DPI is a printing term, for “dots per inch.” Generally, photographers don’t care about this, as it is not a factor of the photo itself just how the photograph is used. However, the topic often comes up and is confusing, so here’s a primer.
“Forget about the dpi of an image. It’s pretty much irrelevant until ink hits paper. What is important? Total number of pixels.”
DPI is a printing and display term, meaning Dots Per Inch. 72 dpi is for web pages where each inch of an image requires 72 pixels. The Apple iPhone displays images at 132 dpi. 200 or 300 dpi is for printing on paper which requires a finer resolution of 200 or 300 pixels for every inch. This does not affect the file size (resolution, image size) of the photo itself, just how it is displayed.
For example, imagine a photo which is 600 pixels wide. On the Web, that would be 600/72 pixels=8.3 inches wide. That is the size of the photo above. But if I was to print that same file at 300 dpi, it would only be 600/300=2 inches wide. Same file, different size, due to a different display resolution.
My coffee-table books are 12"x10" printed at 350 dpi. So, for a full-page picture at full resolution, I would need a photo size of 12x350=4200 pixels wide, and 10x350=3500 pixels high. That’s a total of 4200x3500=14.7 million pixels. The Nikon D5100 has a sensor with 16.2 megapixels, so that’s suitable.
But what if my camera only has 6 megapixels? I can still print at 10 inches high but at a lower resolution (quality). A 6MP camera produces an image 2000 pixels high. Spread over 10 inches that is 200 dpi. Not the full resolution that could be printed but enough to look OK.
What DPI Should I Use?
As a photographer, it doesn’t matter. The image will be the same. The dpi setting is just a data field.
As a publisher, set the dpi data field to whatever you need. This may be 72 dpi for web, 200 dpi for inkjet printers, 300 dpi for prints.
I Licensed a High-Res Image But It’s Low-Res!
Is it? Whether the file says 72 dpi or 150 dpi or whatever doesn’t matter. What you want is the pixel dimensions. 3000 pixels wide and 2000 pixels high is a high-res, 6 megapixel image, regardless of the dpi setting it was saved with.
“The dpi of a file is just a way to measure it. When someone says they need a 300 dpi file, EVERY file can be measured at 300 dpi, whether is happens to be saved that way or not.”
Why Don’t Stock Agencies Use A Standard DPI?
Because resaving a JPEG image reduces its quality. Sometimes the agency can change the dpi but often it is left at whatever setting the photographer used.
PPI is Pixels Per Inch. For our purposes we’ll say that it’s the same as DPI.
I’m European and I agree that the metric system is smarter. PPC is Pixels Per Centimetre. (British spelling there!)
“Images may not be upsized more than 5% — we perform an interpolated upsize to sell the ‘supersized’ version — if you upload upsized images, you risk your account being suspended.”
You don’t want to do this if you’re uploading photos to a microstock agency.
“iStock will not accept files that have been up-sampled or “rezzed-up”. In other words, you can’t increase the pixel dimensions to be larger than the file’s native size.”
If you are controlling the printing, you can use software to “up-sample” the image to a higher resolution. This intelligently guestimates what the missing data should be. Products include Perfect Resize and PhotoZoom.
|Example Sizes for Photo Files|
|Size||Pixels*||Approximate Print Size*|
|XSmall||300x400||1"x 1.5" @ 72dpi|
|Small||600x800||2"x 3" @ 72dpi|
|Medium||1200x1600||4" x 5" @ 300dpi|
|Large||1920x2560||6" x 8" @ 300dpi|
|XLarge||2800x4200||9" x 14" @ 300dpi|
|XXLarge||3300x4900||11" x 16" @ 300dpi|
|XXXLarge||3700x5600||12" x 18" @ 300dpi|
*Sample dimensions only. Actual dimensions depend on the size of the original uploaded file.