Resolution and Printing

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I am planning on purchasing a digital camera as well as a printer. Most likely it will be a photo-realistic inkjet and maybe later a dye-sub. Here's the question -- what are the limits to the size at which an image can be printed without pixelation given the image resolution? For example, how big can a 640x480 image taken with no compression be printed before you start noticing jaggies and other image distortions? 1024x768 and so on.

-- Andrew Y. Wang (wangay1@texaco.com), June 21, 1998

Answers

This is a complex issue, really deserving a whole separate FAQ. (In fact, my next column for Petersen's Photographic is going to be a condensed treatment of the resolution/printing issue.)

Andrew's right that it takes 3 pixels on the sensor to get full RGB information for a final image pixel, but the actual situation is more complicated. This is because it turns out you can extract useful luminance information (where the dark/light boundaries are) with more resolution than if you had to strictly pay attention to R,G,B data from each pixel.

The printer and printing technology also plays a big part, as the way a particular printer lays down ink dots may either hide or accentuate pixel boundaries. (Eg, "jaggies")

Finally, there's the issue of what you consider to be an acceptable level of distortions/softness, etc.

A *really* loose rule of thumb I've used is that a true continuous-tone printer being fed data at its "true" resolution (eg, no interpolation) will look ok down to about 200dpi on the paper. Most inkjet printers use a dithering process that tends to mask pixel edges, so you can probably go further than this. On the other hand, a true continuous-tone printer (dye sub, etc) printing at 300 dpi will probably reveal pixelation on 200 dpi data. As a lower limit, most people will see pixelation in most high-quality inkjets very clearly at 72 dpi.

So what does this mean? If you use the 200dpi figure, 640x480 translates to 3.2 x 2.4 inches, 1024x768 to 5.1 x 3.8. While I haven't experimented heavily, my gut says this is too conservative for most inkjets, and a figure of 150 dpi on paper may be more representative.

Hope this helps a little. - We definitely have a "printing & resolution FAQ" on the to do list, but new cameras keep walking in the door...

-- Dave Etchells (web@imaging-resource.com), June 26, 1998.


Divide the first figure of the resolution size by 3: It takes 3 CCD pixel sensors for color photography. Example: 1280x1024 camera. Divide 1280 by 3. You'll get an answer of 426 pixels. It takes a figure of at least 300 to make a photolike print on a photo inkjet printer using top quality glossy paper, such as Champion or Epson. So, your photo, using such a camera and all other things being equal, will take a full image that size with near photolike quality.

-- J H (wa5tum@bcc.cc.tx.us), June 22, 1998.

Maybe I'm not understanding your answer or I didn't ask my question clearly -- it's probably me! :)

If you take a picture with a camera with a max resolution of 640x480, you can still print a copy of it that is photographic quality but just rather small, right (l x w)? Likewise, a camera with a max resolution of 1024x768 that takes a picture of the same subject would yield a larger output print with photo quality resolution (l' x w' where l'>l and w'>w).

What I'm after is, what would be the sizes of those images when printed (no scaling within the software)? I hope this is clearer.

-- Andrew Y. Wang (wangay1@texaco.com), June 22, 1998.


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