Many people seem to get hung up on DPI (dots per inch) as a measure of quality of their digital photos. To set the record straight, DPI has NOTHING to do with digital image quality!.
As a general rule, there are four main factors that determine image quality: 1) The quality of the recording device (camera's sensor, scanner).Several other factors also come into play, but the above are the main factors that determine the overall quality of the original digital photo.
DPI is a measure of how a image is printed to a medium such as paper (or conversely, scanned from paper). Many software programs call DPI a measure of "resolution" which leads to more confusion since it is the resolution of the printed output, not anything to do with the "resolution" of the digital image. In fact the software programs that use this setting are really just doing so because they lack WYSIWYG (what you see is what you get) output. If you do use WYSIWYG software such as most word processing, desk top publishing or graphics programs such as Corel Draw, then DPI is really a meaningless figure - the quality of output will be determined by the pixel size of the original image and the quality of the printer. However, for programs that still cludge along with DPI (most photo programs), the use of DPI is a method to determine the size of output on paper. In our example above, the photo set at 1000 dpi will print out from a non-WYSIWYG at 0.3 inches in width (300 pixels divided by 1000 dot per inch = 0.3 inches). Whereas the same photo set at 10 dpi will print out at 30 inches in width. The print previews of an 8.5" x 11" page below illustrate the effect:
So - as can be seen DPI is really only used to set the size of the printed output from certain photo programs. If we wanted to print this photo (300 pixels in width) at say 5 inches in width using a program that used DPI to set the dimensions of the printed output, then we would set the DPI to 60 (300 pixels/60 dpi = 5 inches). If we took that same photo into say Word or Corel Draw at whatever DPI, we would simply size it to be 5 inches in width and the print result would be identical. Photo software that comes with most recent digital cameras simply allows you to specify the output size you wish to use (i.e. 4" x 6", 5" x 7", etc.) and then automatically prints it at that size. Note that the internal DPI (which we now know is not relevant to digital image quality) of most digital camera photos is 72. So - why should I care about DPI? Well, obviously if you are printing with a program such as Adobe Photoshop or Corel Photopaint you will have to use the DPI setting to adjust the size of the printed output. Some of the newer photo programs simply allow you to set a size output (i.e. 5" x 7") for the image and it adjusts the DPI accordingly. With most current photo printers, photographic quality is generally achieved at a DPI output of 200 or above. Note that most printers lie to you when you set the DPI of output. You will often see figures in the printer settings of 720 dpi or 1440 dpi. Keep in mind that these printers use a blended dot, so a 1440 dpi setting may in fact be only 240 blended dots per inch. If you want a photographic quality image at 8 x 10, then the original digital photo should be at least 2000 pixels (10 x 200 dpi) by 1600 pixels (8 x 200 dpi) in size. Check your camera to see what maximum size of photo it can take - divide the pixel count by 200 to give an approximation of the maximum photographic quality image it can produce. Scanning - DPI Does Count Scanning in the process of converting paper to digital and in this process DPI is used to adjust the amount of detail of the scan. The DPI setting of the scanner relates to the final pixel size of the scanned image. If you put a 5 x 7 photo on the scanner and scan it at 300 dpi, the resulting digital image will be 1500 x 2100 pixels in size. In this case, DPI does relate to quality, since the higher the scanner DPI setting the more information is being collected. Keep in mind though the 200 ppi = photo quality concept, a minimum of 200 dpi should be used in scanning. Best results for paper photos are generally achieved within a range of 300 dpi (sufficient for most photos) to 600 dpi (if you want to enlarge the image). A Word of Warning Some programs will resize a photo when the DPI is changed. Be very careful of this. To change the DPI without changing the pixel size of the photo you should click on the "maintain original size" (i.e. Corel Photopaint) or similar option that some programs offers, or click off "resample image" that other programs offer (i.e. Adobe Photoshop). Make sure that you select/deselect the appropriate option before changing the DPI so that the pixel size of the image does not change. At least one program, Irfanview, by default allows the easy changing of the DPI with no corresponding resizing of the photo. If you do want to change the size of your digital image, get in the habit of either changing the pixel size directly or use the percentage (%) change option provided by most programs. When doing this make sure the option to "maintain aspect ratio" or "constrain proportions" is selected so that the image get evenly changed in size both vertically and horizontally. Those Pesky Print Shops Print shops and many photo editors still live in the mythical world of "DPI is resolution." In the past, when a photo was requested from me "at xxx DPI" I would sometime put on my "DPI is meaningless" hat and rant a bit. Now I just give them what they want. I usually ask for the intended size of output and using the 200 rule (photographic quality is achieved at about 200 ppi) let them know if the photo I have would be suitable (based on pixel size). Then I set the DPI of that photo to whatever they want (it doesn't change the quality of the photo in any way) and send it along. I haven't had a complaint yet. A Note to Print Shops and Graphics Designers I keep getting emails from poor folk who say that their print shop or graphics designer keep asking for digital phots at xxxDPI (usually 300 DPI). Please STOP DOING THIS to your poor clients - ask for what you really need, xxxx pixels in your prefered format (i.e. low compressed JPG or a TIF). If your need is for a digital photo that can be printed at high quality at a width of 6 inches, and you think that 300 ppi is what is required to do this (based on your equipment), then you need a digital photo that is 1800 pixels in width (regardless of its DPI setting since it's a meaningless figure). So, ask for a digital photo that is at least 1800 pixels in width. Tell your client not to resize the photo, that if it is larger than 1800 pixels it's okay (so they don't resize a larger photo down to your "at least" pixel figure). If you are running older equipment/software that needs a particular DPI setting, then set it in the photo after you have received it. Also, please don't ask for a digital image of xxx Kb in filesize (I've had print shops do this). This is even more meaningless than the DPI figure since the filesize depends not only on the format (JPEG, TIF) but the compression used with that format. If you are in a long term relationship with the client, educate the client to take photos at the highest quality setting their digital camera allows (and then ask for a copy of the original photo) or if they are scanning (where DPI does count in converting paper inches to digital pixels), specify the resulting pixel dimensions that you want. If everyone simply worked with pixels we'd have a happier digital photo world. "All About Digital Photos" website. |
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