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These days, most entry-level scanners are able to scan at 1200 dpi (pixels per inch) without even breaking a sweat. For not a lot more money, you can get scanners capable of scanning at 4000 and even 6000 dpi.
But remember, just because you can (scan at ultra-high resolutions), doesn't mean you should. The trick is to create only as many pixels as you need.
(Forgive me but I am going to use ppi (pixels per inch) uniformly on this page instead of constantly switching back and forth between ppi and dpi (dots per inch.)
What different scanning resolutions will give you - Let's first look at images scanned at different resolutions. All have been enlarged to better show preserved details.
At this resolution, practically every engraved line is visible. This is an excellent resolution to see details on certificates.
Oblique lines (like the angular edges of the stack and light) look a little jaggy at this resolution, but major picture elements are easily discernible.
Major details are completely visible, including all but the smallest printers' names. Individual engraved lines are gone at this resolution. When this happens, you often pick up odd, and perhaps undesirable, interference patterns. (Note the shadowed area of the building behind the stack)
All major details are very soft. At this resolution, details are gone, but so are the interference patterns. This resolution is too coarse to be considered 'high resolution' but is perfectly adequate for less intense illustrations.
Your purposes determine how much resolution you need when scanning. Your possible uses might include:
Scanning for printing in a publication - If you are scanning a certificate for printing in a book or magazine, you first need to discover the minimum number of pixels you will need. There is no point in scanning at 1000 dpi when you need only a fraction of that resolution.
Magazines will often give you guidance on their particular needs. They know you will only contribute a few images and they will probably need adjustment. Consequently, they usually allow you to scan at resolutions greater than they really need. When they receive your images, they will modify, retouch and re-size your images for their specific space needs. The high-resolution image you sent will probably end up much smaller, sharper and brighter prior to printing.
When publishing on your own account, you will quickly discover the need to tightly control your images. While tempting to scan all images at high resolutions, file sizes quickly become unmanageable. The closer your resolutions are to final printed resolutions, the fewer surprises you will have in your final product.
Let's say you have a stock certificate you want to place in a catalog similar to mine. You want the certificate to fill up the entire width of a column. What is the minimum resolution you REALLY need?
You need to discover three things: the resolution your printer will use, the width of your certificate and the column width. That's all.
My commercial printer has the capability of printing at 2400 dpi (dots per inch). Unless very near-sighted, most people cannot readily distinguish the differences between images printed at 200 dpi and those printed at 2400 dpi. For general purpose catalogs such as mine, anything printed at greater than about 200 dpi is overkill.
Next, get out your ruler. If your certificate is 11 inches wide and the column width is 3.4 inches, then the math is a simple two-step process. Your minimum resolution is the number of pixels needed divided by certificate width.
Pixels needed: 3.4 inches x 200 dpi = 680 pixels wide. (I am using width for this example, but you can calculate based on height if that is your crucial dimension.)
Minimum resolution: 680 pixels / 11 inches = 61.8 pixels per inch. Yes! That is all the resolution you need to put illustrations in a catalog like mine. I suggest you scan at a little greater resolution, maybe 100 to 150 dpi, but any resolution higher than that is overkill in ordinary, everyday printing.
A formula - From this example, we can deduce the relationship as:
Scanning for printing for photo album - Many collectors print copies of their certificates and save the prints in photo albums. Then they store their originals more securely. Their photo albums can get abused and their original certificates remain in good conditions. Prints can also serve valuable double duty as proof of ownership for insurance purposes.
Most of today's ink jet printers will print at 600 dpi. I stress once again that printing capability is far beyond the ability of the average eye to discern. A printed image of 300 dpi is perfectly adequate.
The math is only a little more complicated. Let's say your certificate is 12 inches wide. You want to print on a piece of photo paper that is 8.5 inches wide. Your printer can print to within about 0.5 inches of the edges of the paper, meaning you get to use 7.5 inches of the paper.
Pixels needed to fill up the width of the paper: 7.5 inches x 300 dpi = 2,250 pixels.
