01 July 2002
Posted in How To...
NZ Macguide Issue 4
Choosing the correct resolution and size for your final image when you scan is the best idea for smart scanner usage
There's plenty of confusing advice out there about scanners, so here's the real oil: you don't necessarily need the highest resolution scanner you can afford, and you almost certainly don't need to scan at the highest resolution you can.
It works like this: when an image gets printed, it's broken into tiny monochrome dots. Glossy magazine covers might manage 175 of these dots per square inch, measured as Lines Per Inch, but normal magazine printing (the pages between those 175lpi covers) is usually 120-150lpi, and newspapers and newsletters are about 100.
Times that final resolution (the lpi) by 1.5 to get your scanning resolution - to be safe, I usually scan at 1.6 - then if the image needs to be 'stretched' a little (see below) in a layout, it can be - but only a little. In other words, if you're scanning for a glossy magazine cover, scan at about 280-300dpi, if you're scanning for the body of a glossy magazine, scan at 190-240, and if it's for the school newsletter or a newspaper, 160dpi is plenty.
Why? Two reasons. Crudely, once the printer puts those dots into your scan, quite a lot of information will be thrown away - the spaces created between the dots (look at something commercially printed through a magnifying glass if you don't believe me). Secondly, a 300dpi scan takes up more than twice the physical space of a a 200dpi scan - believe me, when you supply your 600dpi scan to a commercial printer, any prudent operator will open it in Photoshop or whatever and 'resample' it to a decent resolution. This is both to save hard drive space and to considerably speed up their raster image processing (to make it into film or plate).
For photos destined for the web, 77dpi is plenty, as this is all monitors can show. A 77dpi scan is typically less than a 20th of a 300dpi scan, so it will draw a lot faster in a web browser. For archiving, 100% in size and 300dpi is pretty safe. A 600dpi scan is physically four times bigger than a 300dpi scan and this kind of resolution is only used for scanning little transparencies which will need to be made much bigger than their original size, for instance to be printed commercially.
Size, stretching and resolution
Another trick is, find out how big your scan will need to be to display when it's used for whatever you're using it for, and scan to that size right off. If I give you a 5cm wide scan for your page layout and you need it bigger, grabbing a corner on the page and dragging it bigger is literally pulling those pixels (picture elements) apart, so you are lowering the resolution below that recommended - jagged steps on diagonal lines and various blocky patches will result. If I supply you a scan too big and you don't resample it smaller (for example, open it in Photoshop, choose Image Size from the Image menu, check 'Resample Image' and type in a lower resolution), dragging it smaller on the page will compress those pixels and make the printer, or raster image processor, work overtime.
Why high resolution at all?
Firstly, if a scanner can handle transparencies, high optical resolution is important. If it's not, look for high 'bit depth'. Secondly, the higher resolution, the more the scanner can see, although this isn't as important as bit depth in terms of capturing colour and subtlety.
In other words, the scanner can register and reproduce more colours and subtle tone variations if it has a higher resolution - that said, a 48-bit scanner with 600X600 resolution may 'see' more tones and colours than a 36-bit 1200x1200dpi scanner, so choose wisely for your money.
© Parkside Media 2002