Caution when resizing

I will use the term "dpi" (dots per inch) on this page, full well knowing that the proper term should be pixels per inch ("ppi") when talking about images viewed on monitors, tablets and smartphones. I do this because scanning software usually uses the term "dpi."

Users have two ways to change display sizes of image files. They can scale images during the scanning process or they can use image editng software. Editing software is preferred because it can accomplish dramatically more than mere scaling. However, many users don't have software and believe there scanners are adquate. I warn that results can be baffling.

Warning: beware when changing sizes with scanning software

I ask contributors who own scanners to help increase the knowledge of railroad stocks and bonds by sending images of certificates they own. I ask them to scan certificates at full size and save them as 300 dpi images. I suggest 300 dpi because that is a resolution sufficient to read printing company names and other small details.

Contributors have occasionally sent images at very odd resolutions. I found that those images often converted to 300 dpi precisely, so it took a while to reason that image resolutions must have altered at the time of scanning. It took another while to determine how that had happened. Ultimately, it appeared contributors had changed the output sizes of images with their scanners which then affected the resolutions of the files they sent.

Why change sizes of images at all?

This question also took a while to answer. While I cannot say I have determined the exact reasons, my working theory is that those contributors did not have image editing capabilities, but wanted to print certificates at specific sizes. For instance, if they had a large bond and wanted to print a copy to fit on a smaller piece of paper, they could tell their scanner to reduce the image to some specific size. Images of small stock certificates and related documents could be enlarged in similar manners. 

How did the dpi change?

Practically all scanning software allows users to scan at a specific resolutions (dpi). Some programs go further and allow users to output files at specific image sizes. Let's say someone has a bond that measures 9" x 13" and they want to print a copy for their insurance records. They set their scanner to 300 dpi and tell their scanner to output the image at 5" x 7" so it will fit easily on 8½" x 11" paper.

Man trying to understand re-sizing of images (Photo courtesy Ba Tik from Pexels)

One would think that the final image would come out at 300 dpi. Like me, they would be wrong. It can be frustrating.

Actually, the scanner captures the entire image at 300 dpi which means it samples color information every 1/300th of an inch, both vertically and horizontally. The software takes that information exactly as it comes from the scanner and does not change it in any way. The software merely records information in the file that says the image size is 5" x 7". The scanner would have captured 10, 530,000 pieces of information (2700 x 3900 points) and the scanner software decreased the size by 53.8%. The net effect of squeezing all that information into half the space made the resolution 557 dpi when the user was expecing 300 dpi. Look at the file properties and the file says exactly that.

So what is the problem with having the scanner resize the output?

There is no problem as long as scanning software gets amnesia every time it is turned off. Conversely, if software records scanner settings, users may not remember the settings they used the last time they scanned. That is when they end up with unpredicted results.

Most of the the scanning software I have used remembered settings from session to session. I currently scan with four programs and three save all my settings from one scan session to the next. One program has so many "professional" settings that it sometimes takes a few minutes to determine whether my old settings are still what I need. That program has tricked me with unexpected results so many times that I now try to re-set everything back to "basic" before turning it off.

The problems that I have seen from contributors revolve around discovering results different from what they expected. Users get surprised when they scan at 300 dpi, tell the scanner to reduce, check the resulting files, and discover higher than expected resolutions. As long as resolution is higher than expected, no harm is done. Their files may be larger, but their image qualities will be equal to better than expected.

Conversely, if they ask the scanners to enlarge, their qualities might end up much worse than expected.

My warnings really apply to letting scanners do the enlargements

Imagine someone wanting to enlarge an 8" x 11" stock certificate to 11" x 17" paper. They scan at their normal 300 dpi. 17" is 54% larger than 11", so the scanner will fit the same number of sample points (3300 in the long dimension) into 17". The user might expect the final resolution to be 300 dpi, when, in fact, it is really 195 dpi. That might be okay, but then again, maybe not.

