Scanning Negatives with Vuescan, Part 1: Scanning in General

0. Introduction - The Problem

Scanning of color negative film (henceforth referred to as CN) is generally perceived as being more difficult than scanning color slide film. The most obvious among several possible reasons is the ability to directly compare a slide with the resulting scan. In order to do this with high precision, a scanner may be profiled. This means comparing the scanned image of a standardized color chart on slide film, known as an IT8-target, with a file containing the ideal values of the individual color fields.

For CN this is not possible, in particular, no IT8-targets on negative film are available. But there are further complications:

- the exposure range for CN material is considerably larger than for slides. Hence one has to either forgo details in the shadows and in the highlights, or one has to accept reduced contrast.

- CN film by default has an orange mask. This has to be removed, either while scanning, or afterwards.

Often the software of scanner manufacturers, but also programs like Silverfast, attempt to minimize the number of choices for the end user. This can lead to good results, if the motifs are of a standard nature and your ambitions are limited by the quality of prints from a regular lab.

Vuescan takes a somewhat different approach by offering the user full control for all settings, and by trying to extract a maximum of information from the negative even when largely automated standard settings are used. Hence scanning negatives with Vuescan may initially produce results that are not quite satisfactory.

The first part of this tutorial is intended to
- clarify the way Vuescan works;
- help finding settings that yield more satisfactory results;

On the other hand, this tutorial is _not_ intended
- to show the one and only or the best way to scan color negative film. It is the result of my personal findings when trying to make use of the whole dynamic range of CN film.
- to be a user manual for Vuescan; instead, it presumes some familiarity with Vuescan's supplied manual and the knowledge which questions are covered where;
- to serve as a guide for purchasing a scanner;
- for photographic novices.

1. Prerequisites

For the first part of this tutorial we need
- a camera that allows time and f-stop to be set manually, and a basic understanding about the interplay of these parameters;
- color negative film;
- a gray card or an IT8 camera target;
- a film scanner or a flatbed scanner with transparency unit;
- a recent version of Vuescan, preferably the professional version (I do not know the restrictions of the standard version);
- to select 'More Options - All' in all tab-pages of Vuescan;
- to be familiar with the basic use of Vuescan.

2. How does Vuescan scan negatives (automatically)?

The basic settings for scanning negatives are found in the Device Tab. Selecting 'Media Type - Color negative' determines how the program proceeds.

Vuescan will then attempt (scanner permitting) to raise the CCD-exposure of the green and the blue channel, in order to at least partially compensate for the orange mask.

In purely automatic mode Vuescan will determine the correct CCD-exposure during prescan. Directly after the prescan, the program evaluates 'Film Base Color', the color of the carrier material, in order to remove the remaining orange mask. Then the image is inverted, and according to the selected brightness and color space a gamma correction is performed and the colors are balanced.

2.1. What do the entries in the Color Tab mean?

Vuescan offers some parameters to influence color and brightness even in automatic mode.

- Black point (%) and White point (%) are available in all "Color Balance" modes except "None". They determine, which percentage of pixels are to be regarded as "Black", respectively, "White" (where the meaning of "Black" and "White" may depend on the selected color balance, see below).

Actually, selecting the value 0 in these fields does _not_ mean, that the values of "Black" and "White" are not assigned to any pixels, but rather only to those pixels of minimal respectively maximal brightness.

The default settings are 0 for the Black point and 1 for the White point. This means that about 1% of the picture's pixels (starting at the bright end) are interpreted as "White", the remaining 99% of the pixels are then spread out with those of minimal brightness being "Black".

More precisely, Vuescan generates a histogram of the image that for each brightness value records the percentage of pixels at least as bright. Now the largest brightness value such that at least 1% of the pixels are this bright or brighter serves as "White". ("Black" is handled accordingly. Hence for a picture where all pixels have the same brightness, "Black" and "White" agree.)

The default setting provides a rather elegant method of handling differently exposed pictures. No matter how bright the brightest pixels are, they will be assigned the value "White". Dark pixels are treated correspondingly.

Values different from 0 may cause some clipping at the corresponding end of the spectrum, and a loss of details, if more than just the darkest and brightest pixels are assigned the values "Black" and "White", respectively. This can nicely be demonstrated in pictures with bright clouds.

What exactly happens with the other colors is determined by the various 'Color Balance'-algorithms, which Ed Hamrick of course does not publicize ;-) Balancing colors is a field that has seen a lot of research.

Easiest to understand is 'Color Balance - Neutral': all color values of the picture bounded below by "Black" and above by "White" are then evenly spread across the whole interval from 0 to 255=2^8-1 (in case of 8 bit color depth). In particular, this leaves the ratios between Red, Green and Blue for each pixel intact, and gray pixels stay gray.

'Auto Levels' is also quite simple: disregarding the values "Black" and "White", the interval [r_min,r_max] is evenly spread across the whole interval from 0 to 255. The same occurs independently for Green and Blue. Hence pixels with maximal color values in all three components end up as pure white, while those with three minimal color values end up a pure black. Notice that gray pixels need not stay gray with this procedure.

