic paper emulsion, if that's what you mean. Just as transparency
is matched to approximating subject brightness range via projection.

> Neg film should actually be much more prone to
> posterisation/areas of uniform tone when scanned -
> especially if you try to capture the full dynamic range.

I can see why people think this should be so, but given a competent
scanner, and especially >8bit/channel working, it just is not a significant
problem in real life, in my experience.

In the end, most of us are aiming for a print or screen display, both of
which exhibit a vastly smaller gamut than reality does. We are trying to
reduce a scene brightness range of maybe >10,000:1 to maybe <20:1 in a
print. I've got a rather superior Trinitron monitor here chosen for its
good contrast, which measures at 8 stops exactly, 256:1. Most will be much
less.

I mention all this for context, just in case we forget the real constraints
we live with whilst arguing the toss over theoretical limitations which may
be annihilated by real ones.

A contrasty slide on material like Velvia requires wide ODR in the scanner
just to capture the full range. Once captured, image info must be discarded
or compressed to fit within a 24bit scheme which is what monitors or
printers require, and which encompasses more colours than our vision can
discriminate.

If you discard it, information is lost from the highlights and shadows, but
midtone slope is preserved. If you compress it, contrast is reduced
overall, and so inevitably are the number of colours. Either way, the scan
of a slide cannot look exactly like the slide itself, it's a quart into a
pint pot. Still, it can look good in it's new medium, whether screen or
print.

A contrasty colour neg's ODR will almost fit within 8bit/channel, so there
is less need for compression, or chucking information away. The ODR
of neg is a better match to the scanner limitations and the eventual 24bit
destination. Except the downside of neg is that the image will require
colour correction, changes to gamma, etc. You may have to expand (increase
contrast of) what is left to achieve a more acceptable perceptual result.
This *could* create posterisation, could enhance grain and so on.

In the days when the orange mask was removed by software rather than extra
complementary exposure, when drivers reduced everything to 8bit/ch before
applying any corrections, colour neg certainly was problematic. You still
see people hitting these same gotchas using cheaper flatbeds with film
adaptors. But in several years of scanning and working with predominantly
colour neg ISO100-800 using modern film scanners and sofware that works
properly, I have never come across significant visible artifacts like this,
unless performing extreme manipulations in PS which would damage any scan
from any material. So I don't think theory is a reliable guide to practical
use.

The major, major advantage of slide is that there's an immediate reference
right there, you can see what it should look like. For many people that's
so conclusive a USP that nothing else matters. For me, that is a
disadvantage!  I'm generally photographing where slide's limitations, its
intrinsic contrast and restricted dynamic range, are a pain, and whatever I
get on slide is far away from what was there and what I want.

Both materials have their good and bad points WRT scanning. It doesn't help
that the problems which arise with one are the inverse in the other. Any
comparison cannot be like with like, so you end up with an endless argument
like 'paper...scissors...stone'. You just have to decide which set of
limitations better suits what you are trying to do, but I *STRONGLY*
recommend people to play with both and see what works for them rather than
rely on my or anybody else's claims, opinions, or (worst of all) theories.

I'll post some examples sometime soon, with workflow descriptions, as
dealing with colour negs seems to be a problem for many people, and it
certainly took hundreds of hours to get on top of it.

That's my last word on this subject for some time :)

Regards

Tony Sleep
http://www.halftone.co.uk - Online portfolio & exhibit; + film scanner info
& comparisons

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