Let's imagine a collector wants to print an image of the same stock certificate image on a 24" x 36" poster. The collector scans at 300 dpi and has the scanner enlarge to fit on a poster. (Okay, few scanner programs will enlarge to poster size, but bear with me for the sake of this example.) The collector sends the file out to a local service to make a glossy print and mount on foam board. Under ideal circumstances, the printer will call back and ask, "Are you sure you want go ahead and print this? It's going to look a little rough." What happened was that the scanner's enlargement gave an unexpected result. To calculate, the long dimension of the scan was 3300 pixels at 300 dpi. If the poster is 36" long, then the resulting print would have the same quality as a 92 dpi image (36" ÷ 3300 = 91.7). That is a low resolution for a photographic print.

The conclusion

It is important to be able to predict final outcomes when asking a scanner to re-size images. Otherwise, surprises will occur when files are printed. 


Enlargement - The first set of images below shows the effect of enlarging an image during scanning. The information gathered at each sample point (pixel) remains the same, just assigned to a larger area. A printer will smooth out the existing information, but cannot add any information between points. If not planned properly, an image enlarged without forethought might appear terrible up close. The further a viewer moves away from the image, the better it will look. That is acceptable if intended, but many users get surprised by such results. I prepared the examples below for viewing on computer monitors, tables and smartphones. Up close, viewers can see individual pixels. Move further away and the images become more  and more understandable. At a substantial distance, even the content of the rightmost image can be deciphered. In general, letting your scanner enlarge more than 2x is probably a bad idea.

Original at 600 dpi Native pixels 600 dpi dpi Image enlarged 2x Image enlarged 4x
Vignette of Mercury scanned at 600 dpi Native pixels of Mercury vignette scanned at 600 dpi 600 dpi pixels enlarged 2x 600 dpi pixels enlarged 4x


If enlarging, whether through software or photo editing software, it is advisable to increase resolution before scanning. To calculate how to set your scanner:

  • determine the final resolution you really want
  • calculate the scaling factor: divide the longest dimension of your final size by the longest dimension of your scan item
  • multiply your scaling factor times your final resolution

Example: You decide your final resolution should be 200 dpi. Your item is 10" long and you want to enlarge to 24". Divide 24 by 10 to get the scaling factor (2.4). . Multiply 2.4 times 200. Set your scanner to scan at 480 dpi. Scanner software usually offers narrow sets of choices for setting dpi. Pick the first choice available above 480 dpi (most likely 600 dpi.)

Reduction - The second set if images shows the net effect of asking your scanner to reduce your scanned image by 50% and 75%. That would be like reducing an 11" x 8" stock certificate to 5.5" x 4" and 2.75" x 2". The second reduction seems extreme, but that is still larger than many thumbnail images you see in auction listings. As in the case of enlargement discussed above, scanner software preserves actual pixel information during reduction. That means scanner reduction squeezes pixels together tighter. In the illustration below, the head at left was scanned at 600 dpi. I zoomed in very tight on Mercury's eye in the left image in order to show individual pixels in Photoshop. I then decreased the image size by half which pushed the pixels closer together for the second image. The third image was reduced by half again and the face becomes recognizable. Look at the square of four gray pixels in the center of the first enlargment. Notice how they and their surrounding pixels remain exactly the same in the two reductions.

Original at 600 dpi Native pixels at 2400 dpi Image reduced to 50% Image reduced to 25%
Engraved head of Mercury scanned at 600 dpi Native pixels of engraved image scanned at 2400 dpi 2400 dpi pixels reduced by 50% 2400 dpi pixels reduced by 75%

Since identical color information is being squeezed into smaller areas during reduction, overall file sizes do NOT decrease with reduction. If file size is an issue, however, you may use the same calculation method shown above to calculate lower resolutions for scanning. The only difference is that the "scaling factor" is less than 1.

The only time reduction calculations might be needed for certificate scanning is when creating thumbnail images for some purpose. Thumbnail images are meant to load quickly, so it may become necessary to decrease file sizes. You are advised that the whole process of enlargement and reduction might be easier through image editing software. (See Image manipulation.)

One last note: Photoshop depicts pixels as squares to make photo repair easier. Pixels, however, are neither squares nor dots. They are strictly locations where color information is gathered by scanners.