Scanners capture digital images of prints or images on film. When they scan the image on print or film, they break it down into pixels. This process has an associated spatial resolution that is related to the original surface being scanned. If the scanner breaks each linear inch of the printed photo into 1200 pixels, then the scan is being conducted at 1200 pixels per inch.

This resolution is sometimes also referred to as the sampling rate. Most commonly though it is referred to in terms of DPI, even though PPI or SPI are far more appropriate terms. Hence DPI in scanning actually refers to PPI or SPI as printed information is getting converted to pixels per inch from the printed photo surface. Note that the scan resolution has no bearing on the print resolution, unless one wishes to match the print size to the size of the original image scanned, in which case the print resolution should be set to match the original scan resolution.

Quick answer is higher DPI leads to better quality. It’s always better to go with higher DPI and get it right the first time to capture as much data as possible. We recommend photos to be scanned at 600 DPI. Higher DPI also enables the scans to be printed on to larger sizes at a later stage. Look at the numerical example below to understand DPI, PPI and SPI better.

We use best in category Epson Perfection V800 flatbed scanners.

The file size of the scan again depends on the scan resolution or sampling rate. Higher the resolution bigger will be size as more information pertaining to clarity, sharpness and color is captured. The format of the scanned image can be jpeg or TIFF. TIFF is the raw format without any reduction in quality and will be the largest in size generally. Jpeg would be ideal for sharing and storing purposes as they are about 5 times lesser in size compared to a TIFF.

Retouching is an art of enhancing an already existing print photo. Print photos would have faded or might have yellowed. Sharpening such digitized print photos by color correction is called Retouch. The digitized content is brought back to its glory. If you choose just scanning for print photos the digitized content would be a replica of the print photo and inherent defects. We charge a very nominal Rs.2 per photo for retouch and this is done manually and takes approximately 3 minutes for each scan. We are the only brand where retouching is done manually and our expert team is adept at the procedures. Let’s consider a printed photo of size 15cm*10cm that needs to be scanned to digital format. For calculation purposes, you will be scanning at 300DPI.

15cm10cm (WidthHeight) printed photo = 6″ * 4″ (approx)
Scanning @300 DPI

Scanner breaks each linear inch of the printed photo into 300 pixels.

On the width of the photo you will end up with (6” 300) = 1800 pixels On the Height of the photo there would be (4” 300) = 1200 pixels
The printed photo is converted to pixels and to the digital medium.

The resolution of the digital photo is 1800*1200 = 2.16 Mega Pixels

Till now we spoke about Scanning resolution and we ended up with a 2MP digital photo. In the digital medium DPI has no meaning as there is no physical dimension and there is only pixel density. Also now you can see that actually scanning resolution is in SPI (spatial pixel per inch) and not DPI as the pixels are picked up after breaking the linear inches of the physical photo.

Now if the scanned photo needs to be printed at a later stage, the size plays a very important role. We already know that this photo can be printed at 6” * 4” size as we scanned it from the same size. If you choose a printer with 1200 DPI resolution for this size then the printer will lay 4 dots per pixel. The PPI (pixel per inch) would be 300 DPI which is the SPI at which we scanned.

Please note that the required standard regulation for printing 300 DPI or in fact it should be 300 PPI. However printer shops will always use DPI interchangeably for PPI. DPI is only for the printer which denotes the dots it lays per inch.
However, if the same digital photo needs to be printed at 7” * 5” size then the issue props up. The PPI reduces from 300 to (1800 pixels/ 7” or 1200/ 5”) to 250 PPI approx. This means that the pixels per inch have reduced and even if you use a 1200 DPI printer the resolution at this size will not be good and will lead to a pixelated picture.

This is the reason why you need to scan the pictures at a higher DPI (or SPI – you know why). You wouldn’t want to regret at a later stage where the scanned image cannot be printed at bigger sizes.