Ask An Expert: Here a Pixel, There a Pixel

We asked for your questions, and you’ve responded! Welcome to “Ask an Expert.” Can’t figure out how to create a rounded rectangle in Photoshop Elements? Wondering what paper to use to print your digital layouts? Or maybe you just don’t understand what “resolution” means. Just Ask an Expert!


I’ve just started playing around with digital scrapbooking and love it. My problem: My program works in pixels. How can I figure out how many pixels equal an inch? I think I’ve decided that 3600x3600 pixels equals 12x12 inches. After that, I have no idea. What would a 4x6 inch photo be in pixels?
      Judi Goins
The answer is, it depends.
Wouldn’t it be great if there was a standard measurement of pixels for each inch? Something we could all remember like 12 inches equals one foot. Unfortunately, that’s not the case. The good news, though, is that you can figure out for yourself how many pixels equal an inch for any document size you want. You just need to understand a few key concepts first.
What does resolution mean and why should I care?

Resolution generally refers to the sharpness and clarity of an image. It’s measured in dots per inch (dpi) for printers and pixel dimensions (the number of pixels wide by the number of pixels tall) for monitors.
If you have an 8x8 inch digital layout that you are printing on your inkjet printer at 600 dpi, it will apply 600 dots of ink per inch to print the image on paper. On the other hand, if that same image is printed at 72 dpi, the printer will only apply 72 dots of ink per inch. Both images will print at 8x8 inches, but one will have more dots of ink applied more closely together and will be of higher resolution and quality than the other.
Monitors, on the other hand, use pixels instead of ink. In fact, most digital imaging software (like Photoshop) refers to screen resolution in pixels per inch instead of dots per inch. This simply refers to the number of pixels the imaging software will use to display one inch of your document on your screen
If you’re importing a photo from your digital camera, the camera’s setting determines the resolution. On the other hand, you, the user, get to decide what screen resolution you want to use for a new document or layout.
Digital scrapbookers usually set their resolution to 300 pixels per inch (ppi) to achieve a high level of quality. The lower the pixels per inch, the blurrier the image will appear on paper. With most imaging software, you can set this when you open a new document window and can also change it at any time. Here’s an example using Photoshop Elements. Your imaging software probably uses a similar approach.

Here I’ve created a new document that will be 8x8 inches at 300 pixels per inch.











Figuring out the pixel dimensions
Since monitors cannot spray the page with ink like a printer would do, the only way to display an image at different resolutions is to give it more or less screen real estate–to make it larger or smaller on the screen based on the number of pixels per inch.
So exactly how much screen real estate, or pixels, does it take to display my 8x8 image?
Since I know the size of my document and the pixel resolution, it’s a simple arithmetic exercise to figure out the pixel dimensions of this document.
Multiplying the width (8 inches) by the resolution (300 pixels) yields a screen width of 2400 pixels. Since this layout is square, the height is the same as the width. So the pixel dimensions of this image are 2400x2400 pixels.
If I change the resolution of this image to 150 ppi, my pixel dimensions will change accordingly to 1200x1200 pixels. That doesn’t change the size of my original 8x8 inch document; it only changes the screen resolution and pixel dimensions.
You can also open digital photos you have uploaded to your computer within your imaging software. In that case, if you open a 4x6 photo that was captured at 72 pixels per inch with your digital camera, the same formula applies:
6 inches x 72 pixels = 432 pixels (width)
4 inches x 72 pixels = 288 pixels (height)
The photo’s pixel dimensions are 432x288 pixels.
Hope this answers your questions and then some, Jodi. Now you’re armed with the knowledge you need to figure out the pixel dimensions of any image!

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