I work for a commercial printer and I have a customer that wants to send me files from a digital camera rather than photos to be scanned. My understanding is that the dpi is supposed to be double the line screen. So the line screen for this job is 133, they sent me a test photo that was a 1280x960 ppi jpeg, the problem was the resolution in photoshop said 72dpi. Is that what 1280x960 converts to, is there a chart that converts these two things. Any help would be greatly appreciated on this confusing subject.

thanks

Deirdra

-- Deirdra Kolb (mgd@hdc.net), June 21, 2000

Answers

With a 133 line screen and an image of 1280x960, you can use a print size of 4.8 x 3.6 @ 266ppi (2x line screen). You might be able to print it a little larger if you go 1.5x line screen, depends what you want, I hope not full bleed 8.5x11. In photoshop go to image, image size, turn off resampling, and type 266 ( or whatever resolution you want) in the resolution box and it will change the print size to the appropriate number. All images off digital cameras will come in at 72ppi, so you'll be resizing these images every time

-- Cris Daniels (danfla@gte.net), June 21, 2000.

Cris pretty much nailed it. The only other thing that comes to mind is the issue of resolution not really being related to PPI or DPI.

The resolution of an image is simply how many pixels were used to "describe" an image when it was captured. The PPI is how many of those pixels you wish to use in one inch(on the output device, be it printer, monitor, etc.) when rendering the image for viewing. So first you figure out how large you want the image to be depicted and then choose a DPI that reflects the line screen & multiplier that comes the closest to what you want. Let's say you had a 1280x960(looks familiar?) image and wanted to print it as a 5x4" image.

These are the formulas you can use:

length = pixels / PPI PPI = screen x multiplier

5 = 1280 / 256 256 = 128 x 2

Now 128 isn't a usual a screen value, so you might have to settle for 133. With 133 you end up with:

266 = 133 x 2 1280 / 266 = 4.81" picture height

Not the 5" you wanted, but you've managed to preserve the 133 line screen that you may need for a commercial printing process. Of course ,you could specify the 256 DPI value and still use the 133 line screen, just realize that you will be getting slightly less than 2 pixels per line for a multiplier. Really, the best way to go with these things is to do some test prints and see how it works out for you. If you do them "in shop" with a couple of standard types of images from digicams you'd probably have a very good reference that'd last you some time. Bear in mind, that dot gain will play a role in a commercial process and must also be compensated for.

One other thought is that if it's just a sample, you might want to render the image at a couple of different PPI values to allow the customer to judge which works best with your 133 line screen. 1.5 & 2 pixels per line of screen are the most common, but lower than 1.5 is probably pushing things.

There's also a really good reference article on this type of thing at: http://www.imaging-resource.com/TIPS1.HTM under "The Lawler Essays" title. You'll probably also want to see the articles on Dot Gain and RGB-CMYK Color Separation directly below it. As I recall, they're a good read!

Good Luck.

-- Gerald M. Payne (gmp@francomm.com), June 21, 2000.