Some Notes on Sizing Images: Part 3, Image SizeWe've looked at the image resolution. Today we start to include image size, which means the size at which the image displays. Image size is a second constraint put on an image - its display size. It's fine to know the resolution, but you also want to know how large the image will actually be. Note that you can theoretically display an image at virtually any resolution and any size - a 500 foot wide 60 dpi imge, or a 5 inch wide 300 dpi. So, when resizing an image, you will need to specify this in the image editor.
Resolution and image size are inversely related. The lower the resolution, the more you can spread out the pixels, and so the bigger the image. Similarly, the smaller the image, the more you can crank up the resolution.
This will be a trade-off: You only have so much image data and so must make due with what you have. If you have a 300 dpi image that is 4x5 inches in size, without adding to or subtracting from the image data, you can have an image double that size - 8x10 inches - but the resolution is only going to be 150 dpi, because you're spreading the dots farther apart to get them to fit in the new image display size. Similarly, you can take the 300 dpi image and squish it down to 2x2.5 images, only the new resolution will be 600 dpi. Even if you print the image to see at arm's length, you'd only need 300 dpi. In other words, you have more resolution than you really need, and that means taking up more storage for the one photo than is necessary.
What you want to do is give an image the resolution and size you really need. An application like Photoshop Elements gives you a lot of options on that front. More on that tomorrow.