Say what? Don’t you mean dots per inch? Well not exactly. They are two different items and I have spent a lot of time trying to figure things out with the two items in the last week. Actually I have been battling this monster ever since I started working with digital images almost twenty years ago. So it’s pretty easy really, pixels per inch or ppi refers to how your digital image is displayed. If your image is 3000 pixels long and you want to size it to 10 inches, then you have 300 pixels per inch. Dots per inch or dpi refers to how the image is layed down on paper with your printer. But this is where all the confusion has come for me over the years. If my image is sized to 300 dpi then my printer should have a 300 dpi setting right? Nope! Usually you have printing settings of 360, 720, 1440 or 2880 to choose from. Well if I want the best print should I choose 2880? Nope to that one as well. And if that was the best setting for my image should I resize my image to 2880 ppi? AAAAAHHHHH the confusion!!!
I’m not sure what all the technical reasons are for why one setting works better than another but I will tell you what my results are. I have used mostly Epson printers and I can tell you from my experience of printing thousands of images of all different scene types that 1440 dpi is the best choice for high quality images. 2880 dpi just uses more ink and takes more time to print. 720 is a very good alternative if you want to print faster and the utmost quality is not the #1 priority. So set you printer setting for 1440. Got that 1440.
Now what about your ppi? This is something else that is very confusing but I have always heard of two magic numbers; 240 ppi and 360ppi. Not sure where these come from but I know they are truthfully the best range for your image to be sized at. But Mike if 360 is good then wouldn’t 720 be better? Well no it wouldn’t. I made prints with increasing ppi to see if the prints got sharper as the ppi went up. A very strange thing happened. Parts of my images actually got pixelated as the ppi increased. I think what was happening is that the printer was trying to squeeze the pixels onto the paper and they got all bunched up. Conversely when the ppi went below 240 the printer had to fill in the missing pixels with dots and didn’t always get it right so there was pixelation at lower resolution also.
The one thing that I should mention is that these images were of a building that had a lot of horizontal, vertical and diagonal sharp lines which were perfect for checking pixelation or the jaggies as they are sometimes called. I also viewed the final images with a 8x loupe. So if you are hanging the pictures on a wall and aren’t right up against them then you can usually print at a lower ppi. But you can’t go too low. Go below 150, 125, 100 and you will see image degredation no matter what size print or viewing distance.
One more thing to take note of. Using Photoshop to upsize your image so you can make a 40×60 inch print doesn’t really work. Neither do any of the other programs out there. I usually just let the printer do the upsizing. Sometimes I use Photoshop to upsize and then use the Unsharp mask filter to control things a little bit better but that’s pretty rare.
So I hope this has helped to de-confuse you at least a little bit. Good luck pushing those pixels.