Ernst and everyone
Testing has proceeded this evening, and I do have some interesting results. My output was from Lightroom 3.4.1, running on Win 7 64-bit, and printing to my Epson R2400. The 2400 was set to its best output resolution, Photo RPM (5760 x 1440), High Speed off, and the paper was Epson Premium Glossy. The test file was sized in the print module to produce output at exactly 720 ppi initially, and then resized to produce other output resolutions for comparison. LR output sharpening was turned off.
I do understand that the original purpose of the restest file was establish what ppi and printer settings were appropriate for a given paper and print size combination. What we are trying to establish here (does the printer resample input data or not) is a bit different and not easy to pin down.
* If the test image is sized to exactly 720 ppi and sent to the printer direct so that there is no resampling whatsoever taking place, either in LR or at the printer (we assume), then we get a clean printed output with no artefacts, as you would expect. The difference between the 720 and 360 line widths in column ‘D’ is clearly apparent.
* I could not try Ernst’s 1440 ppi test, because LR will not report resolution settings above 720 – it just says 720+. So I had no way of accurately resizing the image to 1440.
* I could not try the 720 to 360 test, either, for reasons which will become clear.
* If the file is resized to any other value than 720 or 360 ppi, it results in strong vertical banding in columns A and D. The “wavelength” of the banding seems to vary according to the mathematical relationship between the resolution of the file and the output resolution at 720. The banding happens whether the file is downsampled to 720 or upsampled from a lower value. So any resampling results in artefacts, whether the resampling is done in LR (598 to 720, say) or we just send the file straight to the printer and let the Epson sort it out.
* If we take the image sized to say 686ppi and resample to 720ppi in LR, then send it to the 2400, or alternatively send the file to the printer at 686ppi with no processing in LR we get exactly the same banding, except that we can see even with the naked eye that the artefacts are more tightly defined and crisper in the LR processed version and the overall print quality is better. From this I infer that the printer does indeed resample a non-720 input, but that the LR sampling algorithms are superior.
* I also resampled the file in LR to 360 ppi and there were no artefacts, but the printed result was clearly inferior. The 720 and 360 lines in column ‘D’ become the same width, and the text becomes very smudgy.
* By now I had got rather curious, and tested the file with all the various quality settings on the 2400 for photo output on glossy paper. From the best downwards, they are called – Photo RPM, Best Photo, Photo, and Fine. Your Epson may have different names, but they probably do the same thing. I expected the output to shift from 720 to 360 somewhere down this list, but no – all of these are 720 ppi output. The quality does reduce a little, especially in the text, but the difference between the line thicknesses in column ‘D’ is still clearly apparent.
* So it seems that the Epson R2400, at any rate, when printing photos on glossy paper is always a 720 ppi device. Any other resolution you send to it gets resampled, and some quality is lost.
These tests back up my own observations when printing real photographs – a subject which we can expand on a bit tomorrow, perhaps. By the by, my “naked eye” may well be a bit different than yours, because I have been short-sighted since the age of 5 and I focus without spectacles at 6 inches. The results were also checked with an 8x loupe.
John
