How much image resolution is too much for printing?

[Durant Philosopher]

Hi,

I'm new to the forum, so I'm not sure if this is the right place to post. At any rate, I have several questions regarding image resolution.

Everything I read says it is best to scan slides at at least 2400 dpi in case you ever want a large print. However, I have found if I scan a slide at 2400 or 3200 dpi and resize the image (using Photoshop Elements 6) for a 4x6 print, this results in a very high image resolution of 540 ppi for the 2400 scan and 707 ppi for the 3200 scan.

My question is, since the recommended print resolution is 300 or 360 dpi (not sure which is best), how will printing at the larger resolutions affect the quality of my prints. Should I resample them to a lower resolution of 300, and if I do, how will this downsampling affect the quality of my prints? (I will print some of these photos on an inkjet printer, but may send some of them out to be printed).

Ultimately, I am wondering which of the following scanning methods would result in the best quality 4x6 print:

1. Scan slides at the 4x6 scanner setting with an output resolution of 300 (or should I make it even higher, say 360, 400, or 600?)

2. Scan at 2400 or 3200 dpi (not sure which is best), resize to 4x6, and print at the resulting high image resolution (540 - 700 ppi).

3. Scan at 2400 or 3200 dpi, resize to 4x6, then resample to a lower image resolution (and if so, how much? 300, 360, 400, etc?

Thanks in advance to anyone who might be able to help me out here!

Linda

[Colin Southern]

Hi Linda,

How much image resolution is too much for printing?

Well ... you can't really have "too much" - it's really a case of "above what resolution does it not really make any real difference". And in my opinion that equates to approx 180ppi, unless your either literally viewing each shot with a magnifying glass or - arguably - producing images for a high-end glossy magazine.

The BIG variable with print resolution is viewing distance; the further the viewing distance the lower the resolution you can use and still have it look sharp (even 1ppi would probably look sharp if viewed from 1km away).

So to address your question regarding what resolution to scan at ... it really comes down to how much bigger than the original are you planning on printing (or viewing) them. Just as you can't "print at too high a resolution", you can't really scan at too high a resolution, but the price to pay is in the resultant file size. Additionally, all scanners only scan to a certain physical resolution - after that it's all done with "smoke and mirrors" using interpolation which (in my opinion) really doesn't gain you anything that you couldn't achieve in Photoshop later on.

So to give you a "real world" example ...

If I had a 6 x 4" image and I knew that it's unlikely that I'd ever want to print it any larger than 12 x 8" then - personally - I wouldn't get too excited about scanning at much over 2 x 180dpi = 360ppi (or "samples per inch" if one were to be pedantic). or perhaps 720ppi if you wanted to have a bit more headroom.

If your scanning negatives then same process applies, but obviously that ratio of final printed size to nexative size is greater - in which case you need to up the scanning resolution accordingly (I'd suggest as a starting point, 180 x the the ratio of film size to desired output size).

Having just said all that, there is an even easier way ...

... Scan a few at different resolutions - print them at the sizes you want - and view them from the distances you want - and see how they look

Does this help?

[Durant Philosopher]

I would have sworn that I read somewhere that you actually lose print quality if your image resolution is too high (something about squeezing all of those pixels so closely together?) Are you sure about this?

I am in the process of doing a scan test as you suggested, but I ran into the problem of needing to sharpen the scanned images before printing, and since I seem to need a different amount of sharpening for the various scan resolutions, it occurred to me that I wouldn't really know if the difference in print quality (if any) would be due to the difference in sharpening or the larger image resolution. Any suggestions?

[Dave Humphries]

I am in the process of doing a scan test as you suggested, but I ran into the problem of needing to sharpen the scanned images before printing, and since I seem to need a different amount of sharpening for the various scan resolutions, it occurred to me that I wouldn't really know if the difference in print quality (if any) would be due to the difference in sharpening or the larger image resolution. Any suggestions?

For the purposes of the tests, (using the same picture) don't sharpen.
Or sharpen to a consistent formula (but I've no idea what that might be).

As you have gathered, the sharpening is potentially going to have more of an effect than the scanning resolution.

As Colin says, (he's very rarely wrong) the filesize is the main downside to scanning at higher resolutions. If you can find what the scanner's "native" resolution is, use that - as he says, everything else could be done later if you ever need to (upsize for a mega print).

In your options 2 and 3, is that a non-resampled re-size you're doing?

For printing at home on an inkjet, I have to say I wouldn't bother with that 're-size' step at all; I take the file at whatever pixel resolution it is, then just print it and choose the media size I am printing to in the Printer dialogue. I do a bit of 'head maths' and ensure that I am going to exceed 100dpi first though, just so I don't A4 print something that's only say, an 800 pixel original (due to excessive cropping). Warning: maybe I am going to learn something in Colin's next reply as to why I shouldn't be doing this. So far I've not had any problems and have always been pleased with the results, but I don't do that much printing.

[Colin Southern]

I would have sworn that I read somewhere that you actually lose print quality if your image resolution is too high (something about squeezing all of those pixels so closely together?) Are you sure about this?

Yes, I'm sure - the actual PRINT resolution as opposed to the IMAGE resolution is usually much higher anyway (the printer driver up-samples the image "on the fly), eg my Epson 7800 prints at 1440 or 2880 dpi regardless (and I can't tell the difference between the two modes). Conceivably you could get to a point where the drive was down-sampling, but you'd be working with a very big image (file size wise, assuming the actual output dimensions were something less than trivial), in which case your better off down-sampling in Photoshop and applying output sharpening before it hits the printer.

I am in the process of doing a scan test as you suggested, but I ran into the problem of needing to sharpen the scanned images before printing, and since I seem to need a different amount of sharpening for the various scan resolutions, it occurred to me that I wouldn't really know if the difference in print quality (if any) would be due to the difference in sharpening or the larger image resolution. Any suggestions?

The bottom line is "it doesn't matter"; ultimately your striving for the best quality - if that happens to be a particular scan resolution / sharpening combination, then so be it.

Keep in mind also that it's not a case of "only 1 thing being right, and all the others being wrong" ... there will be many (most?) combinations that you won't be able to tell the difference with.

[Durant Philosopher]

Thanks so much Colin and Dave for your helpful responses. I'm sure happy to have found this forum!

Dave, I can't seem to find the quote button to place your question in, but you asked: "In your options 2 and 3, is that a non-resampled re-size you're doing?"

When I said resize, I meant going to Image>Resize Image (in Photoshop Elements 6) and changing the document size from the original scanned slide dimensions (0.898 x 1.343) to 4x6 for printing. Doing this changes the image resolution from 2400 ppi to 536 ppi but does not change the actual pixel dimensions or file size. My question with options 2 and 3 was whether it would be better to resize without resampling (downsampling) and print at the resulting 536 ppi resolution (option 2) or to both resize and downsample to a lower image resolution (option 3). And the answer was that there is no need to downsample (other than file size) apparently which is good news for me.

The problem with my testing is that I haven't found a slide that I feel is adequate for the test without sharpening, but I'm sure I'll come across something.

Thanks once again for your help. Would welcome any other comments. Linda