There are two basic types of files – Raster (bitmaps) and Vector (outlines). Examples: a photo is usually a Raster image, and a font is usually a Vector or outline. The most common formats for vector files are EPS (Encapsulated Postscript), AI (Adobe Illustrator), or PDF (Acrobat). Common bitmap formats include TIF (or TIFF – Tagged Image File Format) JPG (or JPEG – Joint Photographic Experts Group), and GIF (Graphics Interchange Format. Be careful of GIF files as these are most commonly used for websites and are usually not high enough resolution to be enlarged significantly.) There are a couple of additional considerations to keep in mind – While EPS, PS, and AI files commonly contain vector data, they can also include bitmap data as well. The opposite is not true. TIF’s, JPG’s and GIF’s only carry raster data and do not carry any vector data.
The biggest difference between Raster and Vector file formats is that a vector image can be infinitely enlarged since the image is created with outlines or line segments and curves. The amount a Raster (bitmap) image can be enlarged depends on the original resolution, and accepted loss of clarity versus the accepted viewing distance. Raster images most commonly used for photographic images, but they are also a bit more complicated to set up and produce a quality large format print. We will concentrate a bit more on setting up Raster files.
A raster image (bitmap) is made up of tiny dots, and each dot can be one of many thousands, if not millions of colors. There is also two color modes or pallets – CYMK (Cyan, Magenta, Yellow, and Black) or RGB (Red, Green, Blue). Generally, the RGB color space is larger, and many RIP programs like this format to produce the most vibrant colors. I know this might seem odd, as the printer is a CYMK device. Eventually all files will be converted to CYMK by the RIP (Raster Image Processor) program when it outputs to the printer. Non the less, starting with RGB files and letting the RIP make this conversion will generally produce the most vibrant images.
Raster images also have limitations on how much it can be enlarged depending on the original file’s resolution. We recommend to keep the final output resolution between 100 and 150 DPI (dots per inch) at FULL SIZE. Below 100 DPI and you will start to see a sacrifice of quality. Over 150 DPI, you’re probably wasting hard drive space and processing time This file resolution is not to be confused with print resolution. Even though our input files are 100-200 DPI, we typically print at at least 360 DPI and more often 720 DPI. This ensures the smallest droplet and lack of graininess. One exception might be on a billboard or a tractor-trailer. These areas are so large, a 200 DPI input file will be too large for most computers to deal with. When working in Photoshop, these files can swell to well over 1 Gigabyte. In this case, we may start an input file size as low as 72 DPI to reduce a little overhead.
How can I determine how much I can enlarge a Raster image? In keeping to out 100 DPI at full size guideline, here’s an example: If you take a file that starts out at 600 pixels wide, the largest you can print keeping to the 100dpi rule is 6 inches. (Divide 600 pixels by 100 = 6″) Of course it is possible to enlarge the file much larger in Photoshop, but this just multiplies the pixels around each other, and you will just create a fuzzy mess. Basically Photoshop will just take each pixel and create a new pixel next to it halfway in color to the next, and so on. This is why most website images are not useable for large format output. There is just not enough pixel data. Website graphics are created at as small a resolution as possible to create a reasonable image to view on a monitor and reduce bandwidth. This makes the website load as quickly as possible.
So what about that uber expensive digital camera I just bought? Can’t I just use that? A high end digital camera on the market today may be as high as 12 Megapixels or more. (a megapixel = 1 million pixels) A 12 Megapixel camera, for example, may have a resolution of 4288 pixels wide x 2848 pixels high. In keeping our file within our 100 DPI guideline, the maximum size we can output with minimal distortion is 42.88″ wide.(4288 pixels wide divided by 100) The larger the original and more pixel data you have, the more leeway you have to work with. With this much pixel data, we can go a reasonable amount larger than 42.88″ and still achieve good results. Keep in mind, the larger we go from the original, the more fuzzy and pixilated the final output will occur. The next consideration is how much “fuzziness” is acceptable. Well, it depends on how the image is viewed. Just as you would not be viewing a 8 1/2″ x 11″ piece of paper from 40 feet, you would also not expect to view a tractor trailer from 6″. Billboards would be another extreme and their resolution might be as low as 2 to 20 DPI. But the billboard might be 40 feet in the air, and viewed from many hundreds of feet, the resolution can be much, much less.
There are also filters that can do a nice job smoothing out the “jaggies” of less that optimal files. One that we like is called Photozoom. It’s proprietary algorithms filter a less than optimal artwork to result in a more acceptable image than Photoshop when enlarging artwork, as long as there is sufficient pixel data there to start with. It can’t perform miracles, but it can make a significant improvement.
A special thank you to Ordway Sign Supply, www.signsupply.com for this article.