Do pixels matter? What 20" x 30" prints reveal.

Started Jun 14, 2013 | Discussions thread
TTMartin Veteran Member • Posts: 7,304
Re: printer res

Mako2011 wrote:

rwbaron wrote:

I know the difference and I'm talking about PPI not DPI.

I take a RAW file from my 7D and open it in PS at 240 PPI. The native image is 5184x3456 which equates to a 14.4x21.6 inch print and the file size is 102.5 meg as a Tiff. If I change the resolution to 720 and keep the print size the same the file becomes 922.6 meg. I've never printed anything at greater than 300 PPI and large prints typically at 240 and even less with excellent results. Maybe there's something I'm missing but what benefit is it to interpolate a file to that size for printing?

The printer has a native resolution as well. By sending it a file in the same native resolution, it can then dither the resulting print more accurately. Will only come into play where there are stark contrast/color transitions. When printing something very cam make a difference in the transitions if the printer driver is not re-sampling to get to its native 720 or 1440 dpi.

Better Explanation:


A photo printer is dithering between 4 and 7+ inks.


Color printers are similar to B&W printers, in that they must print several of the printer's dots for each image pixel (except for dye sublimation printers, which can make any color on any printed dot). Inkjets have only 3 or 4 colors of ink, a few have 6 colors, and this is all they can print. They CANNOT print any one of 16 million colors on any one dot. So to represent each image pixel in various colors, shades, and intensities, the image is dithered, meaning the printer uses a pattern of several of its dots to simulate the color of each pixel in the image.

For example, to print one "pink" pixel on our inkjets, we know it must mix some red and some white. There is no white ink, white is the bare paper color, no ink at all. To make red, the printer only has the CMYK (Cyan, Magenta, Yellow, and blacK) ink colors, and so must use a few magenta and yellow ink dots, not necessarily equal numbers of each, to achieve a certain shade of red. To make lighter shades of red, blank white space is used in the right amount. Black ink dots are used to darken some colors. The average visual effect of all these individual magenta and yellow ink dots, white paper, and sometimes perhaps black ink too, looks pink to us. But all of these multiple ink dots represent or simulate the color of only ONE pink image pixel.

So it is clear that we don't get anywhere near 600 or 720 or 1200 or 1440 dpi of "image" resolution from our printers in Color mode. This requirement for multiple printer dots for one image pixel greatly reduces the printer's real image resolution capability to a fraction of the printer's advertised dpi. Printer specifications are real and accurate and meaningful, but are not to be confused with image resolution. Printer ink dots and image pixels are simply very different things, and one color image pixel requires many printer ink dots. This is why we need a 1200 dpi printer (ink dots) to print an image at 250 dpi (pixels). And like B&W printers, attempting higher resolutions on color printers simply limits the pixel size area, allowing fewer ink dots, which then limits to even fewer possible color tones. We need the several ink dots in that space to simulate the correct color of the pink image pixel.

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