Aspect Ratios and their relationship to print sizeAn aspect ratio is just an awkward way of describing the shape of your image. Rather than use colorful imagery (as we do in discussing the "bouquet" of wines or the "temperature" of color), we use, uh, numbers. Like 1:1.3. Ugh. They're rectangles, yes, but a 4x6 is quite a different shape than an 8x10. Rather than divulge our colorful language for describing rectangular shapes, we'll offer a simple mathematical proof that print sizes are different shapes. Just for fun, call the short side 1. How many short sides are in the long side of a 4x6? That would be one and a half (it takes one 4 with 2 left over, which is half of 4, to make 6). To make this easy, just divide the long side (6) by the short side (4) to get 1.5. The mathematical description of the shape of this 4x6 print is, therefore, 1 to 1.5 or 1:1.5. That's its aspect ratio, in fact. How about the 8x10? The short side is the 8, so 1 is 8 inches here. How many short sides in 10? Well, it takes one 8 plus 2, which is a quarter of 8, to make 10. Or (more quickly) 10 divided by 8. So it's 1:1.25. You can see by this what a square would be. The short side is ... well, the four sides of a square are all the same length. By definition. So a square's aspect ratio is 1:1. Those are all not just different ratios but different shapes. The 8x10, for example, is closer to a square than the 4x6. "Fine, fine, fine, what's this got to do with my 480 x 640 image," you say, throwing your voice where we can't find it. Well, what works for inches works for pixels. 480 is the short side. There is one 480 in 640 and 160 left over (which is a third of 480), so it's 1:1.33. Which is not a perfect fit for either a 4x6 (1:1.5) or an 8 x 10 (1:1.25). Something has to go. Either you fit the short side or the long side but you can't match both. Ofoto uses "a zoom-and-trim technique similar to what a traditional photo lab would use, which automatically adjusts the image's dimensions to fit the desired print size." Which means they print the image large enough that it spills over all four sides and then they trim to size. Exactly 4x6, 5x7 or 8x10 (which conveniently slips right into inexpensive prebuilt frames). So to minimize cropping on a 4x6 print, make sure you have a 1:1.5 aspect ratio (and expect a little trimming). If you want a 5x7 print, make sure you have a 1:1.4 aspect ratio. And if you want an 8x10, make sure you have a 1:1.25 aspect ratio. We thought we just heard a little voice somewhere say, "How do you make sure?" Ventriloquists can be a tough audience. Select your Crop or Rectangular Marquee Tool and enable the Fixed Size option, entering the pixels or inches of your target print size. That restricts the tool to a crop of that size. If you'd like to see at a glance the exact number of pixels your
image requires to match the aspect ratio of the four most popular sizes of
prints, just visit http://www.imaging-resource.com/IRNEWS
for the online version of this issue. So what will be the maximum print size for your pixel size? Just type in your image dimensions (the number of pixels on the
short side and the number on the long side) and it will instantly calculate
the pixel dimensions required for everything from a 4x6 to an 11x14. And
even calculate a maximum recommended print size based on 250 dpi printer resolution. |
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| File Size The raw (uncompressed) file size of a bitmap image is the result of its pixel dimensions multiplied by its color depth. For example, the size of an 100 pixels by 80 pixels B&W image is 1,000 bytes (100 x 80 x 1 bits). At the same pixel dimensions, grayscale image takes 8,000 bytes (100 x 80 x 8 bits) and RGB true color image takes 24,000 bytes (100 x 80 x 24 bits) to store precise information. There are several image-compressing formats that can reduce the image file size dramatically. However, please be aware that those formats only reduce the size for storage. It will require at least the space of the raw data file size on the computer to process bitmap images, the raw file size still is the key factor of image processing performance. For example, if you scan a letter-size (8.5" x 11") document at 300 dpi resolution in true color mode, you are generating a bitmap image of 24MB (8.5x300 x 11x300 x 24 bits) in size. The actual buffer space needed on your computer for scanning is even greater than that (sometimes more than 3 times as big as the raw file size). In the same example, if you save the image in JPEG format, it may takes less than 1MB disk space. But when you open the file in an image-editing program, it'll require more than 24MB buffer space again for the program to display and process the image. The following is the chart for file sizes of 24-bit color images:
