by Zimmerman Skyrat, 101Bananas.com, October 2005
The "35" in 35mm film refers to the width of the film, which includes the little holes that your camera's sprockets fit in to advance or rewind the film. Each individual exposed frame on a strip of 35mm film is 24 x 36 mm, a 1:1.5 aspect ratio — in HORIZONTAL format, the width is 1.5 times the height. (We're talking about still cameras here, NOT 35mm Hollywood movie cameras, which are another matter entirely.)
2.54 cm (or, 25.4 mm) = 1 inch
24 x 36 mm = .944 x 1.417 inches
A standard 4 x 6 inch print has a 1.5 aspect ratio; in horizontal format the width is 1.5 times the height. This is exactly the same ratio as a 24 x 36 mm negative. Thus the whole frame of the negative fits perfectly, when enlarged, on a 4 x 6 print.
Since the standard sizes for photographic enlargements larger than 4 x 6 have different aspect ratios than the original negative or slide, something has to be cropped somewhere. Assuming a horizontal format for the picture (width larger than height), and assuming you want the full height of the frame printed, it is one or both ends of the picture (the width) that are usually cropped slightly.
A 5 x 7 inch print has a 1.4 aspect ratio; in horizontal format the width is 1.4 times the height. That's slightly different than the 1.5 aspect ratio of the original negative. This is why when you get a 5 x 7 enlargement from a negative or slide, the ends of the picture must be cropped slightly. About 7% to be exact. You can only fit 33.6 of the 36 mm width of the negative image, when enlarged, on a 5 x 7 print.
An 8 x 10 inch print has only a 1.25 aspect ratio, a little closer to being square than a 4 x 6, so the ends must be cropped even more, about 16%. You can only fit 30 of the 36 mm width of the negative image, when enlarged, on an 8 x 10 print.
An 11 x 14 inch print has a 1.27 aspect ratio, very close to the ratio of an 8 x 10 print. You can only fit 30.5 of the 36 mm width of the negative image, when enlarged, on a 11 x 14 print.
TV Screens / Computer Monitors
A standard (not widescreen) TV screen or CRT computer monitor has a 3:4 aspect ratio (or 1:1.33, the width is 1.33 times the height). This is the same as old Hollywood black & white movies. Newer flat-screen LCD monitors that are not widescreen can vary but will generally have an aspect ratio of either 1.33 or 1.25 (width is 1.25 times the height, a tiny bit closer to square than a CRT monitor). Something that should be mentioned here regarding CRT vs. LCD monitors is that, to vastly oversimplify things, a CRT is simply better at changing screen resolutions than an LCD is. Due to the inherent technological differences in the way CRTs and LCDs create a pixel on your screen, all LCD monitors have a built-in hardware screen resolution and will always look best when set to that resolution. Newer LCD monitors are quite good and the very slight degradation in picture quality when screen resolution is changed to something other than the actual hardware resolution is barely noticible to most people, but it is there. CRTs do not have this problem, and the higher you set their display resolution, the sharper and clearer a picture will be (and the smaller text will be). There are many sites on the Internet with in-depth explanations and pictures and diagrams detailing how CRT and LCD monitors work and display images.
The most commonly used Windows desktop screen resolutions, in pixels, are 800 x 600 and 1024 x 768. The older standard VGA resolution was 640 x 480. All three of these sizes have aspect ratios of 1.33, exactly the same as the physical measurements of a standard TV screen or CRT computer monitor. This was obviously by design, because if the aspect ratio, in pixels, of the "virtual" screen you are visually displaying does not match the aspect ratio of the actual physical dimensions of whatever display device you are using, pictures will be compressed either horizontally (they'll look taller and thinner) or vertically (they'll look shorter and fatter). Depending on how powerful the video card in your computer is and what version of Windows you're running, you may have several other screen resolutions available, including a 1280 x 720 widescreen 1.77 aspect ratio resolution. The next higher "standard" Windows screen resolutions are 1152 x 846 (1.33 aspect ratio), and 1280 x 1024 (1.25 aspect ratio). The visual difference between aspect ratios of 1.33 and 1.25 is negligible to most eyes, but remember, for PERFECTLY proportioned pictures with ZERO distortion, the aspect ratio of your Windows desktop screen resolution should match the aspect ratio of the physical dimensions of your monitor.
