DPI - How much is enough?

From time to time we get asked about something called “dpi” - dots per inch. It’s a technical measure of how much data is used to convert a physical (analogue) image into a digital file. In some ways its easy to answer but often it gets very technical.

The easy answer is “enough”, and if you’re not technically minded feel free to click away now.

For the more geeky among us our basic print scanning service can deliver 300 dpi, 600 dpi or 1200 dpi. Let’s start with the original photos that we scan, they are printed at what is in digital terms 300 dpi. If you want to view images on a computer or TV scanning at 300 dpi will give you a more than acceptable result and will load pretty well in a photoframe. If you make reprints of the file at or a bit bigger than the original the image will be fine.

At 600 dpi you have a much bigger file, if you remember your geometry it’s four times bigger so that will be tougher to load into a photoframe, somewhat slower to load on a PC or DVD player but will deliver prints capable of being enlarged.

What about 1200 dpi? Yes, we can offer this but there’s a big “but”. We’ve done tests, as have a few clients, and in viewing terms on PC, TV/DVD, on Apple TV or similar photo streaming systems, there is no benefit. Technically your files will be 16 times bigger than those at 300 dpi giving you a big jpg file. It’s slow to load and hard to handle. Compared to scanning speeds of 300 dpi or 600 dpi at 1200 dpi you think the scanner has broken down it goes so slowly. For that reason we would typically charge more for 1200 dpi photo print scans. Frankly, it’s not worth the bother.

Yet if you look elsewhere you’ll see we offer very much higher dpi rates with jpg and tiff files when scanning negatives. How do we reconcile this? Well it’s all to do with the size of the original. Take a 35mm slide or negative, that’s very small so to get a decent size image or print a degree of enlargement is necessary. If we scanned that at 300 dpi it wouldn’t enlarge, so a four times enlargement would still only be a modest print (around postcard sized) and you’d need 1200 dpi for that. For that reason our Home slide and negative scanning runs at 2,000 dpi.

Is there a maximum? Well many people have suggested that the maximum amount of data that can be extracted from a 35mm slide or negative is 4,000 dpi. Nikon, one of the foremost names in photography and the maker of our 35mm scanners offer a maximum of 4,000 dpi on both their 35mm and medium format scanners. So that’s why we don’t go beyond 4,000 dpi.

What if the client is adamant in wanting a 4,000 dpi scan of an A4 sized print? I can’t think why it would be wanted or needed but we’re service business and in the final analysis the customer is always right.