Some basic terminology
PPI. Pixels per inch. Each file contains a more or less arbitrary number, the pixels per inch, used to convert from pixels to inches. For example, if that number is 100 PPI, a 3,000 x 2,000 pixel file would have a default size of 30 (3,000 divided by 100) x 20 (2,000 divided by 100) inches. But the exact same file could be assigned virtually any other PPI number. If 50, then the size shows up as 60 x 40 inches. If 300, then 10 x 8 inches. We ignore the ppi number in the files you send us. Instead, we ask you to tell us how you want your files scaled. So you could, using the same example, use the same file to make one 60 x 40 at 50 ppi, one 30 x 20 at 100 ppi, and one 10 x 8 at 300 ppi.
Aspect Ratio. The ratio of the number of pixels in one direction (e.g., horizontally) divided by the number of pixels in the other (e.g., vertical) direction. Traditional 35mm cameras had an aspect ratio of 3:2 (1.5), e.g., 3,000 x 2,000 pixels. Many digital cameras have an aspect ratio of 5:4 (1.25) or 4:3 (1.333). Prints also have an aspect ratio -- the number of inches in one direction divided by the number of inches in the other direction (e.g., 15:10 for a 15" x 10" print). If the desired aspect ratio of the print is not the same as that of the file, then something has to be done to allow for the difference -- either some of the image will need to be cropped, or some white (or black) will need to be added to make up the difference. At the time of ordering, our system attempt to help you thru this issue.
Gamut, Out of Gamut. Printers (subtractive color devices) and monitors (additive color devices) have a range of colors that they can, and cannot, handle. There are a large number of colors that monitors can display that printers cannot print accurately, simply because the dye set that is used has limitations. If your file(s) contain out-of-gamut colors, our software will attempt to come as closely as it can, but it will be an imperfect match. If you use Photoshop, go to view --> gamut warning and turn that on. The out-of-gamut colors will turn gray. The dyeset that Photoshop uses for determining out-of-gamut colors might not be the same dyeset as is used by our printers, but it's a good approximation.
There are various subtypes of each file type. In an RGB file, the higher the, e.g., red number, the more red the eye will perceive. But do equal steps of red equal equal steps to a photometer, or equal steps to an average observer, or some other standard? The answer to this question is linked to the concept of Color Space. There are various standards, each providing various advantages and disadvantages. Examples of color space standards are Adobe 1998 and sRGB. We prefer sRGB.
ISO Number. Digital cameras can be set to be more sensitive (simulating a highly sensitive albeit somewhat grainy film) or less sensitive (simulating a low sensitivity fine-grain film). Graininess in film is analogous to noise in digital files. When high ISO numbers are used (e.g., 800 and above), shots can be obtained with fast shutter speeds despite less than optimal lighting. The resultant image will probably contain significant noise. The higher the ISO setting, the greater the amount of noise. The noise is often inconspicuous in screen images and/or small prints, but is usually much more objectionable in large prints. It is good to examine image files at 100% or greater magnification before ordering prints. And, when shooting pictures, consider the resultant noise when choosing an ISO setting.