Stephen, I too would like to hear (read) the expanded version you were
going to post, on subject of lock film base, raw files, etcetera.
Thanks in advance if you can put it up.
Mendel Leisk
Hi Bill and Mendel
I'm glad my previous post was helpful. Here is a longer posting then,
since there are now at least the two of you who would like to see it.
I don't think it is inappropriate, Bill. Nobody is obliged to read it!
(So no flaming from anyone please! If it is useful, fine. If not,
ignore and move on. Though constructive comments would of course be
welcome.) First though, a caveat. What I say here is merely my own
understanding of VueScan and has no official authority or approval.
There are other people who post here who have a longer and deeper
understanding of VueScan. There is only one "official" guide to
VueScan and that is the User Guide that comes with the software. Also,
there are aspects of the product for which no detailed public
explanation exists because there is absolutely no reason why Ed
Hamrick should reveal his commercial secrets!
On that basis, here goes.
1 General aim of scanning a negative with VueScan
What it is trying to do is capture as much data as the scanner is
capable of extracting from the negative; correct for the orange mask
of the film; correct for how the film responds unevenly to different
wavelengths of light; correct for the scanner's uneven response;
adjust the color balance of the image so that grey looks grey, black
looks black and white looks white. It may also correct faded colours
(!!), reduce film grain and, if your scanner has an infrared channel,
remove dust specks and scratches from the image.
2 The stages of the process
2.1 Setting exposure
VueScan corrects most of the orange mask of the film at the exposure
stage by not exposing R G B equally (I'll use R G B instead of Red
Green Blue from now on). It exposes them in a ratio of R1 G2.5 B3.5
(figures taken from the VueScan User Guide). You cannot alter those
ratios.
As well as getting the ratio correct, it has to get the absolute value
of exposure correct, keeping RGB in those ratios. It calculates the
correct exposure from the preview. It looks at the image and sets the
exposure as high as it can without clipping R G or B.
Imagine a completely black opaque film (which is impossible, but never
mind). No light would get through that at all, however long the
exposure. For nearly completely clear film, it will be possible to
find an exposure long enough to make R (or G or B) reach the maximum
value a pixel can hold. If that exposure is given, clear film could
not produce a higher value, since maximum has been achieved already
with the nearly clear film. That loss of ability to distinguish a
nearly clear part of image from totally clear is "clipping" and
VueScan tries to avoid it in all of R G and B. So it increases the
overall exposure, preserving the ratio of R G B until clipping just
occurs and then backs off a bit.
If you Input| Lock exposure using a frame with a substantial
transparent area of film, VueScan will thereby set the highest
exposure that does not cause clipping in R G or B and so that value
can be used for the whole roll of film.
2.2 The Raw file
If you choose to output raw file with scan (on the output tab) then
the data is saved to the raw file exactly as it comes out of the
scanner. It has been correctly exposed (either following a preview or
because you have locked exposure). None of the remaining steps have
been applied to that data. Therefore you can experiment with all the
remaining settings by scanning from the saved raw file. I find that
extremely useful.
The remaining steps are performed on the raw data held in memory. That
raw data arrives there either by scanning from film in the scanner or
by reading in from a raw file you have saved on disk. I now make a
shot at describing those remaining steps.
In effect VueScan consists of two parts. The first part controls the
scanner by correctly exposing the scan and creating raw data. The
second part behaves as a dedicated image editor to "correct" that raw
data and generate an image. And for that part, the scanner is not
needed.
2.3 Film Base Color
The correction of orange mask by using the R G B exposure ratio is not
going to be perfect. So VueScan detects any residual color cast in
clear film and corrects it by setting film base color. Once it has
detected it, you can lock it for the whole film by Lock|Film base
color. Note that the film base color it will see is dependent on
exposure so it cannot be set until a preview (or lock exposure) has
been done.
Note also that when I say "lock it for the whole film" that applies
just as well to the frames on a raw file you have saved to disk. As I
said, all steps apart from setting the exposure can be carried out on
a saved raw file.
2.4 Film type
If you choose a film type other than Generic, VueScan can correct for
the way that film (e.g. Kodak Gold) responds unevenly to different
wavelengths of light. That is nothing to do with the orange mask –
film base color takes care of that.
2.5 Scanner response.
Scanners also have an uneven response. VueScan holds a profile of each
scanner type that indicates how that model of scanner generally
responds to different colors of light. You can also generate a profile
of your own particular scanner and get VueScan to use that. You will
see lots of posts on this Newsgroup about profiling scanners. Most
people don't do it though.
2.6 Color balancing (Color Tab on VueScan)
I am not going to go through them all and what they do. The VueScan
User Guide is pretty complete on that. The important thing to remember
is that you are trying to make black, grey and white look right. Then
the other colors should be reasonably good too. (If your image is a
sunset, you want to keep the color cast and Neutral is a better choice
than White Balance).
You can do that after a preview or even after the scan – just refresh
the display after making any changes. If you let VueScan do White
Balancing, it looks for what it thinks is supposed to be grey and
adjusts the image to make it grey. Exactly how it decides what is
supposed to be grey is something Ed Hamrick will quite reasonably keep
secret! You can also do Manual color correction and choose the grey
bit yourself. If you then look at the color tab, you will see that it
has indicated the R G B values that will be made to look to grey
(neutral R, neutral G, neutral B). It does not show those if you let
it do White Balance itself.
But if you refresh the display after White Balance or Manual or any of
the other options, and then Input|Lock image color, you will see that
it has set white points for R G B. That is because the way it
implements the correction of grey is by altering the white points in R
G and B. The white point for R, for example, is the input value of R
that will get converted to the maximum value of R displayed in the
image. How it actually calculates those values is another Hamrick
Software secret. But you can, if you wish, overtype them with anything
you wish and see what it does to the image. (Values at the high end
(close to 2) have little or no effect since all you are doing is
mapping maximum input R (for example) to maximum output R. As you
decrease R, say, progressively lower values of input R will maximise
output R, so the image gets redder. Similarly for G and B. This
applies until you reach zero, which, for some reason, VueScan seems to
treat as exactly the same as 2).
I hope it is now clear why Input|Lock image color is only appropriate
for images taken under the same lighting conditions. The values of the
white points in R G B are the result of correcting a particular image
color to grey. And that image color depends on the lighting conditions
rather than the subject of the photograph.
2.7 Practical application.
To get good colors, you can follow the advanced workflow as far as
locking exposure and then scan the whole film, outputting raw file
with scan.
Then you can scan from the saved raw file and follow the advanced
workflow steps to lock film base color.
Working now on the saved raw file, you can experiment with different
film types (particularly with old film, it is often not obvious what
film type it is) or generic.
You can White Balance (or manual or neutral etc) a frame at a time if
the lighting conditions are different from one image to the next. Or
you can balance for one image and then lock image color and apply
those settings to the other images that were taken under similar
lighting.
You can also experiment with the various filters for grain, faded
color and infrared cleaning (if your scanner has an infrared channel,
the infrared values are saved to the raw file if you selected 64 bit
RGBI on the input tab) when scanning from the raw file because none of
those things have been applied before the raw file is saved. Even if
you experiment with filters and color correction while you have the
film in the scanner, the raw file will still be raw and none of the
color settings or filters affect it provided you output raw with scan.
Right then. That's it. If it's any use, all well and good. If not,
just ignore it. If (unbelievably) you want still more, ask and I'll
try. Additions and comments from anyone are welcome of course. But as
I said at the start, no flames please!
Stephen
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