Size and Resolution:
In the last article I started chomping around the edges of digital photography by tackling the question of how much resolution is enough. Specifically, how much is needed to fully capture the detail in your original 35mm slide -- and my somewhat guarded conclusion is that that the 6 million pixel file (and that's 18m of RGB, since each color has a 6m file) found on the Kodak Photo CD should do the job.
So lets say you've put a hundred of your best images on a PCD ( by far the best bargain in image scanning), you pop the CD disc in your CD drive, turn on your image editing program (Photoshop is the king here but some excellent competing programs can be had for a tenth the cost), and "open" the images in the editing program. First thing you have to do is pick the image you want by clicking on the chosen thumbnail or number. Right away you are faced with more than one level of puzzling size vs. resolution choices. In the first box you will see a thumbnail image and a small box showing the dimensions in pixels, for instance it might say 768x512. If you clicked on the small arrow, surprise, it will open up and reveal four more levels of resolution, a really neat feature of the Photo CD. You can click on any one of these resolutions to bring up the full image. You'll notice at the top of the image box there is a percentage shown. As it is displayed the image is at that percent of the size it would be if each pixel were represented by a dot on the screen - usually 72 dots per inch. The 768x512 file might say 66% for instance, so It's at 2/3 the size it would be if each pixel were represented by a screen dot.
That still doesn't tell you the actual size of the image does it? OK now click on image on that top line, go down to image size, click, and now you get the image size box full of detailed numbers. First, in pixel dimensions you will see the 768x512 file size repeated. The really juicy box print size just below it is where the action really is. If you intend to make an ink jet, or whatever print, the dimensions appearing in this box directly control the size of the output. For instance the size from that 768x512 file would be 10.6 by 7.1 inches. If you had chosen the highest resolution of 3072x2048 when you opened up that first box the image size would be an astounding 42.6 by 28.4 inches. You will notice that in both cases the resolution (third line in the print size box) is 72 dpi - it's really just a default setting for opening up the image. No one really expects you to limit yourself to the image size that first comes up and anyway printing at 72 dpi would yield a really poor image. Choosing a smaller image file or scanning to a smaller image file is NOT the way to control image size.
Getting the size of your dreams turns out to be as easy as apple pie (eating it of course). In the print size box in image size you'll notice the linkage symbol to the right of all three items -width, height and resolution. You can change any one of them and the others will change proportionately in a blink. You may decide that you want an image just 10 inches wide because that's the biggest your printer can deliver. Just click into the box, or "tab" into it (or it may already be active when you opened the box) and write in the 10.0 inches. Right away the height drops to 6.67 and you'll notice the resolution zoom up over 300 lines (if you chose the top level file to start with)
Some people find the fact that the resolution goes up as the size goes down confusing but if you switch back into your darkroom mode of thinking the relationship should be fairly obvious. A medium size print from a small negative might look fairly sharp but if you blow it up too far it looks less sharp and may "fall apart". Since the resolution is in "dots per inch" imagine dots or lines drawn on a small balloon. When the balloon is inflated the dots spread apart so the "per inch" count drops. The total amount
f information (detail) stays the same however - that's purely a function of file size.
Of course you could have sized the image to 10 inches wide even if you started with that, medium level, 768x512 pixel file but the resolution would have only gone up to 77 dpi, still very inadequate for good quality. The good news is that there does not have to be any strict numerical relationship between the dpi, as expressed in the print size box, and the dpi of the image scan or the printer output or the printers (very high) ink dots per inch. Since more is better, at least as far as dpi of resolution is concerned, you can just feed the highest resolution file you have to the printer and let the printer software sort it all out - which it does quite nicely.
So what are the possible uses of lower resolution images? If you're just browsing and looking at pix on the screen the middle resolution would be fine, even the one just below it would be passable and would appear in a blink when clicked on. If you opened a PCD image at the top resolution of 3072x2048 and then decided to make some 5x3.5 inch prints from it the resultant resolution would be over 600 lines, a serious case of overkill since anything beyond 300 just would not be utilized by the printer (again there's no direct correlation between the inkjet dpi and the image resolution dpi). Large files are just slower to open, move, edit and maybe print and if you try to open a lot of images at the same time large files may lead to serious slowdowns or crashing.
And then there's putting pictures on the net or in your e-mail. Since 72 dpi is the resolution common to most monitors we finally find a use for those small file 72 dot images. If you took the second res from the bottom in that 5 pack in the PCD, at 384x256 you still get a 3.5x5.3 inch image on the screen a fairly nice size in that environment. Actually, for web use smaller is better since even that small image is a 288K file which might be slow to send and to open when received. If an image of just under 2x3 inches would do then the smallest file would be the way to go.