Magic DPI numbers?

This is what we taught in the Epson Print Academy (http://www.epson.com/cgi-bin/Store/PrintAcademy/pa_home.jsp): Decide what size you want the print to be. If the resolution of the image as specified in Photoshop’s Image Size dialog falls between a low of 180ppi, a high or 480ppi, just send that to the driver and off you go. IOW, if you want an image that is 13x19 and the resolution falls at 197, or 223 or any odd number, just send that off to the driver as long as it doesn’t fall below 180 or above 480ppi. You’ll get the size you desire and there is no reason to interpolate the data (which is just made up pixels when you resize up).

Andrew: Just to be clear, when you say "any odd number" you don't mean "odd" in the sense of "not divisible by 2," but in the sense of "any number in that range.". Right?

I mean any value between 180 and 480.

Could you explain what you mean by "send to the driver"?

Large format Epson printers native photographic resolution is 360 dpi. It doesn't matter what you send the file to them at, you can send a 10dpi or a 1000dpi file to them, it will resample to 360. Some people think they can see a difference if other numbers are used, others don't. I have never seen, or heard of, a disadvantage (other than the inconsequential file size) to sending a file resampled to my satisfaction at 360dpi. Some might argue it is overkill, but that is what the printer is going to do anyway.

To prove it to your satisfaction try a test. Resample an image to 500dpi, 360dpi, 250dpi and 180dpi. They only need to be small crops. See if you can tell the difference.

Could you explain what you mean by "send to the driver"?

Take the data as it is (at say 207dpi), click Print and let the driver do the heavy lifting in terms of handling this data.

So conclusion is any print res between 180 and 480 is no better or worse than the traditional 180, 240, 288, and 360 on Epson printers that we have traditionally used?

Large format Epson printers native photographic resolution is 360 dpi. It doesn't matter what you send the file to them at, you can send a 10dpi or a 1000dpi file to them, it will resample to 360.

It is true. If you print borderless the PPI value is slightly higher.

Some people think they can see a difference if other numbers are used, others don't. I have never seen, or heard of, a disadvantage (other than the inconsequential file size) to sending a file resampled to my satisfaction at 360dpi. Some might argue it is overkill, but that is what the printer is going to do anyway.

The difference may be more visible on regular structures.

I heard from another source that with newer releases of Photoshop (CS4 and 5)

The resampling is performed by the printer driver.

The Epson driver doesn't interpolate in the same sense as Photoshop interpolates. It is more of a mapping and truncation action.

If you print a 100 ppi image, which is coarse enough that individual pixels can easily be seen, the Epson will faithfully reproduce those pixels so that they can be seen in the print as well, only with the pixel edges resolved at 360 ppi. They will be especially visible in diagonal lines and curves.

The same image resampled (interpolated) in Photoshop may show some pixelation in the output, but the edges will be blurred, with considerably less staircasing in diagonal lines and curves.

If the original resolution is greater than about 200 ppi, pixels will be too small to see and there is little staircasing effect. There is some argument that even multiples of 180 ppi will require less math to map to 360 ppi, but I've not seen any difference. The driver probably uses floating point math, which will take the same time even if modulo(n, 180) = zero.