rmalarz wrote:
The professional printing house I use requests 300ppi. I would imagine that works across the board.
--Bob
The terms PPI (Pixels Per Inch) and DPI (Dots Per Inch) are often confusing because they are interrelated and often misused. Pixels most correctly refer to discrete units as in individual receptor cells of the CCD chip in your camera (how many of the cells are placed in a lineal inch (PPI) and the overall size of the chip determine the megapixel rating of the chip. The same is true of video displays.
The term DPI usually refers to the resolution of an output device -- an inkjet printer, laser printer, or printing press. Big numbers are impressive in specifications or advertising, but often you have to dig a little deeper to find out what really is happening. An inkjet printer may have a specification of 9600 dpi, but did they get that number by multiplying actual number of dots per inch by the number of cartridges in the machine? So we may already be down to 2400 (per color). Then does the printhead have the ability to print a dot in every one of those positions in one pass, does it take multiple passes, or are there limitations like printed dots no closer than every third position or more. Then, how many different sizes of dot can the inkjet physically produce? The math can get complicated and explains why a change in quality levels can change the output speed immensely.
I've been in printing for over 50 years, so am very familiar with that form of output. Nearly gone are the days when a photograph made from a negative was photographed through a screen with a graduated grey pattern to convert it into a "halftone" negative to make plates for letterpress or offset printing. The presses only print one color of ink from a plate, but the various sizes of dots allow for changed of tone from a solid color to very light tints of the solid color. Overprinting properly made plates in Yellow, Magenta, Cyan and Black (CMYK) allows for the vibrant color printing we know today. The final product hasn't changed, but now the plates are made by digitally controlled lasers instead of photographic methods.
This brings me to the "300ppi" mentioned above. What isn't specifically stated is that usually means 300ppi per inch of lineal dimension of the final reproduction. So for a 4" x 5" printed photo, the digital image would be a minimum of 1200 x 1500 pixels; an 8" x 10", 2400 x 3000.
The vast majority of commercial printing is done with 133, 150, and 175 dots per inch (DPI) of printed screen resolution. Each of those dots may vary from 100% ink coverage down to 0%. Those dots are usually created at a resolution of 2400 dpi and up which allows for a 256 or more sizes of dots which results in that many possible shades of the color being printed. Multiply that by the four process colors used in printing and you can see how millions of shades of color are possible.
With an original at 300 ppi and an output at 150 dpi, the computer would average a 2x2 pixel area in the original file to determine the size of the printed dot to represent that area in the final print. Larger files will work but it just takes more disk space and computer time to use a larger sample of original pixels to determine the size of the dot on the plate.