What You Should Know About Resolution and Photo Scanning

When it comes to digitizing images, resolution is key. It’s the number one quality factor in a photo scanning project. And it’s easy to see why. Although they may appear to be of reasonable quality when viewed on a screen, low resolution images begin to show their flaws when printed or enlarged. As soon as the image is blown up, the pixel squares in it turn it from a photo to something that resembles a screenshot from a Minecraft game.

Decoding DPI

Quality in digital images is measured in two ways: megapixels and dots per inch (DPI). The megapixel metric is most commonly used on the optical side of the spectrum (to assess the output of digital cameras, for instance), and the DPI number comes into play when printing images. For example, when printing a document or image, you will likely see various DPI settings in your print options.

A higher DPI normally means a better digital image to work with. But DPI is more than just an indication of how many dots or pixels are concentrated in an inch of graphic space. It also gives us a sense for the level of detail available in an image as well as the size of the final image file once it is scanned. When there’s not enough detail available, the quality quickly plummets as soon as you start zooming in on the image. If there is a lot of detail, on the other hand, the image is likely to be very good and will hold up to zooming but the image file you end up with may be too large for practical purposes.

The right resolution for your photo scanning project

Before you begin a photo digitization project, it’s a good idea to first conduct an inventory of all your image sources. As a general rule, slides or negatives make for much better scanning sources than actual prints. The crisp details in a photographic image may get blunted in a print. Prints are also more prone to aging, discoloring and other blemishes that tend to make their way into digital images. These then have to be digitally erased using photo editing software, a time consuming and not entirely easy task for someone just getting started with Photoshop. It’s true that film does gather dust and grime over time. But this is easily wiped away with a lint-free cloth. In terms of DPI, most prints are in the 200 to 300 range. The typical negative or slide is usually well over 600 DPI.

It should come as no surprise that scanning resolution is what sets good scanners apart from mediocre ones. Most people want to scan images in order to capture as much detail as possible for digital preservation and for making enlargements. In most cases, the higher the scanning resolution, the better the quality of the final image. However, in cases where there is insufficient image detail to work with in a print, the scanner may not extract much of substance at a higher setting. In other cases, it may interpolate and fill in the gaps with empty or ‘’fake” pixels that take away from the sharpness of the image. So, choosing your resolution setting based on the source image is an important factor. Another point to keep in mind is that a higher resolution number not only involves more scanning time but also calls for more disk storage space.

Just to provide an example, a small 4 x 4 print that is scanned at 4800 DPI will yield an image that can occupy a whopping 2.2 gigabytes of hard disk space. This may be an extreme example but even lower resolution settings can create image files that are a few dozen megabytes in size.

The best scanning resolution setting for your project tends to be determined by what it is you’re trying to do. Time, quality, storage and your overall intent will ultimately dictate whether you scan your photos at 300 DPI, 600 DPI or higher.

Here’s a ScanCafe quick guide with some optimal resolution settings for different image formats:

FORMAT

SCANNABLE IMAGE SIZE

SCAN RESOLUTION

MAXIMUM PRINT SIZE

Any Photo (Varies) 600 dpi At least twice the scannable image size
4″ x 6″ Photo 4″ x 6″ 9 Megapixels 15″ x 10″
35mm Negatives 1.4″ x 0.92″ 10 Megapixels 13″ x 9″
35mm Slides 1.4″ x 0.92″ 10 Megapixels 13″ x 9″

A final note on backing up your digitized photos

You can’t be too careful when it comes to backup. Your laptop’s hard drive may be an obvious place to save your scanned images but it’s not necessarily the best permanent home for them. Secure your memories by doubly backing them up on another physical disk drive as well as online. With online storage costs having gone down considerably in recent times, there are many increasingly affordable options from the likes of Google, Apple, and Dropbox.