Best Photo Scanning Resolution
One of the most common questions people ask us is “What’s the best resolution for scanning photos?” I’ve been scanning for more than 30 years and this excerpt from my book Declutter Your Photo Life is my best answer to that question:
Image resolution is the level of detail in a digital image, and capturing the appropriate resolution for different photo formats is essential to getting a good scan. Unfortunately, this fundamental aspect of scanning is a common source of confusion. To make matters worse, even scanner manufacturers add to the confusion with their imprecise use of technical terms such as DPI and PPI. Fortunately, looking at the words clarifies our understanding.
DPI (Dots Per Inch)
Most people use the term DPI when they talk about scanning photos. For example, clients might ask, “Should a photo be scanned at 300 DPI or 600 DPI?” However, there are no dots when you scan a photograph. Dots of ink are used in the professional printing process and DPI is a description of the density of dots of ink on paper, not the quality of a scan. If you use a magnifying glass to inspect a cereal box or newspaper, you’ll see this pattern of tiny dots. When viewed from afar, your eyes mix those dots together and you see a smooth image, but when viewed up close, you can see the individual dots of colored ink from the printing process. While DPI is a commonly used term, it measures the quality of printing, not scanning.
PPI (Pixels Per Inch)
When you scan an image, the correct technical term to use for image resolution is PPI. This measures how many pixels are captured in one inch when you use a digital capture device such as a scanner or camera. For example, if you scan a 4-inch x 6-inch photo at 2400 pixels x 3600 pixels, then you have a 600 PPI scan.
I don’t mean to be overly particular about the technical terms, but I’ve always found it helpful to use the correct terminology when discussing technical subjects. It helps you sound like an expert, and you can communicate clearly with others about the quality of your scanned photographs.
Now that you understand the difference between DPI and PPI, let’s review recommendations for optimal resolution for scanning different photo formats.
Recommendations
For reflective media such as photo prints, I suggest a resolution of 600 PPI. This allows you to reprint the image at original size and up to two times the original size if you want enlargements. If you never plan to reprint an image larger than its original size, then scanning at 300 PPI is sufficient.
Now, if 600 PPI is better than 300 PPI, then wouldn’t 1200 PPI be better than 600 PPI? That sounds logical, but it’s not how it works in the real world. The reality is that 600 PPI is the upper limit of detail that can be captured from most photo prints, regardless of the scanner, software, or computer you use. If you decide to scan a photo at 1200 PPI, the process will take longer and your files will be four times larger, but you’re unlikely to capture any more useful visual information. If I was going to err on one side or the other, I’d rather scan at a higher resolution than a lower resolution, but 600 PPI will be the highest effective resolution you’ll want to use for scanning photo prints and other reflective media.
The other major category of photo formats is transmissive media, such as negatives and slides. For these items I recommend scanning at 4000 PPI, which aligns with the US federal guidelines for digitizing cultural heritage materials. This captures all the usable visual information in these formats and allows for larger reprints and many creative possibilities. Scanning slides and negatives at resolutions higher than 4000 PPI is possible, but is probably a waste of time and hard drive space.
Pro Tip
Many scanner manufacturers advertise a maximum resolution such as 9,600 or 12,800, but upon further inspection you might find that it’s only an interpolated resolution instead of an optical resolution. Interpolation in scanning is similar to digital zoom in a digital camera, it’s just made up information. When you scan important photos, make sure you never go beyond the optical resolution of your digital capture device.
Conclusion
These recommendations are based on industry best practices, US federal guidelines (FADGI) for cultural heritage preservation, and our 30+ years of experience scanning photos. If your photos are scanned at these resolutions it will be the last time they need to be scanned because you’re capturing all the available detail for today and generations to come.
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