Table of Contents
Optimal Scan Resolution
The optimal scan resolution for a photo is one of the native resolutions of your scanner, with a minimum of 400 dpi and a maximum of 1200 dpi, chosen according to the size of the photo to achieve a scanned size of about 5,669 by 3,780 pixels for normal 35mm prints.
Always scan at a resolution that your scanner natively supports, or a whole-number ratio of it.
If you don't want to have to think about which resolution to use, scan at 800 or 1200 DPI, whichever your scanner natively supports, and always use that resolution.
It is always better to scan at “too high” of a resolution (DPI) than too low.
Never scan at 300 dpi or below.
Anything above 1200 dpi is overkill for prints.
DPI Based On Photo Dimensions
If your photo has no physical damage, you can choose a resolution (DPI) based on the dimensions of the photo, with a minimum of 400 DPI and maximum of 1200 DPI. If the desired DPI is not natively supported by your scanner, choose the next higher setting:
| Dimensions (inches) | DPI |
|---|---|
| 2×3 (wallet) | 1200 |
| 3½×5 or 4×6 | 1000+ |
| 5×7 | 800+ |
| 8×10 | 600+ |
| 11×14 | 400+ |
An explanation of these numbers is given below. But:
If the photo is cracked or damaged, scan at 800/1200 DPI. This makes it easier to digitally fix the damage.1)
Theory
Modern photo labs print your photos at 300 DPI. However, the Nyquist Theorem says unless you can perfectly align the printed dots with the pixels of the scanner imager (which is not very likely), you should scan digital prints at a resolution of at least 600 DPI.
Older photos that were printed with a completely analog process are not limited to 300 DPI. One limit of the resolution of an older printed photo is the resolution of the film itself. When scanning film, 4000 DPI is preferred as it is the resolution limit of film under real-world conditions with a good lens and a skilled photographer2). For 35mm film which is 36x24mm, 4000 DPI comes to 5,669 by 3,780 pixels.
This becomes even more important when you need to crop or digitally straighten photos.
There are smaller films such as 110 (13x17mm frame size, 2,047 by 2,677 pixels) and larger films such as medium format (60x60mm, 9,448 by 9,448 pixels) so you may need to adjust the DPI setting, especially for professional photos or contact prints3). But for the most part, when scanning old photographs, try to keep the long edge at least 5,000 pixels.
There is one exception: if the photo has numerous small cracks, scan at 800/1200 DPI because the higher resolution will make it easier to digitally remove the cracks.
And why a minimum of 400 DPI? Because below that, aliasing can become a problem4).