What is resolution?
We'll keep it simple. For our purposes, resolution is the number of samples the scanner takes of a defined space of your photograph. So, for a 300 ppi scan, the scanner will divide your photo into 300 boxes (pixels) for every inch in one dimension (so a 1 inch square photo would have 90,000 pixels or 300 X 300). For each of these pixels, the scanner determines the average red, green, and blue level for that 1/300" by 1/300" box, and displays your photo as a series of pixels. Unless your eyesight is a heck of a lot sharper than mine (not saying that it isn't of course), at 300 ppi, the series of boxes of different colors looks like a continuous tone photograph. To make the concept obvious, I simulated a scan of the cabinet card at 5 ppi below:
For each inch of the card in length or height, the scan makes 5 boxes of average color and brightness over the area of the box.
A brief word here about display resolution. The images you see on this site are set to display at 72 ppi. That is because most browsers will display images on your monitor at this resolution, so if I want something to display at a particular size, I assume your monitor is going to display it at 72 ppi. I wanted the image on the prior page to be 6.5 inches wide (the width of the card), so I resized it in my photo editor to 468 pixels wide (=6.5 inches X 72 ppi). If I had left it as scanned at 300 ppi, the card would have been 1950 pixels wide. Most browsers would try to display it as a 27" wide photo (= 1950 pixels / 72ppi), which would be really annoying.
This leads us to a couple of impromptu formulas, although typically I avoid math on this site:
Number of pixels per dimension = length of photograph (in.) X Scan resolution (ppi)
- and -
Resolution of enlargement (ppi) = Number of pixels / length of enlargement dimension (in.)
Why do we care about all this? Well, it matters when you are trying to enlarge an image, and when you are trying to print. In general, I try to print out an image at about 240-300 ppi - this will give you a decent print. With interpolation, even 150 ppi will give you pretty good results. Much below this, and the image starts looking either "grainy" and pixellated, or "soft" with detail smoothed over, depending on how your program manages your print. So if you want to take that 2" by 3" photo of Aunt Martha, scan it, and blow it up 20" by 30" for the living room wall, what's going to happen? Let's say you scan at 300 ppi. This will give you a scan that is 600 pixels (=2 in X 300ppi) by 900 pixels. When you enlarge the photograph to 20" by 30" (i.e. each dimension increases by a factor of 10), you do not increase the information in the photo (regardless of interpolation, more later), which is still 600 by 900 pixels of separate color dots. Your 20" by 30" photo will print out at 30 ppi (=600 pixels / 20in, or = 900 pixels / 30 in, your choice), which will produce a crummy print.
Wait a minute - My new flatbed scanner will scan at 2400 ppi. How about if I scan the photo at 2400 ppi. This will give me a 4800 by 7200 pixel scanned image, and that 20 by 30 inch portrait will print out at 240ppi. That will give give me a great print, right?
Why? Because, as I hope to demonstrate, the photograph itself only contains about 300 dpi (or perhaps a touch more) of information, so, no matter how fine you set the resolution, you aren't going to get any more information out of the photo. My new flatbed does 2400 ppi (optical resolution), my old one did 1200 ppi. I have never even used the 1200 ppi setting until working on this section because it wouldn't be useful for photos. So even if you had a scanner that had 50,000 ppi resolution, you wouldn't get a better scan because the photo itself is limited. Note: we are talking specifically about photographic prints; slides and negatives are a different story and do contain more information (I drool over the 4000 ppi nikon negative scanner, but I'm still happy with my 2700 ppi model).
How will I demonstrate that you don't get much more than 300 ppi from your photo? Well, I did a little experiment, so that I could prove it to myself; hopefully it will convince you, too.