Pixel vs DPI
A recent discussion about image theft from internet galleries spurred an impromptu lesson in image formatting for the internet. We talked about how to re-size and watermark images to deter illegal use of images. The conversation is worth expanding on and repeating here.
DPI stands for dots per inch and refers to a small microscopic drop of colored ink deposited onto a substrate (paper, card stock, vinyl, cloth, etc.) through a series of ink jets. These drops are deposited in a prearranged order that, when viewed along side thousands of others, creates an image.
Each pixel or dot refers to a corresponding color captured on your camera's digital sensor, translated into binary code and stored as a digital file. That file is then read by your computer screen, laser printer, inkjet printer or other raster based output device as a photograph. In short, a pixel is an individual point of light on a digital screen while a DPI refers to a point of ink on a printable substrate.
The issue of the discussion was how to determine what size is suitable for a screen but unusable for print. The simple answer is dependent on what is being used to print out the image. There are three common output devices used for images; a computer screen, your home printer and a commercial photo printer. Each of these have their limitations in output. That output is measured in dots per inch or how many dots of color can fit across into an inch of paper.
Printers, however, print at a higher resolution than a computer screen. Most home printers will print at 150 dots per inch but can go as high as 300 dpi. Commercial printers will normally print at 300 pdi but can go as high as 1200 dpi or more. So what does this mean for your images processed for video display?
In order to have the same 4x6 inch image on paper from your home computer the digital file size needs to be 900 pixels wide by 600 pixels tall. That's more than double the size in pixels.
If you want to print that at your local photo kiosk that digital file now needs to be 1800 pixels wide by 1200 pixels tall. Twice over the size of the original file.
Let's reverse that; an image set for screen display at 4x6 (432x288 pixels) will print at a tiny size of 2.88" x 1.92" on your home printer (150 dpi) or the miniscule size of 1.44" x .96" at the kiosk (300 dpi). That last one is about the size of a large postage stamp and is not even a full inch tall.
So if you are formatting your images for your on line portfolio I usually recommend an image size between 600 and 800 pixels on the long edge. This gives enough detail to be appreciated on screen but will print a slightly pixelated 4x6 standard photo.
Keep in mind that if you upload your image at the native size (straight from you camera) most galleries will down-size the display size but may keep the original image size as is. So while on screen it may appear to be a small image, the downloaded size may be of higher quality suitable for printing at better resolutions. Always process your images before uploading to the web and make sure you scale your images accordingly.
Not only is this a deterrent for theft, it also associates your name with the image for potential clients. You never know who might be looking at your images.
To determine the final output size, in inches, of an image simply divide your image size in pixels by the dpi of the printer;
( px / dpi = x" )