Newer multi-megapixel cameras offer high enough resolution to preserve image quality in offset printing for most image sizes (except posters and exhibits). To take high quality pictures for print, you’ll need at least a 2-megapixel camera to reproduce small images. A 2-megapixel camera has an image resolution of 1600 x 1200 pixels. (By comparison, 35 mm film has an image 36mm x 25mm or apr. 1.4in x 1in. If Scanned at 1200 ppi [Pixels per inch] that is 1680 x 1200.)
A digital camera with 8 to 12 megapixel capabilities are now common and can be reasonably priced. Also coming into consideration is the quality of the lens. A 8mp camera with a quality lense can create a better image than a 12mp camera with a cheap lens.
To determine the maximum image reproduction size at 300 pixels per inch (ppi) recommended for offset printing, divide the file’s resolution size by 300. For example, a file with 1024 x 767 resolution (divided by 300) will provide a 3.4" x 2.5", full frame—uncropped image for offset printing. The 300dpi resolution figure comes from the fact that we print with a line screen of approximately 200 lines per inch. Years ago the rule was 2 pixels of resolution for each line on print. We used 400dpi for many years before we tested this rule and found little quality diference between 1.5 to 1 and 2 to 1. But the print quality did suffer at 1.2 to 1 and below. This is why we use 300ppi as our standard for print.
Tip: Another problem that comes up now is using a large image, say a 21mp image (5616x3744) sized small say 3x2 inches. That comes to 1872ppi. In this instance it is better to use photoshop to sub sample the image to 900x600 or something similar and using this image as it creates a sharper image in print. This is because the screening engine in the RIP (Raster Image Processor) just averages the extra pixels often times creating a gray area that contains no detail. Photoshop does a much better job of keeping detail when sub sampling. (Unless you use the averaging setting.)