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Pixel dimensions When working with a digital image, the quality of that image is based largely on the number of pixels that make it up. This is called resolution, and is measured in pixels per inch. You may see an image’s resolution described as 72 pixels per inch , or as 640 x 480 pixels (640 columns of pixels by 480 rows). To calculate an image’s size in inches based on its pixel dimensions, divide the pixel dimensions by the pixels per inch in the image (ie.: a 640 x 480, 72 ppi would calculate: 640/72= 8.89”; 480/72 = 6.67”). When scanning or creating images, the resolution you set determines how many pixels are in the image. If set at 72 ppi, there are over 5,000 pixels per square inch (72 x 72). If set to 300 ppi, each square inch contains 90,000 pixels (300 x 300). This all has an effect on the size of the file containing that image. File size in a bitmapped (raster) image is based on the number of pixels. The greater the number of pixels, the larger the image’s file size. There’s also another factor that comes into play here and that is bit depth; 32 for a CMYK image, 24 for a 24 bit RGB image, 8 for an 8 bit grayscale image , or 1 for a black and white image. To estimate an image’s file size, multiply its dimensions in pixels by its bit depth. This provides the number of bits in the image. Then divide this number by 8 to calculate the number of bytes (remember ?, 8 bits = 1 byte). For example, the file size of a 640 x 480 pixel 24 bit RGB image is approximately 920KB (640 x 480 x 24/8). Monitor resolution determines how many pixels are displayed on the screen (72 ppi Macintosh, 96 ppi SVGA [PC]). If your image resolution is the same as the monitor, it will appear at actual size. If the image resolution is greater, the image will appear larger onscreen, because one inch in the, say 300 ppi image, can’t fit into one inch of the 72 ppi screen. Printer and imagesetter resolution Output resolution is measured in dots per inch (dpi) when printing to a laser printer or imagesetter. The greater the number of dots per inch, the higher the quality of the image. Black and white and color laser printers often have 300 - 600 dpi output, inkjets typically up to 1440 dpi. Imagesetters most often found in service bureaus and offset printers range from 1,200 to 5,000 dpi. When an image is printed on a printing press, color and black and white images are created using tiny dots called halftone screens. These halftone dots create shades of grays and, using cyan, magenta, yellow and black (CMYK) overprinting each other, colors. The number of lines in a halftone screen is measured as lines per inch (lpi). Newspapers use typically an 85 line screen, magazines usually around 133 - 150 lpi, while high quality color printing often uses over 150 lines per inch. Following is a comparison of output in dpi to output in lpi for offset printing.
When scanning images for printing it is important to either scan at the size and resolution they will be printed or do the calculations to insure that they will be consistent with the final output resolution desired. File formats Here are some of the more common file formats encountered in working with digital image, publishing, and multimedia productions that relate to digital images. TIFF (.tif) - Tagged Image File Format. Widely supported format for full-color and grayscale halftone images. May be compressed using LZW compression, a loss-less compression method. JPEG (.jpg) - Joint Photographic Experts Group. A lossy compression format available in varying grades of quality. Usually used with raster images. GIF (.gif)- Graphics Interchange Format. Widely used for internet graphics. Limited to 256 colors. Works best with vector images. EPS (.eps)- Encapsulated PostScript. Widely used vector file format. May be saved with a tiff preview file (enables file to be seen onscreen). Can be saved in ASCII(text) or binary format. ASCII files are larger (ca. 2x). The DCS (Document Color Separation) option, if available, allows exporting CMYK files into page layout programs. Native - Each different software manufacturer has its own “native” file format (ie.: Corel = .cdr, Photoshop=.psd, etc.) Some other file formats include BMP(Windows Bitmap), CGM(Computer Graphics Metafile), DXF (Drawing Interchange), IVUE, PCX, PNG, RIB, Scitex CT, SVG, TGA, QuickTime movies and Video for Windows. |
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Links Understanding Bitmap Image Resolution Making the Most of a Small Number of Pixels |
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