Digital Photos are made up millions of tiny little squares called pixels. Every time you take a photo with your digital camera, you are collecting a couple million organized pixels as captured by the sensor behind the lens. If your camera is a 10 megapixel camera, you are collecting 10 million of these pixels. If you have a 5 megapixel camera, you are collecting 5 million of them.
Another piece of information, in addition to those millions of pixels, is also passed along to the jpg or tiff file your camera creates, and that is the resolution of your photo. Because pixels only exist on a screen in the digital world, we need a translation to be able to take those pictures from the screen to a print. So our camera provides a number, usually 72, though sometimes 100 or 240, that corresponds to the number of pixels that a printer will pack into each printed inch of your photo. To find this information in Photoshop with an open photo, go to Image > Image Size.
Those are the specific numbers that I get directly out of my camera. The Pixel Dimensions refer to the total pixels captured in my camera – 3888 pixels across by 2592 pixels down equals 10,077,696, or approx 10 megapixels. The Document Size area refers to the translation of that information into real life. My camera provides the arbitrary number of 72 ppi (pixels per inch) as its default resolution. So dividing our width and height in pixels by our resolution we get the numbers shown in the Document size area in inches. In other words, if I were to hit print right now in Photoshop, it would try to print a 54″ x 36″ print. There are two operations we can do to remedy this situation.
Resizing refers to changing the resolution of the image. For most print applications, the standard resolutions of 300 or 360 are sufficiently high enough that you will have nice smooth images. Making sure that “Resample Image” is unchecked, if you type 360 into the Resolution box, you’ll see the numbers for the width and height change with it:
Nothing about our photo has changed one bit here digitally. We still have the same 10 million pixels we started with, we are just telling photoshop that we want to print more of them per inch. This gives us a totally reasonable size of 10.8″ x 7.2″ and we could print right now if we wanted to.
But maybe 10.8″ x 7.2″ isn’t exactly the size we’re looking for. Lets say we wanted to print out our photo at 360 resolution on an 11×17 piece of paper. Now we need to resample our image, which means we are going to be adding or subtracting information (pixels). With “Resample Image” checked, type in either the width or height of your targeted printed size. In this case, I’ll choose 10 inches as my height:
This time, Photoshop has done a little more than some simple math. If you look at the pixel dimensions at the top, we are now at 5400×3600, or over 19 million pixels. Photoshop has used its resampling operation to add in about double the amount of pixels than we started with. In other words, there is now about one extra pixel between every original existing pixel that we started with. Resampling is considered a destructive operation, since we are creating information out of thin air (albeit very intelligently). Photoshop does a pretty good job upsampling at this rate. However, if you start trying to upsample at a rate of introducing say 10 pixels for every one pixel of information (ie ending up with 100 million or so pixels), Photoshop will struggle a bit more, so it’s up to you to try your resample operation and scrutinize to see if it’s had a detrimental effect on your photo. Similarly, you can also choose to downsample your photo by typing a smaller number into the width or height. This is also destructive because now we are throwing away original information from our camera to make sure we end up with the right number of pixels.
Knowing the difference between resampling and resizing and how to use both of them together are essential basic tools for digital photographers who wish to print their work.