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SilverFast AI & HDR

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Tutorial - 8

Basic Image Editing - Histogram

By Ian Lyons

A Computer Darkroom Review

This tutorial will attempt to help the new user to better understand the range of colour and tone editing tools to be found within SilverFast. I don't intend to give a detailed account of each tool but simply highlight those that appear less than obvious/friendly to many SilverFast users.

 

Editing the Prescan in SilverFast Ai and HDR

This page provides a quick overview of the toolbars and deals with the basic tonal correction tools, Histograms, White and Black Point tools, and finishes with the Auto Colour Correction tool.

Know your tools!

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Adjustments Toolbar

 

B1

B2

B3

No Colour Management Enabled

Colour Management via Embedded ICC Profile

ColorSync/ICC Colour Management Enabled

Variations of the Auto Adjust Tool

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Special Functions Toolbar

Assess the Image

After pre-scanning the image it is generally helpful to establish the tonal range of the original and the location of the darkest and lightest pixels. Using the Black or White point tool B4 located at the bottom of the Special Functions toolbar we can easily find dark and light pixels. The location is identified with a red circled cross-hair. If there is a large numbers of equally light pixels at this point then pressing the White button will cause ALL of these pixels to turn BLACK. Large numbers of equally dark pixels in the image will show as WHITE when we press the Black button. I find this a much more intuitive approach than Photoshop's clipping display since it only effects those pixels that are likely to be of interest.

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SilverFast at Work (Figure 1)

As shown in following screenshot we can also see the actual RGB/CMYK/Lab values for these pixels in the Densitometer window. The group of numbers shown circled in red represent the original values. The numbers on the right represent the corrected values. Obviously since both the left and right number groups are equal this screenshot represents the image before any corrections have been applied.

Densitometer

Densitometer

If a single densitometer was all that we got then there would be little to celebrate as the information and location of the dark and light pixels would be lost as soon as we clicked another area of the image with the mouse tool. However, a really neat trick is to lock the location of these pixels on the pre-scan and provide a dedicated densitometer - How? Easy! - Hold down the Shift key and click on either the White or Black point tool and presto a new dedicated densitometer is created. The location will also be displayed on the image by a small cross-hair with an associated number

Fixed Point(s) Densitometer

Furthermore, we can open the Levels or as LaserSoft call it the Histogram at any time and see the actual spread of data from dark to light for each of the channels (RGB, CMYK, etc.). The following screenshot shows the ideal method of display for best practice colour editing (i.e. all three RGB channels in view). Here we can see the 3 colour channels.

Histogram

Histogram - No Adjustments

What does the above histogram tell us? Firstly that the sample image is not full-scale, i.e. the image has no pixels at level 0 nor 255. We can also see that the shadow and highlights are not completely neutral. So the histogram provides us with confirmation of what the Black/White point tool and densitometer was indicating.

Should I use Auto Adjust?

Lots of  SilverFast users shy away from the Auto Adjust B5 tool thinking that it is going to damage their images. Maybe it will and maybe it won't - but the USER has control. By getting a basic understanding of how Auto Adjust works we can save ourselves a lot of agony. LaserSoft have provided an exceptionally powerful tool and we should learn to use it. Note that in the screenshot shown below I show Auto Contrast as OFF - this is my normal approach - others are free to choose otherwise (Experiment!). The settings shown inside the red boundary are the KEY to ensuring that minimal data is lost whilst getting the best from a well exposed and normal toned original. Read Chapter 5 of the user manual, read it again, then configure the Auto section of the SilverFast Defaults window correctly and you won't be sorry.

Tips

  1. setting the highlight offset to 0 is not a good idea since the highlight pixels will be clipped to level 255 upon application of Auto Adjust.

  2. setting the shadow offset to100% is not a good idea since the shadow pixels will be clipped to Level 0 upon application of Auto Adjust.

