So you thought that file size was confusing... Wait until you get into a discussion about resolution.
Did you ever get a call or an email from a client asking for a 300 dpi image? What does that mean? Here's how it typically goes, "Hey John, we saw this great image on your web site. We'd like to use it in an article in our magazine. Can you send us a 300 dpi version of it?"
If it is early in the morning and the above question is followed by the typical, "oh, by the way, we have no budget, but we'll send you a copy of the magazine" I will probably just say, "sure," and send them a 300 dpi version that will be just what they asked for, but possibly nothing like what they wanted.
If I'm in a good mood and it looks like they may actually have a budget I will ask them for more information. First question might be what is the usage and the budget. But this isn't a business discussion here, so we'll jump to the technical question... "300 dpi at what print size?" is the big question that needs to be asked.
300 dpi on its own doesn't mean all that much. Here are some sample images. One of them is 300 dpi. Another is 72 dpi. And the other one is 1 dpi. Can you tell the difference?
Basically we have three images that are 400 pixels wide by 400 pixels tall. On the web the resolution setting for them is meaningless. They display one pixel to one pixel. Where the difference comes in is in the printing. What would happen if I sent the 300 dpi version of the above image to the client? It would be just what they asked for (300 dpi), but nothing like they wanted. It would print at about 1.3x1.3 inches, not the full double-page spread that it should be printed at.
Anway, here is what happens if we bring the 72dpi and 300 dpi versions of the image into a page layout application like InDesign. The file tagged at 72 dpi comes in at around 5.5x5.5 inches at 72 dpi. The 300 dpi version comes in, as stated above, at 1.3x1.5 at 300 dpi--much smaller. Can you scale the 300 dpi version up to 5.5x6.5 inches? Sure thing! But then the resolution goes down to 72 dpi. Similarly, we can change the 72dpi version to 300 dpi. But then the print size shrinks. Both files have the same pixel count. The resolution number is telling the layout program how much space to stretch those pixels out over.
Another place this gets confusing is in the images coming straight out of a digital camera. For example, take a file from the Canon 5D mkII that is 5616x3744 pixels. What is the resolution?
The answer is 72 dpi. Or 100 dpi. Or 240 dpi. Or 300 dpi. Or whatever resolution you want it to be. The DPI by itself means nothing until we go to print.
That 5616x3744 pixel image will (approximately) be:
at 72 dpi = 78 x 52 inches
at 240 dpi = 23.4 x 15.5 inches
at 300 dpi = 18.75 x 12.55 inches
That's all from the same file with no changes to it.
Some camera files claim to be 72 dpi. Others claim to be 180 dpi. Others might claim some other resolution. This resolution number is not important. The actual pixel dimensions are the important numbers. You can change the resolution at any time without affecting the file size/quality. In Photoshop, go to Image > Image Size and UNCHECK the RESAMPLE box. Then you can type in whatever resolution you want. The document size will change, but the pixel dimensions will not. In Photoshop Lightroom you get to select the pixel dimensions and dpi when you export your image. While working on your image those numbers don't come into play.
So, when someone asks for a 300 dpi image you need to go back to them and ask them for more information. Is this for web or print? What will the printed size be? Then you can work backwards and come up with the pixel dimensions you need to provide.
TO BE CONTINUED
Model in the sample photos is Courtnee Fallon (Zita the Aerialist).