- Settings for 'Neutral red', 'Neutral green' and 'Neutral blue' are only available when 'Color Balance - Manual' has been chosen. Unfortunately, they do not have an intuitive interpretation. However, they are not intended for being set manually, instead they can be set automatically by right clicking with the mouse in some neutral area of the picture that is not too dark. This internally adjusts the White point to make the clicked area appear neutral gray.

- Contrary to some image manipulation programs, 'Brightness' does not simply shift the brightness values, but rather serves as a multiplier for the Gamma value of the selected color space. Hence it has almost the same effect as the central slider in Photoshop's Levels dialogue. "Almost", since Ed has implemented things slightly differently, which may cause strange effects for extreme settings.

- 'Brightness red', 'Brightness green' and 'Brightness blue' act in the same fashion for the individual color channels. They are very useful for adjusting mid tones. E.g., the Gamma value for red is obtained by multiplying the current Gamma value for the color space (e.g., 2.2 on PCs) with 'Brightness' and with "Brightness red'.

- 'Negative vendor', '-brand' and '-type' are best set to 'GENERIC'. My Nikon LS40 produces a red cast for all CN material, even if the results differ widely in other respects. Moreover, all other settings seem to result in a limited tonal range.

2.2 Problems and workarounds in case of the automated approach

Vuescan makes a few assumptions when scanning CN material automatically:

- The correct determination of 'Film Base Color' requires the presence of some unexposed film in the scanned area. For this it suffices to include some part of the film separating the exposed areas in the scanned area.

- This, however, may affect the color balance. Unexposed regions will be set to "Black". Since dark areas in the exposed region are seldom as dark as unexposed film, they could then appear too bright.

To avoid this problem, Vuescan offers the setting 'Buffer (%)' in the Crop Tab. It indicates, how large a margin just within the crop box is to be ignored for the purpose of balancing colors. Since Vuescan shows the mouse coordinates in the foot line, one can estimate the size of 'Buffer' that will restrict color balancing to the relevant portion of the image.

- in any 'Color Balance' mode except 'None', 'Manual' or 'Neutral' Vuescan may badly misinterpret a picture. If no white is present, it is rather difficult to perform a proper white balancing. Possibly the red shades of a sunset will be "corrected" towards gray. The only way of avoiding this is to stick to the modes 'None', 'Manual' or 'Neutral' and to give up part of the built-in automatism.

3. Manual procedure.

You always need a bit luck to get the desired results if you rely on the automatics. There are several reasons for this.

Wrong film base color:
- The unexposed edge of the film border is not scanned.
- The negative is overexposed and very dark (this kind of negatives will play a big role here). Vuescan increases CCD-exposure and overexposes the film border. This gives wrong values for film base color.
- Very bright lights at the image border (sun) or an adjacent very overexposed image cause blooming  of the film border.

Color balance:
- white balance is not possible if the image is mostly the same color (f.e. sunset).
- very high CCD exposure causes color shifts.
- film border is outside the scanned area (wrong 'Buffer' value)
- different areas have different main color.
- there are color casts in mid tones and highlights due to the elimination of the orange mask.

3.1 Determination of film base color

In the manual you find a short description how to determine film base color and use it for the whole role of film.

You need the unexposed, clear film leader. Scan it for preview, check 'Lock exposure', preview again and check 'Lock film base color' (Input Tab)

The determined values show up in the Color Tab. The double preview is necessary because one set of film base color values is only valid for one certain CCD exposure value.

You can avoid doing this procedure over and over again if you note down the values and enter them manually or if you save a raw scan of the film leader to disk (pro version only).

To use the same CCD exposure for different negatives should be no problem for most film scanners. The maximum density of CN film is about 2.8 and thus well within the usual density range.

In the pro version you have another possibility: Vuescan knows two ways of saving raw data. If you choose 'Raw output with Scan' in the Output Tab, the data is written to a TIFF file as it comes from the CCD. If you choose 'Raw output with Save' infrared clean and grain reduction are performed as chosen in the Filter Tab. If you choose a bit depth of less than 48 as 'Raw File Type' a gamma of 2.2 is applied, otherwise the data is written in gamma 1.0 space.

Raw scans can be done with different CCD exposure. Nevertheless you can apply the same film base color values if you choose 'Scan from Disk' later.

There is a small risk if you do that: If CCD exposure and film base color values are not related linearly you might get wrong results. Film base color values too low for a particular CCD exposure cause clipping - you might loose shadow detail.

If you like you can determine film base color values for different CCD exposure, put them in a spreadsheet and draw a diagram in order to see how the values are connected. This is described in detail in part 3 of this tutorials. It is a good way to determine optimal CCD exposure.

Correct film base color values prevent color casts in the dark regions of the result image.
 

