Nothing can confuse even a well trained photographer more than resolution! Back in the day the resolution of an image was something most people never even thought about. You bought your film, stuck it in the camera, and took a photo. Sent it off to be processed and "ta-da" got back a lovely print!
With the advent of digital photography, we have access to so much more information about our images. Just a peek into File>Info in your photo editing program gives you all kinds of knowledge about your image - when and possibly where it was taken, the size of the file and the size of the image to name a few. It's this last one that really messes people up!
An image is made up of pixels, or for our purposes - dots of color. When you talk about how many megapixels your camera is, that just means how many pixels it can capture for each image. An image that is 1280 pixels wide and 1024 pixels high has 1280x1024=1,301,720 pixels, or 1.3 megapixels (MP). A 2580 x 2048 pixel image comes out to 5.3 MP. Many cameras can capture images at 8, 10 or even 12 MP.
While knowing the number of pixels an image is made of is useful, what most people are interested in is the resolution. Resolution is a term of output - what physical size will the image display as, and how much detail will be visible. We refer to this as pixels per inch (ppi) or dots per inch (dpi). PPI refers to screen output (computers, television, phone, tablet), DPI refers to printed output. For clarity, we'll use ppi for both.
Resolution is the number of pixels displayed within an inch. The more pixels (or dots) per inch, the more detail is visible in the image.
Photos are almost universally printed at 300 pixels per inch. This means a 1" square print would be 300 pixels wide by 300 pixels high. A 4"x6" print would be 4 x 300 = 1200 pixels wide by 6 x 300 = 1800 pixels high.
That same image that is 1200 x 1800 pixels and prints at 4x6 inches would be MUCH larger on a computer screen. Why? Because screens only display 72 pixels per inch. So 1200 pixels divided by 72 = 16.6 inches! 1600 divided by 72 = 22.2 inches! Your little 4x6 is now 16x22! Ever get an email from someone with a photo and you have to scroll around to view it? That's why - they put a full resolution image in the email, and it displays bigger than your screen, so you have to scroll to see it.
Now here's where people get really confused. THE RESOLUTION doesn't matter. "What? You just told me it makes a big difference!" Well, yes and no.
It only makes a difference when you go to output the image. If you're printing you want to have at least 300 pixels for every inch of size you plan on printing. For screens you don't want more than 72 pixels for every inch you expect to see. But when you open the image in an editor, the resolution displayed in the size box means nothing except how large that particular image will be when printed or displayed.
WHAT MATTERS IS THE PIXELS. If your image has at least 1200 pixels in any dimension (4 inches, printed) you should be good to do whatever you need to do with it. However if your image has 288 pixels across, you can display it on a screen at 4 inches, but it's not large enough to print.
Where people get confused is here:
Notice the dimensions of the image are 900x900 pixels - and since the resolution is set at 300 ppi, the width and height are 3 inches each.
But when I change the resolution to 72 ppi look what happens:
The image is still 900 x 900 pixels, but the width and height are 12.5 inches! That would fill your whole screen! Yet IT'S THE EXACT SAME SIZE IMAGE. Still 900 x 900. It's just the resolution that it is being displayed at that has changed.
Where this will mess you up is if someone saved this image at 72ppi but still at the original pixel dimensions 900 x 900. Then say you go to upload it to put in your Shutterfly book, and you get an automated warning "This image is too low resolution for a good print result". But wait, we know it should print fine at 3 x 3 inches since it's 900 pixels. So why the warning? Their software just looks for that resolution field, it doesn't consider how many pixels you have and what size of space you're trying to put them in. It sees a 72 and automatically sends the warning.
I've had this happen to me, uploading scrapbook layouts that I KNOW are 3600 pixels across. When I get the warning, I just go to the original file in the folder and hover my cursor over the thumbnail until the file info pops up which shows me the pixels. If there are enough pixels to put 300 of them in every inch of the size I want to print the photo, I know I'm good and I tell it to ignore the warning.
You can fix this by going into your files and changing the resolution. But you have to be VERY careful not to change the number of pixels in the image if you do. In Photoshop and PS Elements when you go into Image >Resize be sure that the Resample box is NOT checked. This will keep the same number of pixels and only change the resolution number so you don't get that warning.
For me, it's all about the pixels. I only consider resolution if I need to resize an image smaller so it fits on a website or blog page properly, or so I can put it in an email the right size for viewing. Then I just do the math - number of inches I want displayed x 72 = number of pixels needed. Otherwise, I ignore the resolution box and keep an eye on the pixels!
Have you ever run into this resolution problem?