Print Resolution

[Martin456]

I am new to digital imaging and have recently purchased a Canon 30D. I am dowloading directly to Photoshop Elements 3. My camera captures at a max resolution of 3504x2336. When downloaded to the computer it is saved at the resolution of 3504x2336 but with an output resolution of 72 dpi. This gives an extremely large physical picture but at a low resolution for printing. I do not know at which resolution it is printing. When printing an A4 print (297x210mm) does it automatically print at the captured resolution of 3504x2336 pixels (abt 300dpi) or at the lesser rate of 72 dpi? And, do I need to resize the image prior to printing?

Martin

[orangekay]

3504x2336 but with an output resolution of 72 dpi. This gives an extremely large physical picture but at a low resolution for printing

The pixel dimensions of the image do not change of their own accord (though the printer driver may downsample for you if you feed it more data than it can handle). Without getting into any of the semantic differences between dpi and ppi:

3504 pixels / 300 ppi = 11.68"
2336 pixels / 300 ppi = 7.79"

3504 pixels / 72 ppi = 48.67"
2336 pixels / 72 ppi = 32.44"

You can re-interpret those values in terms of any ppi setting you want, but until you actually alter the pixel dimensions of the image file, you're not really changing its resolution.

Or to put it another way, until you start printing 32 x 48, you're not printing those files at 72 ppi.

[bjanes]

The pixel dimensions of the image do not change of their own accord (though the printer driver may downsample for you if you feed it more data than it can handle). Without getting into any of the semantic differences between dpi and ppi:

3504 pixels / 300 ppi = 11.68"
2336 pixels / 300 ppi = 7.79"

3504 pixels / 72 ppi = 48.67"
2336 pixels / 72 ppi = 32.44"

You can re-interpret those values in terms of any ppi setting you want, but until you actually alter the pixel dimensions of the image file, you're not really changing its resolution.

Or to put it another way, until you start printing 32 x 48, you're not printing those files at 72 ppi.
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You can see the above changes in resolution (in terms of pixels per inch) with size in Photoshop if you uncheck the resample image option. The total resolution of the image remains unchanged without resampling.

Another question is how much resolution should you send to the printer. According to Bruce Fraser's most recent book on sharpening, this varies with the printer. With a contone device such a Fuji Frontier or Lightjet, if you send too much resolution to the printer your sharpening halos will be downsampled out of existence. For continuous tone printers, he suggests sending approximately the native resolution to the printer, thus avoiding this problem.

For inkjets using diffusion dither output, the situation is somewhat different. With current Epson printers (native resolution 360 ppi) there may be a small advantage in sending up to 480 ppi to the printer. However, sending more than 720 ppi to the printer may degrade the image.

Some photographers like to send even multiples of the printer's native resolution to the device: e.g. 180, 360, etc.

When the resolution in the image is less than the native resolution of the printer, Bruce usually sends the image as is to the printer and lets the printer driver resample as necessary. Others like to do the re sampling themselves.

Any comments?

Bill

[digitaldog]

I use Bruce's techniques because they work really well and its easy.

If the file falls within 180-720 ppi, I just let the driver take the data and sample to the size I specify (you can do this in Print with Preview/Print in Photoshop). The above values are for an ink jet.

[Martin456]

Thanks for that information. It has given me the answer to my concerns plus more.

Regards
Martin