Now that your screen resolution is set correctly so you know that Playmate of the Month you're going to scan won't be taller and thinner or shorter and fatter than she should be, actually scanning a picture to use as Windows desktop wallpaper is simple. With your scanner's software, you can scan it at exactly the resolution you want; or, you can scan it at a higher resolution, load it into a photo editing program for touch-up, and then re-size and/or crop it down to exactly the dimensions you want. When scanning a 4 x 6 print or a negative or slide, which have 1.5 aspect ratios, remember the ends will have to be cropped a little (just as if you were getting an 8 x 10 print enlargement) if you want it to completely and exactly fill your screen for perfect-fit wallpaper.
Taking Digital Photos for Prints
For the moment, note that we are talking only about prints made optically on photographic paper at a photo shop or drug store. We are NOT talking about prints made on your color ink-jet printer at home; there is an important difference that will be explained later. The commonly accepted standard for the resolution required in a digital file that you want to have a print made from is 300 dpi (dots per inch) for whatever size print you are making. (More than 300 dpi is wasted and will not increase the quality of the final print. Normal photographic paper does not have a fine enough grain to resolve more than about 300 dpi.) The dpi on photographic paper translates exactly into number of pixels in a digital file, so if you want some 4 x 6 prints made at 300 dpi, you would need a digital file that is 1200 x 1800 pixels in size. (4 inches X 300 dpi = 1200; 6 inches X 300 dpi = 1800.) For an 8 x 10 print, you would need a digital file that is 2400 x 3000 pixels in size. HOWEVER, like all so-called "commonly accepted standards" there is wide variation in what standard is accepted and by whom, and for good reason. You can get excellent results with 250 or 200 or even 100 dpi, especially if the original image or scan is very sharp and clear. Anyone who disputes this and claims you need 300 dpi for best results doesn't know what they are talking about, and there are a lot of experts that don't know what they are talking about. The proof is always in the pudding (preferably banana), not in a theoretical abstract discussion. I have actually tested this, and I urge you to do the same to convince yourself. I scanned a colorful 8 x 10 picture at 300 dpi, sharpened it a bit, cropped it to exactly 2400 x 3000 pixels, and it looked perfect onscreen. Then using Paint Shop Pro (you can use PhotoShop or any other of the many photo editing programs) I resized the picture to 2000 x 2500 pixels (250 dpi when printed at 8 x 10) and saved it as a separate file. Beginning with the original again, I resized the picture to 1600 x 2000 pixels (200 dpi when printed at 8 x 10) and again saved it as a separate file. I repeated the same thing to get files of 1200 x 1500 pixels (150 dpi at 8 x 10) and 800 x 1000 pixels (100 dpi at 8 x 10). I put a small text identifier in the corner of each picture so I could tell which was which. I put the original 300 dpi file and the other four lower resolution files on a CD and took it to Wal-Mart and had one 8 x 10 print made from each file. After years of reading the propaganda in photo and computer magazines about the 300 dpi requirement, I was quite surprised at the results. I could not tell the difference between any of the five prints at first glance, and only after very close examination could I tell a tiny difference in sharpness around edges between the 300 dpi and the 100 dpi print. I even looked at the prints with a magnifying glass, and only when you get down to 100 dpi is there an almost insignificant difference in sharpness or clarity. This obviously shows up more on some types of photos than on others, for instance if there is a sign in the photo with black text on a white background, you will be able to see a difference between the 300 and 100 dpi prints. But for most general snapshots that you're not going to have blown up very large and framed, you won't notice any difference. If you don't believe me, do a test yourself. The small cost of the prints to actually prove this for your own eyes will convince you that for anything 8 x 10 or smaller, you'll never need anything more than 200 dpi, assuming the original is fairly sharp and clear.
