What Resolution do I scan at?

Today's scanners have many features, some of which are very confusing this article attempts to clarify the features found in your scanner and simplify the decision making process when scanning your photos, art and other items.

Dots versus pixels Scanners and monitors use pixels as their base unit whereas printers use dots. Scanner resolutions vary greatly hence the pixel per inch or dot per inch value. For this article we will be referring to pixels per inch (ppi).

Scanning modes
Scanners support several modes; typical modes are line art, halftone, grayscale, and color. Each of these modes has features which allow you to customize the effect or result of your final scan.

Line art is used for when the item being scanned contains solid areas of black and white. This mode is ideal for scanning text, pen-and-ink illustrations and black and white logos. Do not use this mode for Black and white photos.

The halftones setting is used when you need to scan something from printed materials. These materials include sources like books, newspapers, and magazines. Printed images tend to have a dot pattern which will create an undesirable effect if you use one of the other modes. This effect called a moiré will detract from your image. Some scanners and software will have a feature called de-screening which can reduce or eliminate this effect.

For scanning colour or black-and-white photos, it is best to use the color and grayscale modes respectively. Scanning quality in colour or greyscale can also be affected by the bits per channel you select. Most scanners support either 8 or 16 bits per channel, most RGB images are 8 bit per channel yielding a 24bit image. A 48 bit image will yield more detail and colour acuity but bear in mind most imaging or photo applications may not support more that 24 bit.

How should I use my Scanner?
One of the features that confuse users is resolution. You have a photo you want to scan so that you can look at it, email it or print it. What resolution and settings should you use for your photo? My basic rule of thumb is to scan for size as, at 300 pixels per inch or ppi. This size as rule allows me to optimize my images depending on the final application. A 6x4 photo can be scanned to generate a 12x8 by doubling the 300 ppi setting and using 600 ppi. So why do I use 300ppi? This resolution is a throwback to when scanners first started being used for generating files for graphics studios. The graphics studios scanned all photos for a 150 line screen or LPI.

Line screen values relate directly to the halftone dot patterns mentioned above. This setting of 150 LPI was equivalent to what would be used for a high quality magazine or art book. The optimal setting for scanning for any line screen was usually a ratio of 2 to 1, so for a 150 line screen you would scan 300 ppi, for lower lpi values you would typically down sample or resize the scanned photo. This LPI value has very little to do with the DPI of your desktop printer. If you wish for your image to be used only on your screen you may scan at lower resolutions. I prefer to scan my photos so that they generate a 12x8 image at 300 ppi.

Transparency vs. Reflective.
Some scanners have the capacity to support transparency scanning. When scanning transparencies your scanner should support much higher resolutions. The active image area in a 35mm slide is approximately 1.5" by 1", in order for it to be 12"x8" at 300ppi it needs to be scanned at a minimum resolution of 2400ppi. This setting should be an optical resolution and not interpolated. Interpolation allows you to add additional pixels to an image in order to increase the resolution. When scanning you should avoid interpolation as this will increase your image and file size without really giving you any increase in detail.

If my scanner supports 3200 ppi should I use it all the time?
The simple answer is, no, 3200 ppi is overkill for the vast majority of scans. If you were to scan a 5"x4" inch photo at 3200ppi it would yield a 614.4mb file. This large file could of course be reduced substantially if you saved it as a jpg file, but would still be approximately 50mb and would soon eat up all your storage if used for every scan. A 600ppi scan of the same 5"x4" image would yield a file size of approximately 21.6mb. In this case the lower resolution is ideal and the 600ppi setting will still yield a file that can used for a 10"x8" at 300ppi and produce a sharp, clean image on your photo quality printer.

Other Factors.
One very important feature to consider when buying a scanner is its Dynamic Range. This feature relates directly to the scanners ability to detect brightness variations. This feature is very important, a scanners dynamic range can be measured based on a scale of 0.0 to 4.0. 0.0 represents perfect white, while 4.0 represents perfect black. Everything in between represents graduated levels of gray. Reflective scanners should typically support a dynamic range of about 2.7 whereas a transparency scanner should support a dynamic range of at least 3.2.

What file format should I save my images and photos to?
The file format you select for your image can greatly affect the quality of your image. The most common image file formats on the internet are jpg and gif, although great for small file sizes these file format have many problems, both use a lossy compression system. Lossy means that you will lose detail through compression and colour reduction. Lossy compression also introduces artifacts into your images which further degrade quality. The ideal file types to use are file types which use lossless compression. TIFF or Tagged image file format is one of my preferences along with PSD which is the native file format of Adobe Photoshop. Tiff files support lzw based file compression which allow for files to be reduced to about 10% of their full size.

Things to consider, when buying a scanner.
Your scanner should have a good optical resolution ideally it should be anywhere from 75ppi up to 1200ppi for reflectives, 2400ppi if you'll need to scan transparencies. The scanner should also have a good dynamic range, this is also essential if you are scanning transparencies. Look for a scanner that supports multiple interfaces, most scanners these days support usb or firewire, the better units support both.

Writing this article brought one of the most fundamental rules into my thoughts. In order to do any job correctly you have to understand what it is you're trying to achieve. Once you know, it's important to get the right tool for the job. When buying a scanner don't skimp and buy the cheapest, do your research and chose wisely because you will most likely be using the scanner you buy to record your memories or to produce content for your work.

Also remember that to minimise clean up of your images after scanning clean your prints and negatives. When cleaning please ensure you use methods which are non destructive.

Good luck and happy scanning.