Collectible Stocks and Bonds from North American Railroads     by Terry Cox

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Auto-exposure vs manual exposure

Most scanner manufacturers design their software with default settings that work adequately well for average uses. Defaults, of course, work less well for non-average uses. Unfortunately for people who want to scan certificate, certificates are NOT average.

Why? Because certificates usually have printing (or handwriting) on both sides, so show-through is common. Words are usually engraved with very fine lines, so letters are usually thinner and less dense than in normal typewritten documents. Vignettes are made up of very thin dark lines, instead of shades of gray.

The software supplied with most scanners normally allows users to adjust numerous image qualities. Unfortunately, the software provided with entry-level scanners often makes it difficult to discover how to alter settings. Even help screens don't help much.

To show you a few capabilities hiding in your scanner, I scanned this certificate with a Hewlett-Packard scanner priced about $100 above entry level. I scanned this certificate against the white scanner lid.

This image certainly appears acceptable. If you want to be picky, though, look more critically at the vignette and you will notice the woman's face and body are a little dark. The gray panel behind the company name is also a little darker than it should be.

After searching for an "exposure" setting, we find a screen like this.

Many people with scanners have seen screens like this and almost everyone ignores them because they don't know what to make of the little graph.

Let me explain. It is really not a big mystery.

The graph is a "histogram" that is 256 pixels wide with each pixel representing the number of shades of gray the scanner can distinguish. (In case you're curious, human eye can distinguish about 30 shades of gray.) Total black is on the left. Total white is on the right. Everything in between are shades of gray. The height of the graph measures the percentage of sample points (pixels) at each given gray level.

What does all that geek speak mean? It means there are few total black or total white sample points. The scanner 'sees' the certificate almost entirely in terms of shades of gray.

The two lines (red on the right and magenta on the left) are where the software calculated the white and black points ought to be. The software is saying that it will calculate the proper exposure based on all the points in between the two lines and will ignore the remainder. The scanner will interpret everything left of the magenta line as black and everything right of the red line as white.

Of course, since we collect certificates, we know that much of the certificate (all the lettering and most of the vignette) is total black. The scanner resolution is set too coarse to actually detect much genuine black. I 'sees' the vignette as shades of gray instead of pure black and pure white.

The little sliders below the graph (labeled 'Highlights' and 'Shadows') will alter what the certificate 'sees' as black and white. In other words, the scanner software allows us to force the scanner accept or ignore whatever we want it to.

Does this seem too technical? Well, your eyes do the same thing. Walk out on a bright snowy day and your eyes are overwhelmed by the extreme brightness. Your eyes ignore all the white by closing your iris down to pinholes. Conversely, let your eyes get accustomed to a dark night and your irises open wide. Your eyes will not be able to perceive colors well at either extreme.

Below the 'Highlights' and 'Shadows' sliders is another slider labeled 'Midtones'. The software is telling us what it will assume is a medium gray. By moving this slider, we can force the software to show us more detail in either the lighter gray tones or in the darker ones.

By default, most scanner software is set with a 'midpoint' (or 'gamma') in set to 2.2. It turns out that a gamma of 2.2 looks good on Windows monitors, but a little dark on Macintosh monitors.

By simply moving the small 'highlights' and 'shadows' sliders (also called 'white' and 'black' in some software), we can adjust how the scanner "sees" the certificate.

Using the graph as a guide, I made a slight adjustment and moved thin red and magenta in toward the edges of the big gray hump in the middle of the graph. I also modified the midtones slightly by changing it to 2.5.

Here's the result.

Granted, the average person will not see much difference. However, knowing what we've done here, you will hopefully notice that there is more detail in the gray areas of the vignette. The woman's body looks less dark and we can perceive an attractive improvement in the gray behind the company name. Notice also that there is slightly more detail in the wide brown borders. As you can appreciate, larger images shows improvements more clearly.

What if your scanner will not allow adjustments?

Regrettably, some scanners do not allow exposure adjustments. Many are overly hard to use. Thankfully, there is often excellent third-party scanning software that will drive your scanner much, much better than factory software. Check out Silverfast.

Even without third-party software, hope is not lost. Get some kind of photo/image software that allows you to manipulate images. There is almost always free image software included with most scanners. Buy an instruction book on image manipulation and then experiment. Believe me, even if your scanner precludes much pre-scan improvement, you can make tremendous improvements to the final scan with very little effort.

Now, let me show you what you can accomplish with image manipulation software if you will simply ignore your fear.

Here is an image I grabbed from an ebay auction. (Serial number removed.)

And here is an example of how I was able to improve the image in a mere 12 seconds, including the time to save the file!

The main programs to accomplish such modifications are Adobe Photoshop®, Corel PhotoPaint®, and Jasc PaintShop®, plus various light versions of the same software. I firmly believe that you get what you pay for. However, if you don't think your needs are tremendous, then look around for free versions. Search Google for "free image editing software."

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(Last updated November 15, 2015)


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