All About Digital Photos
 
What Print Shops Really Want

DPI does count

Although this website is about digital photos - this section will briefly deal with the topic of scanning.

DPI, which is a paper dots per inch, does count when scanning. When an image is scanned, one scanning "dot" equals one digital image pixel. You are converting an object with physical dimensions to digital pixel dimensions.

Scanning Paper (photos, documents, etc.):

Before you do your first scan, you'll want to know what DPI the paper object (a printed photo for example) should be scanned at. There are a couple of ways at looking at this:
  1. You can simply pick a number that will provide a good quality scan with a reasonable filesize. This is usually about 600 dpi. That will provide enough pixels to effectively double (or triple) the potential output size while maintaining photographic quality. See item #2 for exactly why.


  2. You can calculate the dpi of the scan based on your desired output size. For instance, say you wanted to scan a 4" x 6" inch photo with a desire to later be able to print it at 8.5" x 11". First, choose your desired ppi for the final print - although 200 is likely sufficient, most print shops are going to ask for 300 - so let's choose 300 ppi.

    To get an 8.5" x 11" print at 300 ppi, you're going to need a digital image that is 2550 (8.5 x 300) by 3300 (11 x 300) pixels in size. To scan 4 inches to 2550 pixels will require a DPI of 638 (2550 divided by 4). To scan 6 inches to 3300 pixels will require a DPI of 550 (3300 divided by 6).

    The two DPIs are not the same since the aspect ratio of the two print sizes is different. The rule is to simply pick the largest DPI number - in this case 638 (you can round up to say 650 since a bit bigger is okay). Your 4" x 6" photo scanned at 650 dpi will result in a digital image that is 2600 x 3900 pixels in size. With cropping, the digital image will print to 8.5" x 11" at 306 ppi.

Okay - #2 sounds quite convoluted, why not just scan everything at a very high number, say 3000 dpi, and be done with it?

The main problem is that you're going to end up with extremely large digital files - a 4" x 6" photo scanned at 3000 dpi (in 24-bit RGB) is going to result in a digital image that is 12,000 x 18,000 pixels in digital image size. That image, stored as an uncompressed TIF will be about 600 Mb in filesize. It will still be very large at 350 Mb for a compressed TIF and still large at about 80 Mb as a print quality JPEG. That same photo scanned at 600 dpi (resulting in a 2400 x 3600 pixel size image) will be about 24 Mb as an uncompressed TIF, 16 Mb as a compressed TIF and only 4 Mb as a print quality JPEG. You can store 22 more compressed TIF images or 20 more JPEG images on the same hard drive if scanned at 600 dpi instead of 3000 dpi.

So, scanning at about 600 dpi is a good compromise between getting a good sized digital image and storage filesize.

You can see a workflow description on scanning photos in my genealogy section at: Workflow Examples using a Flatbed Scanner

Scanning Slides and Negatives:

A slide or negative from a 35mm camera is a tiny (about 0.95" x 1.42") piece of film. We can run the same sort of logic as we did with scanning a paper image.
  1. You can simply pick a number that will provide a good quality scan with a reasonable filesize. For a slide or negative this usually between 2400 dpi and 3200 dpi. That will provide enough pixels to print a decent size photo while maintaining photographic quality. See item #2 for the calculation.


  2. Printing to 8.5" x 11" at 300 ppi will required 2550 x 3300 pixels. Our starting image is 0.95" x 1.42" in size - to get 2550 pixels from 0.95" will require a scan of 2684 dpi and to get 3300 pixels from 1.42 inches will require a scan of 2323 dpi. Same rules apply as with prints - pick the bigger number, in this case 2684 which can be rounded up to 2700. Scanning at 2700 dpi will result in a digital image that is 2565 x 3834 pixels (10 megapixels) in size.
Often with slides and negatives, the final paper output isn't known (you're just digitizing them to get them into a form that easier to edit, distribute and archive). So, any number between 2400 dpi (which will create an 7.4 megapixel digital image) and 3,200 dpi (which will created a 13 megapixel image) is generally a good number to choose. I generally scan my slides and negatives at 3200 dpi.

Note: don't exceed the optical scanning resolution of your scanner. Look for this information about your scanner - what is the maximum optical (not digital) resolution it is capable of. Don't exceed that number. If it is not sufficient for your purposes - buy a new scanner.

Good software is critical. A scanner that comes with software that allows the use of exposure curves, especially ones that enhance shadow detail and allow you to pick the source film type (i.e. Kodachrome, Fujichrome, etc.) will provide better results. A excellent alternative to the software that come bundled with a scanner is the program Vuescan that can be used with most USB and SCSI scanners. It is a commercial program available from www.hamrick.com. Note though that Vuescan is best used by those with some familiarity with computer programs and scanners (it has lots of bells and whistles).

You can see a workflow description on scanning slides and negatives in my genealogy section at: Workflow Examples using a Flatbed Scanner

Other Stuff

Scanners have many settings. The above information is predicated on scanning using 24 bit RGB (8 bits per channel). See the Colour Models page for the explanation of types of colour. For most people, using 24 bit RGB is ideal. Many scanners can also do 48 bit RGB - but don't use this unless you know exactly what you're doing. It will result in a much large filesize and not all software will be able to handle a 48 bit RGB digital image.

For general scanner tips and tricks, view the many internet sites dedicated to this topic.




Home   •   What Is | Filetypes | Colour Models | Acronyms and Nomenclature | Frequently Asked Questions (FAQ)   •   Storage   •   Digital Labelling | Visible Captioning   •   What Is DPI | Myth DPI | Change DPI | Print Shops | Arithmetic of DPI | FAQ | Photoshop Dialog Box   •   Home Printing | Print Shops   •   Geotagging -- Scanning -- Digital Images and Genealogy | Make Your Photos Last Forever | Camera or Scanner | Copying Old Photos | Workflow Examples | Visible Captioning | Choosing a Camera | Print Longevity | Sending Photos to Relatives

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