At 07:52 AM 10/15/2002 +0100, you wrote: >It's an easy way to stop geeks printing your pictures though: embed >in then a ludicrous dpi setting (say 9999.99 dpi) and they are >effectively safe ;o) Only those who don't understand DPI, which seems to be a majority of people. >If you have an image with 4000 pixels across, and you need to print it >at 150 dpi to get the size-on-paper you want, would it make any >difference to the finished product if you resample it up to 8000 >pixels wide and printed it at 300 dpi? Any difference at all that >is. It depends on the output resolution of the print device. Each printer has its own optimum setting. >Is it just a matter of whether PhotoShop handles the interpolation >better than the printer software? A lot of times the print driver does a better job of interpolation since it is optimizing it for the printer. Photoshop's Bicubic resample is pretty generic. >Do printers such as the Epson 1200 have a "native" resolution which >means they "don't have to think"? Yes. Lets look at the two different types of print devices: Continious Tone and Half-Tone. Continious Tone printers are printers that can represent any color in a given "Dot". These include printers like Dye subs, Fuji Frontiers (using LED's to expose real photo paper), etc. For these printers one image pixel is one print dot. So for the Fuji Frontier, the optimum DPI is 304. For the Sony 8x10 UD-P70A, its 300 dpi and for the Sony UP-DR100 5x7 roll printer, its 334. The Sony's for instance don't like files that are too big. If I send a 400 dpi image to them, I will get blocked up colors. I have programmed actions in Photoshop that resample the image file to the right size before printing. Half-tone printers are printers that use a series of small "dots" to represent one pixel. There are also two different ways of looking at half-tone printers as well. Your ink jets, color lasers and such give you a DPI for the printer such as 1440 dpi (which adds to the confusion) and offset printers used for news papers and magazines use a measure called "Line Screen". I'll start with this first since its the quickest to explain. When printing to an offset printer, i.e. submitting an image file to a news paper or magazine, you need to know what their line screen is. For most glossy magazines its 150 lpi. Most news papers run in the 85-100 lpi range. Once you know the lpi, just double it for the necessary dpi of the image. The news paper I string for uses an 85 lpi screen so we submit our images at 170 dpi. Once I've converted my images to dpi (with RESAMPLE OFF) and end up with an 11x7" image @ 170 dpi (still 2000x1312 pixels), the I check RESAMPLE and then change the inches to the size I need to print. Photoshop will then resample the image to make it smaller since the paper wants 8" wide images (downsamples from 2000 pixels wide to 1360 pixels wide). Ink Jets and such, like the Epson family have an optimum setting. It may vary by maker, but for the most part its 240 dpi for the Epson family. The ink jets print one large dot by using several smaller dots of ink. On older printers that print at 720 dpi, each pixel is represented by 9 printer dots (a 3x3 grid) visually blending the three colors those printers typically represent. As the printer technology improved, you went to a 1440 x 720 print head which prints a 6x3 grid of dots (many of these were 6 color printers) and finally 2880 x 1440 which gives you a 12x6 dot grid to represent each color. As ink dot sizes got smaller and the bleed of the ink was better controlled, the more dots per inch the printer could lay down the more realistic the print became. If you try to say, well I'm going to print a 720 dpi image to my 1440 dpi printer, your actually cutting down on the number of print dots per pixel and hurting the image quality (well the print driver probably downsamples for you to avoid the issue). So to summarize, you should RESAMPLE your images to match the print size based on the printer's needs and scale the image to the right size (not all print drivers auto-scale for you). For continuous tone printers your image DPI should match the printers DPI. For offset printers, your image DPI should be twice the LPI screen the printer is using. For ink-jets and such, try 240 DPI. I think most ink-jets are probably based on the 720 dpi system. Thanks, Rob -- Rob Miracle Photographic Miracles 203 Carpenter Brook Dr. Cary, NC 27519 http://www.photo-miracles.com
