Thank you Rob for this helpful information. Richard Cooper ----- Original Message ----- From: "Rob Miracle" <rwm@photo-miracles.com> To: "List for Photo/Imaging Educators - Professionals - Students" <photoforum@listserver.isc.rit.edu> Sent: Tuesday, October 15, 2002 9:45 AM Subject: Re: file formats > 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 > > >
