Originally posted by veezchick
Note: all images shot with K20D at optimal quality - some JPEG, some RAW
Assuming these aren't crops, that means you've got 3104x4672 pixels.
Quote: [*]I need my printed images to be 16x24"
So just ask Mpix to do that. You don't need to mess with anything at all in the file to make that happe. Mpix is perfectly capable of creating a 16x24" print whether or not you fiddle with the file first. In particular, you *definitely* needn't be concerned with what Photoshop says the size would be if you happened to print at the default of 72ppi.
Quote: [*]I've always heard printing at 300 PPI is optimal, so I changed the 72 to 300
That doesn't affect a thing except the canvas size as displayed by Photoshop. Your image still has 3104x4672 pixels, so the actual print resolution won't have changed a bit. Unless you do something to decrease or increase the number of pixels, that means a 16x24" print is 3104/16 = 4672/24 = 194dpi.
While you can certainly ask Photoshop to make up pixels for you to bring the total up to the necessary 16*300 x 24*300 = 4800x7200 you'd need to get to get a 300dpi print at 16x14", it's not really necessary. Made up pixels are made up pixels; you're not *really* getting any higher resolution. And that's fine - at that size, 194dpi is more than good enough. You don't normally view a 16x24" print from anywhere near as close as you would a 4x6 or even 8x10".
Of course, the printer itself *always* prints 300ppi - that's all it is physically capable of doing. So one way or another, someone is going to be making up those extra pixels to get to 4800x7200. The only question here is whether you do yourself in Photoshop (by selecting one of the resizing / rescaling options), whether you let the folks at Mpix do it (also most likely in Photohop), or whether the folks at Mpix just let the printer driver do it automatically.
Of these three options, I think the first is *least* likely to produce optimal results, unless you really know what you are doing and how to optimally control the rescaling process. I'd expect the folks at Mpix or the folks who designed the printer driver for the printer they use to know *far* more about the process than you (or I, or pretty much anyone not in the printing business). So my recommendation would be to not mess with any of this - just give them the file right as it comes out of the camera or as edited within Photoshop but with no attempt on your part to make up pixels yourself (eg, don't try to resize / resample, and again, no need to mess with the resolution figure).
Quote: [*]So I check the "Resample Image" box, which maintains the 300 PPI when I reenter 16x24" as my document size.
For the record, this is indeed how you tell Photoshop to make up enough extra pixels to give you 4800x7200, which is what you need for 300ppi at 16x24". But there may be different options for selecting the specific resampling (= pixel making-up) algorithm used, and different algorithms work better for different images. Which is part of why I say you're not likely to be better off by messing with this. But I'd assume the default resampling algorithm wouldn't be noticeably worse than if Mpix just let the printer driver handle the resampling. So unless Mpix does custom processing, it probably won't hurt to do what you're doing here. It basically works out to a big no-op, though (meaning, a bunch of steps performed with no real effect. Again, just handing them the file straight from the camera would print at exactly the same resolution.
And, BTW, you're right - there have been many existing threads on print resolution. You're not even close to the first person to be confused by all this. But it's really quite simle if you keep in mind that in the end, it all comes down to how many pixels you have and how many inches you print. If you have X pixels and print at Y inches, the resolution is X/Y.