Although the National Digital Information Infrastructure and Preservation Program and the National Digital Stewardship Alliance focus on digital preservation and access, many of the personal digital archiving questions that the general public ask us are about scanning. Though scanning is a separate issue from digital preservation, scanning does generate digital files that need to be preserved. In the interest of helping people create the best possible digitization of their photos and documents for preservation, we have produced a “how to” video that we will be releasing soon. In the meantime here is a brief, basic introduction to scanning that we hope will demystify the process.
When you scan a paper photograph, the scanning device creates a digital version of the photo made up of tens of thousands of tiny dots or squares called pixels. This paper-to-digital conversion process is digitizing, though the act of digitizing is not limited to images or text; you can also digitize video and audio. In this post we will just look at scanning and digitizing photos.
Step 1: Prepare Scanner and Photos
The first step in the process is to clean the scanner and photos. Smudges, dust and hair will scan into your digital photo and ruin its purity. Wipe the scanner glass with a plain, lint-free cloth dampened with water. Do not spray the water directly onto the scanner; spray it on the cloth. Wipe the inside of the scanner lid too.
Next, lightly wipe off the photograph with a dry anti-static cloth. You can find these cloths in a camera store. They not only clean the photo, they help reduce static electricity on the photo and prevent it from attracting more dust particles and hair. Place the cleaned photo face down on the scanner. Do not touch the glass when you place the photo. The natural oils on your fingers may smudge the glass and you’ll have to clean the glass again. Try to slide the photo against the side of the scanner glass and up into the corner for the best alignment.
Some software will detect separate photos or items and scan them as individual files. Leave about one-half inch between the photos to help the software recognize them as separate items. Close the lid gently so the photos remain aligned with the scanner glass edges.
Step 2: Set Scan Properties – DPI and Bit Depth
Once you have prepared your scanner and photos, set the properties for the photo scan. In your computer, open the scanner software. There are two important settings to look for:
- details of the digital image, such as the number of dots per inch and whether the image is color or grayscale.
- the file format to save the image as – such as TIFF or JPEG – and the type of image compression (if any) you want on that file type.
Dots per inch – or DPI – is a measurement of pixel density. Image specialists use the more precise term “pixels per inch” or PPI. However, since documentation for commercial scanners almost exclusively uses the term DPI, we will stick with the term “DPI.”
The more pixels packed into a one-square inch space, the greater the potential detail an image can hold. An image with 200 dots per inch potentially displays more detail than the same image with 72 dots per inch. There are optimum DPI settings for different photo sizes and types but more DPI is not always better; there is a DPI limit or threshold. Beyond that limit, there is nothing more of value that increased DPI can add. You can only scan so much detail from a photo.
- For most personal work, 300 to 400 dpi is satisfactory for snapshot prints and for common enlargements at 4″x6″, 5″x7″ or 8″x10″ in size.
- Since very small prints or photographic slides contain a lot of detail in a small area, capture more dots per inch, around 1400 to 1500 DPI.
- Photo negatives also hold a lot of detail, so for negatives, select a minimum of 1500 to 2000 dpi. Remember that increasing dots per inch increases data and increases the file size.
- If you want a close up, such as an enlargement of a face in a crowd, select the area and increase the DPI until you get the height and width that you want. Keep in mind that the quality may not be very good.
Some software may enable you to adjust the bit depth of data per pixel. The more bits per pixel, the more information the pixel contains and the richer the digital palette you have to work with. The most commonly used scan setting is 8-bits per pixel for grayscale (some scanners may also offer you 16), and 24-bits-per-pixel for color (although some scanners may also offer 48). With more bits per pixel you can have a bit more to work with if you intend to edit your digital photos later. But for routine scanning, where you do not plan to edit much, or where the quality of the outcome is not such a big deal, then select 8-bit grayscale or 24-bit color. Remember that increasing bits per pixel increases the amount of data in the file and so it increases the size of the file.
If the paper photo you want to scan is black and white, and you see a menu choice of grayscale or color, select “grayscale.” If the paper photo is color, select “color.”
Step 2: File Format and Compression
Scanner software saves your scanned photo as a digital file and the most common file-type options are TIFF and JPEG. TIFF, the preferred format for digital photo preservation, retains the maximum amount of digital data that your scanner captures. If you have a choice, save your original master scan as a TIFF.
If file storage space is an issue, you can compress a file and reduce its file size. Scanning software may offer an option of LZW compression for a TIFF file, which will cut the size of the TIFF file without the loss of digital data. This is called “lossless” compression. By contrast, saving an image as a JPEG employs a “lossy” compression, so named because a JPEG file, by its nature, is compressed and it loses some of the digital data during compression that the scanner captured. You can select JPEG quality levels and degrees of compression, from “least compression” — the least amount of lost data and the highest JPEG quality — to “most compression” — the most amount of lost data and the lowest JPEG quality.
We recommend that if you intend to modify or work with a digital photo, you save two versions of it: a master version and a working copy. Keep a TIFF file as the master file and store it safely with your other personal digital archives; use a JPEG version as the working copy. The JPEG file will be smaller and more convenient to email or post on social media sites. Edit, modify and work with the JPEG. You can always make a fresh JPEG copy of the master TIFF file.
Once you have selected the file type and set your bit depth and DPI, you are ready to scan your photo. Preview the scan, if you have that option, and look it over to make sure you haven’t picked up any dust, hair or artifacts. And check that the photo is aligned properly. Then select “scan.”
After scanning the file, some scanning software will prompt you to assign a file name. Some software will automatically assign a file name to your file. If it assigns a file name (usually some alphanumeric name like “DC2148793.jpg”), you can either keep that file name or you can change it. To change the file name, right-click –- if you are on a PC –- and select “rename.” On a Mac, control-click and select “rename.” Renaming the file will not affect the contents of the file. We recommend that you rename the file to help you find the file later. Many people include the date in the file name — at least the year or the combined year and month. If your file names lead off with year-month, followed by a descriptive word or two, then — in your computer folder — the files will sort in chronological order.
Remove each photo from the scanner by slipping a piece of paper under it and lifting it. Avoid touching the glass with your fingers.
As soon as possible, back up your digital photos in a few separate places. Every five years or so, migrate your personal digital archives to a new storage medium in order to avoid having your collection stuck on some obsolete media.