3.2 Define color balance.

Color balance can be locked much the same as film base color. You partly loose tolerance of CN material on different lighting this way. Tungsten light will be yellow-orange as it is the case for slides, too. You can make corrections for especially this case of course.
 

3.2.1 Preparation

A general setting of color balance requires some preparation. You need a gray card, a piece of white paper and something dark non-reflecting black (f.e. the inside of a lens blind) or a camera type IT8-target.

On a sunny early afternoon preferable with some white clouds you shoot the the three objects together. Use the gray card for setting film exposure and add the paper and the black thing later.

If you shoot an IT8-target do this from a tripod and have the edges parallel to the image edges.

In both cases the objects should not reflect and the IT8-target should be lit evenly. This makes it easier to use it for profiling later.

It is a good idea to bracket some exposures. Because of the high dynamic range it should be enough to use steps of 3 f-stops. CN tolerates overexposure but not underexposure. I recommend to use 6 shots with  -2EV, 0EV, +2EV, +4EV, +6EV, +8EV

Use time and f-stops to vary exposure in order to not get long exposure times (> 1s). Long times can cause color shifts (Reprocity Failure or "Schwarzschild Effect").
 

3.2.2 Scanning

Scan all images with the same ccd exposure and film base color. 'Color balance' should be 'Manual'. Set 'Grain reduction' in the Filter Tab to 'Heavy' in order to minimize effects of grain and noise. Set 'White Point (%)' to 1.0 (there should be a big white area) and 'Black Point (%)' to 0.1 (black is present, too).

I recommend to start with the +/-0EV image. A right click on the white area (paper or patch 0 of the IT8 gray scale) sets the 'neutral' colors. This eliminates a color cast in the highlights.

Vuescan shows the RGB colors under the mouse pointer in the footer line. The value for the gray card or patch 11 of the gray scale should be pretty neutral. The values for R, G and B should be equal or near to each other.

If this isn't the case you should adjust 'Brightness red' and 'Brightness blue' in order to get the values for R, G and B almost identical. Don't change all three because this changes resulting gamma only.

Note down the found values. I refer to this values henceforth as relative values because they change color balance but contrast is still controlled automatically.

This changes if you check 'Lock Image Color' in the Input Tab. After that the Color Tab shows the absolute values for black and white point. Please note down this values, too.

Finally you take the other shots of the exposure bracketing and adjust 'Brightness' (Color Tab) in order to get the same RGB values for the gray card respectively patch 11 of the gray scale as for the +/-0EV shot.
 

3.2.3 Usage

You can use the found values in two ways:

- The absolute values for black and white point (with 'Lock image color' checked) reflect the object contrast of the IT8-target respectively the gray card image. This way you can produce images with realistic contrast out of the bigger contrast range of the film. With the data for the different exposed images you can extract and correct images of different exposure (shadows, mid tones, highlights) from one negative.

- The relative values ('Lock Image Color' not checked) cause the calculation of individual black and white points for real world images. Results will possibly be washed out if 'White Point (%)' and/or 'Black Point (%)' are set to 0.0 because the whole dynamic range of the film is used. Color balancing only takes place in the range of the IT8-target resp. gray card image.
 

3.2.4 Possible problems

- Due to development film colors may vary between different roles of the same film type. This is most likely the case if film base color changes a lot from role to role. To test that you first must determine the tolerance of the scanner. This is done by scanning the same piece of film leader several times with locked CCD exposure (see 3.1) and comparing the values for film base color. Then you can check different roles developed from the same lab to see how big the differences are.

- While shooting there should be not colored light present. Colored sunlit objects change the light color. To avoid influence of blue skylight for example you can shoot in a dark room where only the objects are lit by the sun. Or you use a day with thin white clouds all over the sky. Be aware of windows glass - it is light green most of the time.

- Even id you use 'Grain reduction' the pixel values vary in an even colored patch. Move the mouse a bit and see how the values change. perhaps it is a good idea to use averages

- The gray card must be real neutral gray. If it has a visible color cast and you have a calibrated monitor and viewing environment you can try to reproduce the color with your settings. If you have a calibrated scanner you can compare the values with those of a direct scan. In this case the procedure is a bit different than described in 3.2.2 (see below).

- The gray scale of an IT8-target is not necessarily neutral. The measured values are stored in the IT8 data sheet you got with your target. You can use Vuescan (Pro?) to create an artificial IT8-target out of this data for comparison. Go to Input Tab, select 'Scan task - Make IT8-target' and in Color Tab 'Color Balance - None'. After a preview you can read the values under the mouse pointer. Try to adjust the values for your shot image to these values.

Brightness of a scanned gray card or a generated IT8-target might be different of a shot one. If you want to adjust mid tone gray to a particular set of RGB values do it as follows: Adjust 'Brightness' only to get the green value right. Then adjust 'Red brightness' and 'Blue brightness' in order to get the other values right. Then set 'Brightness' back to 1.0 or whatever you need.