Demystifying DPI for Scanning Photos

Posted by ed oboyle on Tue, May 07, 2013 @ 06:00 AM

Demystifying DPI for Scanning PhotosNot all photo scans are created equal and knowing the difference can mean the getting better results for your photo scanning efforts. Digitized photos that look great onscreen may be look less sharp in print, for example, if not scanned to sufficient quality.

The Key is Resolution

The key to word to remember is “resolution,” which describes how much detail will be captured from the photo scan. Resolution is measured in dots per inch (DPI), meaning a scanner will capture a certain number of dots for every inch of area scanned. These dots are also referred to as “pixels”, meaning picture elements.

Each pixel is made up of three values -- red, green, and blue (RGB) -- used to determine the color of each pixel. In a color image, each of these RGB values can be made up of 8 bits for red, 8 bits for blue, and 8 bits for green, for a total of 24 bits. Each 8-bit value has 256 possibilities ranging from 0 to 255. Each color pixel, then, is made up some combination of red, green and blue numbers corresponding to those 256 numbers.

A digital image is simply lots of pixels stored in a grid. This grid and the pixels are the basic building blocks of any digital image, whether the digital image is captured using a digital camera or created when you convert slides to digital.  Image size describes the horizontal and vertical lines, like 640x480 or 1024x768.

While it’s not necessary to know the individual RGB values of every pixel, it is useful to know how resolution affects image quality and size. There are many scanning resolutions available, and it’s important to select the right resolution for scanning projects. The trade-off with high-resolution scans is they can be very large and consumer a lot of hard drive space.

Scans of a 35mm negative can be done at several resolutions, which affect file size a great deal. Here is a sample chart showing how a 35mm film frame can grows larger as scanning resolution increases:

Scan resolution               Pixels             Megapixels                File size (JPEG)

2000 DPI                   2700x1800              4.8                         2.2-3.8 MB

3000 DPI                   4050x2700            10.9                         4.3-7.1 MB

4000 DPI                   5400x3600            19.4                        6.7-10.8 MB 

Notice how, when doubling the scan resolution from 2000 DPI to 4000 DPI, the size of the file nearly quadruples.

Scanning photos from prints follows similar guidelines, but at lower resolutions. Typically, prints are made at 300dpi and rarely higher than 600 DPI, so scanning at a higher resolution is unnecessary. And, for some applications, scanning at a lower resolution may be suitable.

How do you choose the right resolution?

The ideal scan size depends on your needs and what your plans are for the scanned images. For example, at the highest resolution, HDTV has an image size of 1920x1080, so if you are looking to display your images only on TV, higher resolution is not needed. On the other hand, large prints require higher resolution than images shown only on computer monitors or TV screens.

While it may be tempting to scan images at the highest available resolution, this may not be the most practical solution. As shown in the table, just one roll of 36 exposure film scanned at 4000 DPI could be as large as 360 MB. Even with hard-drive and cloud-storage prices dropping all the time, this is still an expensive proposition for large film collections. It may be a better solution to scan all the images in a mid-range resolution for catalog purposes, and retain the film for later high-resolution scanning as needed. That’s one of the advantages of analog photography; film and prints can be rescanned as needed.

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Topics: DPI, photo scanning

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