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What is the Best Resolution to Scan Old Pictures?

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By Christian Roemer

Turning things digital from analog is a weird process. The first strange thing is that you’re taking something (analog), which can be any shape in the world, and you’re turning it into a bunch of little digital squares. That’s because, to turn anything digital, it has to be made out of pixels. This is especially weird for printed photos, because they’re printed using dots. That means, to digitize a photo, you have to convert something that’s printed in dots into squares.


This is where things get tricky. The standard size for printed images is 4x6. That means each picture is four inches tall and six inches wide. When we think about an image that small, it seems to make sense that there’s a maximum possible resolution that we can get when we digitally scan them.


And that assumption is correct.

When you scan images, your scanner measures picture quality in DPI, which stands for dots per square inch. Similar to how digital images use pixels to measure resolution, printed images also have to print somehow. The way they do that is with little dots.


Most printed images are printed at 300 DPI. That means if you’re scanning your image at 300 DPI, you’re getting all of the details out of the image that you possibly can. If you scan the image at more than 300 DPI, you’re trying to get more details out of the image than it has. 


I know it’s confusing, but bear with me here.


To think about it a different way, imagine you have a big sheet of legos. If you use a bunch of the square legos, we could think of that like one of the dots in the DPI. If you divide that square into 4, you’ll be left with 4 squares, but they’ll be the same color as the single square was, and the same size. In other words, you technically increased the amount of resolution, but it didn’t change anything, because it’s still a block that didn’t increase or decrease in size.


When you’re scanning 4x6 photos at 300 DPI--the standard size--you’ll end up with a digital image that’s about 1200 pixels by 1800 pixels. Amazingly, that’s the perfect size to fit on a high definition television.


Since we’ve talked about some confusing things with a bunch of jargon, let’s recap. Pictures are printed with a measurement called DPI: dots per square inch. Pictures are printed at 300 DPI by default. If we use a little bit of math, that means a 4x6 photo would be 1200 dots by 1800 dots. Scanners also measure their scans in DPI. When an image is scanned, dots are translated into pixels, which is how you end up with a 1200 x 1600 pixel image when you scan a 4x6 photo.


So what happens when you scan an image that’s bigger than 4x6? Well, it depends. Was the picture printed at a non-standard resolution? Was it higher or lower than 300 DPI? How big is the image?


Basically, to scan an image isn’t as straightforward a process as you might think. At KODAK Digitizing, we specialize in digitization, meaning we use the most ideal resolution for every single image, every time. You don’t need to do any math, because we do all of the hard work for you.


But just in case you were curious how printed images are converted into pixels, now you know!

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