Minimum resolution needed: 2,250 pixels / 12 inches = 188 dpi. If you scan at 200 dpi and your image will look perfect. Of course, you can scan at higher resolutions. But why?
Scanning for printing for displays - If you want to print your certificates larger than real life, the math is still the same. In this example, I wanted to fill about 48 inches of wall space with an image. The original certificate was about 12 inches wide and our plotter is capable of printing at 300 dpi.
Pixels needed: 48 inches x 300 dpi = 14,400 pixels.
Minimum scan resolution: 14,400 pixels / 12 inches = 1200 dpi.
Web images. Scanning for the web is equally simple, except for one thing. You must come to grips with the difference between what you want to show and what you really need to show.
Controlling image sizes for the web is more important than for other purposes we've discussed. The larger your images, the more sluggish your sites will be. The smaller your images, the faster viewers will be able to see what you have to offer.
Ideally, you want small file sizes for images you want to show on a web page. It is difficult to accurately predict image file sizes before scanning. Therefore, it will help to know how scanning resolutions affect file sizes. A typical American stock certificate is about 8" high and 12" wide. A scan of such a certificate will require this much space (in megabytes) when saved in popular formats. (Be aware that TIFs CAN NOT be used for web display but are used in many popular programs including AutoCad, Photoshop, PhotoPaint etc.)
* - medium quality
** - using LZW compression
Typical collectors scan certificates for two major uses on the web
Sales. Whether selling on your own store site or through online auctions, the basic idea is the same: make images large enough to appear attractive and small enough to load quickly. There is one basic truth on the web: the longer people are required to wait for images to load, the more likely they will go elsewhere.
While there are several competitors, eBay pretty much controls the online auction scene. eBay understands the necessity of showing images quickly. Consequently, eBay will display 50 to 200 thumbnail images within seconds of shoppers conducting searches. Anyone who is interested in particular items can then click on tiny images and quickly view larger images. Although larger images take longer to view, images sizes are well researched and planned to give viewers the best balance of information and speed.
These days, eBay allows sellers to post larger images than a few years ago. eBay currently recommends images be between 500 pixels an d 1,600 pixels in the longest direction, but says images can be up to 7 megabytes in size. (Talk about overkill!)
Calculating your needed resolution is simple. If you have a typical 12 inch stock certificate, then
Pixels needed: 1600 pixels
Minimum resolution: 1600 pixels / 12 inches = 133 dpi.
I have looked at tens of thousands of eBay images and I assure you that no certificate needs to be scanned at resolutions greater than 150 dpi. You may scan at greater resolutions, but understand that you will worsening the experiences of your potential buyers. Be careful.
Website or informational blog. The same principles work here. Figure out your maximum width in pixels, and then scan for that width.
As discussed in Pixels vs dots, monitors display pixels. So, the only thing you need to learn is the maximum width available for your image.
This particular page you are reading is designed to display at 725 pixels wide. It will fully fit on any screen that is 12" wide, set at a meager resolution of 72 dpi. There are three columns on this page and the widest column will accommodate any image up to about 600 pixels wide.
In real life, this certificate from the Blue Ridge Railroad is 15" wide, but I only want 542 pixels for display here. I have two ways of accomplishing that with scanning.
I can either scan the certificate at 36 dpi (542 pixels / 15") or scan it at any handy resolution and reduce it to 542 pixels (the quickest way.)
I define archiving as saving images of certificates for some unknown future use. I think this is the hardest possible scanning task. You don't want to scan at low resolutions, because low resolutions will restrict your future usability. You don't want to scan at overly high resolutions because large images require huge amounts of storage space.
Realize that you may never need any of your images in the future and you will probably never need all your images. Consequently, 300 dpi seems like a reasonable balance of resolution and storage space. If you can afford large amounts of hard drive space or stacks of DVDs, you can scan at 400 dpi. Just remember that 400 dpi images require 78% more storage space than 300 dpi images.
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