Q: “I have a dumb question. My client says he needs an image that’s 5×6 inches at 300 Pixels Per Inch, but the file as it came out of my camera is 72 Pixels Per Inch. What should I do? How do I make it the right size? Do I downsample the image? Upsample? I’m so confused.”
A: It’s not a dumb question at all. I think the “resolution” question continues to be one of the most misunderstood and confounding pieces of the digital photography era.
Remember that all Photoshop knows is “how many pixels are there?” It simply counts the total number of pixels (rows x columns) and doesn’t much care about the number of pixels per inch. The pixels per inch is really a “human construct” that simply defines the size of the image as it will be printed. The term “Resolution” is confusing and sometimes misleading, because it’s only when we define how big those pixels are (how many of them there are in an inch) that we have a sense of whether the image has “enough” resolution for a given printed size.
So, a file that is 1500 pixels by 2000 pixels will be seen by Photoshop as 20.8 inches 27.7 inches in size when given a “Pixel Per Inch” number of 72 PPI. To do the math, 1500 divided by 72 equals 20.8 and 2000 divided by 72 equals 27.7.
That same 1500 pixel by 2000 pixel image will be seen by Photoshop as 5 inches by 6.6 inches when the Pixels Per Inch are set at 300 PPI. (1500 / 300 = 5 and 2000 / 300 = 6.6).
In both cases, the TOTAL NUMBER OF PIXELS in the image DID NOT CHANGE. We simply reassigned the total number of pixels to a different number of pixels in every inch, resulting in a file that would print at such dramatically different sizes.
Now, the one piece of information you either left out in your question or that you didn’t get from your client is “at what size will the image be printed?”
If the example I gave above was your image and your client needed it at 5×6 inches and 300 pixels per inch, but your existing file is 72 pixels per inch, you wouldn’t need to “resample” the image at all. You’d leave the image at the same total number of pixels and just tell the file that there is a different number of pixels in every inch (just like in the example above).
You would do this in Photoshop by going to the Image menu and choosing “Image Size…” At the bottom of that dialog box, there is a checkbox for “resample image” which, when *unchecked*, leaves the total number of pixels the same and only changes the number of pixels in every inch.
The dilemma comes when you have either too many pixels or not enough pixels for the desired number of pixels per inch and printed size. At that point, you have to “upsample” (add more pixels to) or “downsample” (delete pixels from) your image to get the size and number of pixels per inch needed. The problem here is that when you do that, you risk having an image that doesn’t look as good as it might because Photoshop has to figure out which pixels to get rid of (“downsample”) or to add (“upsample”). You do this by checking that “resample” checkbox to the *on* position.
This is a task that Photoshop isn’t bad at for a small jump in total number of pixels, but when asked to get rid of a *lot* of pixels or (worse) to add a *lot* of pixels, the results sometimes look soft and lack detail. Think of it as if your photograph was a mosaic picture on a wall and you needed to fit it into either a smaller wall (downsample) or a bigger wall (upsample). Which of the mosaic tiles should you remove or add that would get you the right result but still have the picture look “right.”
So, for your purposes, I’d try using Image> Image Size… with “resample” unchecked to get the right number of pixels per inch (in this case, 300) and see how big your image is. Once you’ve done that and you find that you need to make the image bigger or smaller, call up the Image> Image Size… dialog box again, check the “resample” checkbox and enter the number of inches x inches that you need the final image to be, aware of the fact that if you add or subtract pixels, your image’s quality may suffer.