Just encountered a problem that is a bit puzzling me.
If you create a 1cm by 1cm image in Photoshop, with a resolution of 200 pixels/cm (= 508 pixels/inch), and you save this image to PDF, using for instance the PDF/X-1a presets or one of the Ghent PDF Workgroup PDF presets, the image will be downsampled to 118 pixels/inch (or 47 pixels/cm). This does not compute. When checking the PDF-settings, I would suspect the image would be downsampled to 118 pixels/cm (or appr. 300ppi), as you would believe from the PDF-settings, but not to 47pixels/cm or 118 pixels/inch.
Is seems to me that Photoshop is taking the value (the '118') for the downsampling, but without translating my pixels/cm to pixels/inch first (it just downsamples 200 pixels/cm to 118 pixels/inch), which doesn’t seem very correct to me.
This is a huge problem. If people would accidently use pixels/cm to set their image size, they will end up with a PDF that is not printable and will be rejected by most printing companies because of too low image resolution. I tried to do the same with Adobe Photoshop CS3, but the same problem.
Anyone that can help me try to understand the logic behind this? Below the downsamping settings that would make me believe that I would end up with a 118 pixels/cm image instead of 118ppi and some other screenshots to clarify the issue.
Original image resolution in Photoshop
Image resolution (or preflight error) after saving to PDF
The odd thing is that is doesn't downsamples it to 300ppi (which I would expect too), but to 118ppi! And that's what 's makes me so worried. If you change your image settings (image > image size) to pixels/inch, it will downsample correctly, meaning to 300ppi. I keeps puzzling me .
Because it is only 118 ppi or 47pp/cm. Remember: the original image was 1cm wide and had a resolution of 200 pp/cm. The resulting PDF contains only 47 pixels for the entire width (the entire 1cm), so: 47pixels x 1cm or 2,54 inches = 47 pp/cm or 118 pixels/inch. It looks like there were really thrown away too many pixels during downsampling (calculation error in the PDF presets downsampling algorithm).
Actual number of pixels in the PDF (dimension: 1cm x 1cm):