Now for the difference between "real" photographic prints and color ink-jet prints. The main difference can be explained simply in how the term "dpi" is used and what it refers to. A photographic print is made optically on a special kind of paper, so as noted above, the "dpi" (or resolution, or sharpness of detail), of a photo coincides exactly with the number of pixels in a digital file. If you want a high quality 300 dpi print, you need a digital file that contains 300 pixels per inch for whatever size the final print will be. A printer is a little different. An average consumer-grade color ink-jet printer today might be advertised as printing at a resolution of 2400 dpi, but this refers to the actual number of dots of ink per inch the printer is capable of printing on paper, not to the digital file it is printing. Since it will take several dots of ink to print each pixel of the digital file, the printer obviously needs to be able to print much more than just 300 dots of ink per inch. Different makes and models of color ink-jet printers are different, but they all require several dots of different colored ink to print one pixel. It may be 6, 8, 10 dots of ink per pixel, or whatever, depending on the quality of the printer and the number of different colors of ink it uses, but they all need to print several different individual dots in varying patterns to create different colors and densities of pixels. So don't make the mistake of trying to save ink by setting your printer via software to print at only 300 dpi, thinking that's all you need for high quality. Leave your printer set to print at its highest resolution if you want the highest quality of print.
So now it's easy to see what kind of camera you'll need if you want certain sizes of prints. If you absolutely insist on 300 dpi, an 8 x 10 picture needs 2400 x 3000 pixels = 7,200,000, or 7.2 megapixels. At only 200 dpi it would be 1600 x 2000 = 3,200,000, or 3.2 megapixels. The differences in quality, features, and price between a 3.2 megapixel camera and a 7.2 megapixel camera are fairly large. Also, you can read reviews in photo magazines and online that prove that in actual practice in the real world, many 6 megapixel or 5 megapixel cameras take pictures that can be printed at 8 x 10 inches with close to perfect results. Obviously then, the resolution in pixels is only one factor in the quality of the final print. The quality of the light sensors in the camera, the quality of the lens used, whether a tripod was used if zooming (highly recommended), and the software that runs the camera and saves the picture as a .jpeg or other format, all have a direct impact on the final quality of the print. It is not only the widely advertised megapixel resolution of a camera that determines how the picture looks.
Scanning Photos or Negatives & Slides for Prints
Scanning an old photograph or a picture from a magazine follows the same principles. You should scan the original at a resolution that will give you the number of dpi you want for whatever size print you are planning on making from the scan. As an example let's say you have an old 4 x 6 photo whose negative has long since disappeared, and you would like an 8 x 10 print of that photo. (Remember that enlarging a 4 x 6 to an 8 x 10 will require a little cropping before printing.) I'll use 200 dpi for this example, since if you've been paying attention you should now know that you don't need 300 dpi to get excellent prints. Since you need a 1600 x 2000 pixel digital file for a 200 dpi 8 x 10, divide 1600 (height of the digital file required, in pixels) by 4 (height of the photo, in inches) and you get 400. You would need to scan the photo at 400 dpi, which would give you a 1600 x 2400 pixel file (4 inches X 400 dpi = 1600; 6 inches X 400 dpi = 2400). Most scanner software would then allow you to crop the image to 1600 x 2000, or you can load it into a photo editing program to do it. If you are not printing the picture yourself, but taking the digital file to a photo finisher to be printed, then you definitely want to do the cropping yourself first, so the print you get will be exactly what you see on screen.
For scanning slides or negatives, you will of course need a very high resolution scanner if you want to have some 8 x 10s printed from your scans. If you're not going to buy an expensive dedicated film scanner, make sure the flatbed scanner you get includes negative and slide holders. As noted above, slides or negatives from 35mm film are .944 x 1.417 inches. Since an 8 x 10 print at 200 dpi requires a digital file of 1600 x 2000 pixels, you would need to scan a negative at about 1700 dpi which would give you a digital file of 1604 x 2408 pixels. (.944 inches X 1700 dpi = 1604; 1.417 inches X 1700 dpi = 2408.) The picture can then be resized and the ends cropped, down to 1600 x 2000 to fit the 1.25 aspect ratio of an 8 x 10 print. Many inexpensive flatbed scanners today have at least 2400 dpi scanning ability, which is more than enough to get excellent results. As with digital cameras, the quality of the scanner hardware itself, and the software that runs it, is critical to the quality of the final print, so read reviews in magazines and online from users who have actually used the specific scanner you're interested in.