Auto Adjustment Values

Auto Defaults (Figure 2)

The Histogram

The following screenshot shows the histogram for my sample image after pressing the Auto Adjust button (using the settings I suggested above). Simply pressing the Auto Adjust button will mean that the image highlight and shadows will be optimised such that the spread of image data is more evenly distributed (figure 3). Notice how the shadow and highlight points in the histogram are all equal which means no colour correction has been applied to the image; I'll explain why in a moment.

Auto Adjust Histogram

Histogram after Auto Adjust (Figure 3)

We can also see from our previously created densitometers that the values for the dark and light points have changed after applying the Auto Adjust tool.

If the original pre-scan was assessed as being dark/light by SilverFast then an appropriate correction will automatically be applied (figure 4). In the screenshot shown below I show an adjustment of -4 (brighten the image). Positive numbers will darken the image.

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Histogram after Auto Adjust (Figure 4)

Important note about Auto Adjustment tool!

For SilverFast users with IT8 calibration or a customised ICC scanner profile SilverFast will ignore any colour casts it finds in the pre-scan and so Auto Adjust will simply correct the tonal range of the image. If on the other hand you are using SilverFast without the benefit of calibration then SilverFast will automatically remove any colour casts found in the image (not a good idea for sunsets). The user can preset the amount of automatic colour cast removal by inserting a value into the Color Cast Removal popup shown with a green asterisk (see Figure 2 above for an example).  The default value of 100% will completely remove the colour cast an a value of 0% will leave the cast untouched. Alternatively we can hold down the Shift key when pressing B1 Auto Adjust and the colour cast will be retained in all its glory. We can identify whether SilverFast will or will not automatically apply colour correction by checking which form the Auto Adjustment (aperture symbol) takes in the Adjustments toolbar (see Auto Adjust tool variations on page 1).

Colour cast removal can be fine-tuned at any time using the Color Cast Removal slider. The screenshot below shows how smart SilverFast is at removing colour casts using the Colour Cast Removal slider. I push one slider and SilverFast decides which channels, if any, require adjustment to neutralise the cast. As mentioned above; no colour cast removal is Zero % and complete colour cast removal is 100%. In the example below I show 75% of the cast has been removed. Remember not all colour casts are bad, some are an inherent feature of the image (sunset).

Colour Cast Removal

We can also manually adjust the vertical sliders for each channel by simply grabbing each in turn with the mouse or as a group (highlight, shadow or mid tone groups) by grabbing the appropriate triangle directly under the histogram.. Experiment in this dialog and you will learn lots of neat tricks to refine your images.

So how much damage has been done to my Image?

Are you worried about trashing important image data? Do you need to know how the image data will appear in Photoshop? With the Levels Histogram open simply hold down the Alt/Option key and watch the data in the histogram spread. It's probably better to use the combined histogram for this test as it will more likely be comparable to the Photoshop levels dialog.

Before

Histogram for Uncorrected Image

After

Histogram for Corrected Image

Notice that through careful choice of values for the Shadow and Highlight Threshold in the Auto Defaults dialog (figure 2 above) I have managed to ensure that no clipping occurs at the end points. In this example the end points were defined as 98% for the shadow which equates to Level 5 and 2% highlight which equates to Level 249. If we can manage to avoid clipping at the scan stage we can be confident that saturation of individual colour channels has also been avoided 

Note that any spikes (there might not be any) in the histogram represent values that have been rounded (made the same) and usually results from the high to low bit conversion. Gaps in a histogram represent data values that have been lost. In simple terms pressing the Alt/Option key when viewing the histogram will represent the 8-bit data being output to Photoshop. It's also important that we remember EVERY edit and conversion will loose data. Even applying the basic and very necessary gamma adjustment to the raw scan will loose data.

Conclusion

So what have we learned from this tutorial:-

  1. The basic tools

  2. How to assess the Pre-scan

  3. Find and store the location of darkest and lightest pixels

  4. Configuration of the Auto Adjustment tool

  5. How to use the Levels Histogram tool

  6. How the maximise the range of the scan

  7. How to brighten/darken the scan

  8. Automatic and manual Colour Cast Removal

  9. How using the Alt/Option key and Levels Histogram we can preview the "edited data" being sent to Photoshop.

 

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