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Optimising the Scanner

Response Curve

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(Gamma-Gradation)

By Ian Lyons

 

A Computer Darkroom Tutorial

 

Scanner Gamma is sometimes referred to as Scanner Tone Response and most scanner applications will include a facility for altering it.   The following tutorial makes extensive use of LaserSoft SilverFast, however, even if you don't use SilverFast I recommend that you read through the tutorial and try applying it to your preferred scanner software. With most scanner applications gamma will operate in a broadly similar manner therefore most of what follows should be easily read across to offerings from Polaroid, Nikon, Minolta, Epson et al.

For SilverFast users who are still  unfamiliar with the term Gamma-Gradation (gamma for short) it can be found in a dialog labelled Defaults (click the Option button to open) and more specifically the General Tab.

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The SilverFast User Manual describes Gamma broadly as follows: This function adjusts the general brightness of the image for midtone and shadow. In general, a gamma of 1.6-1.8 is recommended for reflective media. For transparency, the gamma is normally higher than for reflective media with values between 1.8 and 2.0 being normal although values up to 2.2 are perfectly acceptable. Lower values will tend to result in a darker more contrasty preview than higher values. Choosing a higher value also means that the shadows will be more open than for lower values; the image a bit brighter, however, image contrast will tend to be lower. That said some experts do recommend values as high as 2.6 and higher.

Through experimentation I have standardised on a value of 2.0 for my Polaroid SprintScan 120 and 2.2 for my Polaroid SprintScan 4000 and Nikon 4000ED. This is not to say that the higher gamma values aren't equally valid just that I personally don't find them to offer any significant improvements to my scans.

The different values likely come down to how the scanner hardware is internally calibrated by the manufacturer. In my case choosing these values also means that an image will preview almost identically on each scanner. The next section discusses how we go about achieving the optimum Tone Response Curve (shortened to Tone Curve) or gamma-gradation value.

Note: the values quoted below for patch GS11 on IT8 targets are specific to transparency type from manufacturers such as Fuji, Kodak and Agfa. Some reflective type targets differ slightly and so the values quoted for GS11 should be read as GS10.

Determining the optimum Gamma-Gradation value

The SilverFast User Manual goes on to say: "Check that your monitor is set to the correct gamma in order for the tonal values to be displayed evenly. For best results scan a grey calibration strip and adjust your gamma accordingly". I think this is the point at which the wheels come off for a lot of folk. The impression being given is that there is a direct linkage between the scanner gamma and monitor gamma, which is not really the case. Your monitor should be calibrated in the normal way and independently of your scanner - a gamma value of between 2.0 and  2.2 is accepted as being a good approximation of human vision. Gamma 1.8 as used by Apple Macintosh computers is really only used for legacy images. Many Mac users (myself included) have standardised on 2.2 and that is now my recommended value for all systems. There is a lot of material and guidance elsewhere on this web site on how to calibrate the monitor so I won't repeat it here.

The purpose of scanning the grey calibration strip (you're better to use an IT8 calibration target for slide scanners) is to find a the ideal Tone Curve or gamma-gradation value for your particular scanner. The simplest approach is to adjust the gamma value so that the grey patch GS11 on the IT8 calibration slide has an average RGB value in the range 100-115. This means scanning the IT8 at different gamma values then selecting the one that is closest to the optimum. You should also find that the average RGB values for patches GS0 (white patch) is in the range 235-250 and GS23 (black patch) is in the range 10-25. In each case it is better to avoid values outside these ranges.

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IT8 Calibration Target

 

The following series of screen-shots shows the results of my effort to demonstrate the technique and should give you some idea as to how increasing the gamma value will cause all of the values from near black through to very light grey to become lighter. The actual preview image became progressively brighter but less contrasty as I increased the gamma value. The large peak around the centre is actually the grey boundary area of the IT8 and not patch GS11. You should bear in mind that nothing is set in concrete and if you prefer the results with a lower/higher gamma value then use it instead. However, before committing yourself to this value you might wish to scan an image that has real blacks, real whites and a good range of tones between then check the histogram. If the histogram shows any clipping of highlights or shadows you've chosen a gamma value that will likely cause you many headaches in the future.

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Gamma = 1.8

Gamma = 2.0

Gamma = 2.2

This procedure is akin to calibrating the brightness and tone response of the scanner and as with all calibration processes the idea is to bring the device to a predefined condition or setting and leave it there. All future scans should be undertaken using your preferred gamma value with specific image adjustments being carried out using the Colour and Tone editing "tools" provided with the scanner software.

The effect of Gamma-Gradation on IT8 Calibration

The procedure described above will have no effect on how the scanner reproduces colour. If the scanner reproduces colour badly you will need to take further steps to establish the cause of the error and then alter your workflow to eliminate it. The most common approach to obtaining good colour output from a scanner is IT8 calibration and LaserSoft offer this feature as an optional extra. However, if you already have a third party profiling application from companies such as Monaco Systems or Gretag Macbeth these will do fine.

Those of you reading this who already own a scanner profiling application such as those mentioned above will be wondering if the above procedure for determining the ideal gamma is either necessary or desirable. The simple answer is NO and Yes in that order! The reason it's not necessary is the IT8 calibration will always reproduce grey-scale patch GS11, etc as per the associated reference data file irrespective of what gamma value you choose to make the original scan with. In effect the IT8 calibration process will produce the optimum Tone Curve. However, should you choose to use an alternative gamma value at some future time and still retain the original calibration profile you should not expect images to match in terms of brightness and contrast. The calibration profile is specific to the gamma value we used when creating the profile. The following series of screen-shots show the histograms for an IT8 calibration slide that has been scanned using profiles created at different gamma values. Notice that they are identical; meaning that no matter which gamma value I choose to create the profile with the results are always the same; that is so long as I do not alter the gamma value afterwards. Unlike the examples shown in the first section IT8 effectively prevents the gamma value altering the brightness and tonal response of the scanner since they both become a function of the calibration profile.

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Gamma-Gradation = 1.8

Gamma-Gradation = 2.0

Gamma-Gradation = 2.2

So now we're left with why it might be desirable to preset the optimum gamma value. The reason should be obvious in so far as the less work the profile has to do to achieve a target value the less data is lost from the image as a consequence. A simple example should help explain: say we preset the gamma value to 2.2 - we see from earlier example for the SprintScan 120 that the GS11 patch will have a value greater than the desired value and so the profiling application must make a correction to bring it back to desired value. In making the correction some data will be lost and since a fair amount was lost when the original gamma curve was applied we can ill afford to loose more going backwards.

Also, it should not have escaped your noticed that once IT8 calibration comes into play the ability to adjust the placement of the mid-tone value is lost, however, the shadow and highlight values are more accurately placed, albeit fixed. The downside of this accuracy should be plain to see from comparing the the two sets of screen-shots. Each set used an identical slide (Fuji IT8 calibration slide) and we can see from the first set that the shadow and highlights are never in danger of being clipped. Once the IT8 calibration is activated we find that shadows in particular may be prone to some clipping. Using real world images I've not found shadow clipping to be a problem with either the SprintScan 4000 or 120 but I was less fortunate with my Nikon LS30. I think this was more a characteristic of the Nikon hardware than a failure of the profile or SilverFast. Modern 12 and 14 Bit film scanners such as the Nikon 4000ED are capable of rendering highlight and shadow detail in all but the most extreme cases.

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