One of the most important things I know about digital image resolution I learned from baking cookies. Here it is: four tablespoons equals one-quarter of a cup. What does that have to do with resolution you might (logically) inquire? Allow me to explain.
Too many people get all hung up on the wrong detail: the dots per inch (or DPI) of an image file. This measurement is related to the allocation of pixels in an inch of screen on which the image is displayed or the paper on which an image is printed. They get hung up on this because they don’t understand that measurement of resolution—300 dpi, for instance—is only part of the equation. DPI is meaningless without the physical dimensions of an image.
Such specificity is critically important when determining what file sizes are needed for a given purpose. Dots per inch and physical dimensions combine to form the area resolution, and these are the specifics that really matter when talking about resolution.
A very common resolution request is to deliver an image file that is “300d pi.” Knowing that an image needs to be physically printed on paper at 8×10 is very helpful because 300 dpi is typical for printing, so the combination of 300 dots-per-inch and 8 by 10 inches gives you the area resolution you’ll need: 2400×3000 pixels in total. But saying only “300 dpi” is meaningless without that physical dimension. Because any image can be 300 dpi.
If you tell me that you need a file to be 300 dpi at 8×10 inches, now we’re talking. That’s very specific. That tells me everything I need to know to provide an image file that will work for your needs. I’ll determine that file size by multiplying 300 dpi by those physical dimensions (8 and 10 inches) to arrive at the area resolution of the file: 2400 pixels by 3000 pixels in size.
By counting pixels, you know how big an image file is—or how “high resolution” the image will be. More pixels mean more resolution. This is a measurable, unchanging measurement of file size. If you want to know how large or small a file is, count the total number of pixels.
This is what’s being measured when a camera sensor is referred to in “megapixels.” Those megapixels are simply the pixel dimensions of the sensor multiplied to get a number in the millions. 2400×3000 pixels, for instance, works out to an area resolution of 7,200,000 pixels—abbreviated as 7.2 megapixels.
Those pixels can be spaced differently—at 300 dots per inch, let’s say, or just 72 dots per inch. At 72 dpi, that 2400×3000-pixel image can be enlarged to a whopping 33×41 inches in size. That’s huge! But not nearly as “high quality” as if those dots were packed in closer together—which is why the standard of 300 dpi for printing makes it difficult to make quality prints out of small image files. A file of 1200 pixels by 1500 pixels will display large enough on screen at a resolution of 72 dpi, but with those pixels allocated to 300 per inch, the file will only enlarge to 4 inches by 5 inches with any quality. (Because 1200 pixels divided by 300 dpi is 4 inches, and 1500 pixels divided by 300 dpi is 5 inches). If it’s enlarged to print at 8×10 inches in size, it can only allocate about 150 dots per inch—not nearly as high quality and “continuous tone” as when 300 of those pixels are packed into an inch.
Are you beginning to see the similarity between resolution and baking cookies? It’s all about understanding equivalencies and discerning a meaningful measurement from one that isn’t.
Let’s say you’ve got a quantity of sugar; a quarter cup. I can ask for 4 tablespoons of sugar or one-fourth of a cup of sugar or a quarter cup of sugar and it’s all the same amount expressed differently. You could even weigh the sugar to find out that it’s exactly 50 grams or 1.76 ounces. In each of these examples, the quantity of sugar is the same, we’re simply using different terms to describe it.
And so it is with a digital image file.
In the case of our image, “weighing the sugar” amounts to counting the physical pixels, the area resolution. That’s the best way to measures precisely how many pixels make up an image file. Once you know the dimensions—2400×3000 pixels—you know exactly how large the file is. No matter whether those pixels are divided into 300 or 72 or any other number of pixels per inch because, after all, any image file can be 300 dpi—or even 3,000 dpi or 30,000 dpi—so long as you don’t mind printing it very small. You’re simply reallocating the same quantity of pixels in a new way—just like a quarter cup of sugar is exactly the same as 4 tablespoons, 50 grams or 1.76 ounces.
The next time someone tells you they need a “high-res” file or that it must be “300 dpi,” confidently reply that it’s only part of the story. To get the file size right, you need to deal in specifics, and that means the pixel dimensions of the image file.