The relationship between image size in pixels and DPI (dot per inch) or resolution as it's labelled in Photoshop is one that seems to confuse many newcomers. The first point I'd like to address is the peculiar notion that web images must be 72 DPI. This is just nonsense: the size of an image on your screen will be completely determined by the number of pixels on each side.



The image on the left is set to 240 DPI; the image on the right is 72 DPI. Not a lot of difference, is there? The file size is the same too — down to the last byte.

Changing the Print Resolution

Another thing to understand is the relationship between print size and resolution. Here's a screen shot of the Image Size window (click for larger image). You reach this by using the command Image>Image Size or by right-clicking in the title area of an image and choosing "Image Size...”.

I am about to change the resolution (DPI) of this image. I don't want to change the number of pixels, therefore, I have unchecked the "Resample Image " box. This greys out the "Constrain Proportions" option. Below you can see I have changed resolution from 240 to 72: the pixel dimensions have remained unchanged, but the document size has increased from 3.18 cm square to 10.58 cm square. We get a bigger picture, but only when the image is printed. In this final example, I have changed resolution, back to 240, but this time the "Resample Image" option was checked. This increased the number of pixels in the picture so that the physical print size would stay the same.

Note that the "Constrain Proportions" option is checked. This is why the width and height pixel dimensions and document size are shown locked together. If you uncheck this option, then you can change the height and width independently causing your picture to distort.

In this final example, I have changed resolution, back to 240, but this time the "Resample Image" option was checked. This increased the number of pixels in the picture so that the physical print size would stay the same.

Note that the "Constrain Proportions" option is checked. This is why the width and height pixel dimensions and document size are shown locked together. If you uncheck this option, then you can change the height and width independently causing your picture to distort.

Update: I received an e-mail from Jim Emery (thanks, Jim). His mail has prompted me to add a comment reminding readers to consider the size of images — in kb not pixels or inches — that they create for web use or e-mailing to friends and family: an image that you have created for a 6x4 print at 300 DPI will be 1800x1200 pixels and take a decent chunk of real estate on your hard disk. A 600x400 pixel version will be fine to e-mail and consume much less bandwidth.

Updated 26 December 2009