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Image Interpolation
Changing the Size of Digital Images for Digital Prints and Web Pages
The number of pixels in a digital image limits the maximum size of a quality print. For the print to look sharp with good tonal quality and well-defined smooth edges, just as it does on the computer screen, there must be sufficient pixels to every inch.
A reliable photo lab that uses photo-digital lasers to produce high quality prints on photographic paper will probably recommend 300ppi (pixels per inch) for the most common print sizes. This means a 6x4 print should ideally be 1800x1200 pixels, a 7x5 should be 2100x1500, a 12x8 should be 3600x2400, and a 15x10 should be 4500x3000. But less than ideal pixels per inch can deliver fair results: a good quality 3500x2300 digital image can be used to produce an acceptable 15x10 print for the hobbyist. At the other end of the scale, too many ppi will mean the KB size of an image will be bigger than it needs to be and some labs may refuse to process smaller prints from images that contain a lot more information than is necessary.
If you need print sizes significantly larger than your DSLR's output you can use use a special process that adds pixels to the original image, although there is a limit to how far any digital image can be expanded to usefully fit a print size. (Image degradation is inevitable, but large prints are normally viewed from several feet away.) For example, if you are using a digital camera that produces 3500x2300 images and you need a 20x16 print, you should enlarge your image to around 5000x4000. This enlargement process is called image interpolation , or resampling. *
Programs like Photoshop and PHOTO-PAINT can easily enlarge a digital image. You can also buy unique interpolation software, like Genuine Fractals and PhotoZoom Pro . Basically, the better methods of image interpolation cleanly smooth out jagged edges and a variety of irregularities by creating new matching pixels to fill the ‘spaces' produced by enlargement.
The images below show unsharpened image interpolation results from Corel PHOTO-PAINT and Shortcut PhotoZoom Pro. The original unedited image was captured by a DSLR.
How Many Steps for the Best Interpolation?
You may read in books and magazines that it's best to enlarge your images in increments – a little at a time until you reach the size you're after. There are many who favour incremental increases and offer evidence to prove that it's possible to produce better enlargements through additional software techniques, but nevertheless, the images below show that in standard image-editing software, the step method degrades images very slightly. In fact it has been convincingly argued elsewhere that at a mathematical level it is inevitable that an image will degrade directly because of incremental enlargement (see Steve Hynes' article, Professional Photographer Magazine, March 2004).
The original TIFF image of a business card (converted from a RAW file) was enlarged from 3072x2048 to 6096x4064, the recommended size for 24x16 digital prints. |
Understanding DPI and PPI
The resolution of a digital image is measured in pixels per inch, not dots per inch. Software may show the image resolution as DPI, but strictly speaking this is not correct. Unfortunately, the two terms – ppi and dpi – tend to be used interchangeably, even by some professionals. dpi refers to a printer's output resolution, not the resolution of a digital image.
If the output resolution from a film or flatbed sanner is measured as "300 dpi", you might suppose that when the RGB image is resampled down for a webpage it will take longer to download online than if it had been scanned at a typical screen resolution of 96 or 72 ppi. This is not the case, as the two images below illustrate. Assuming compression and sharpening are the same, identical images of identical pixel width and height, regardless of ppi, will be the same kb size. An image is only affected by ppi when it is printed.
The significance of the actual ppi output when scanned is highlighted when text is added: a tiny font size is appropriate for a 300 ppi image, whereas a much larger font size is needed with the 96 ppi image for the text to look roughly the same. The image kb size and resulting download time are determined by the number of pixels in the image, how detailed the image content is and finally, how much JPEG compression/sharpening was applied.
Left: 9,732 bytes @ 300 ppi, flatbed scan; Right: 9,770 bytes @ 96 ppi, flatbed scan.
* RAW files can also be interpolated.
UNDERSTANDING DIGITAL EXPOSURE:
Part 1: Exposure Reduction for Highlight Retention
Part 2: Digital Exposure & Noise
Part 3: When Blocked-up Shadows Aren’t Really
Part 4: Take a Balanced Approach to White Balance
Part 5: Dynamic Range
Part 6: Extending the Tonal Range
Part 7: What’s the Real Difference Between RAW and JPEG?
GENERAL:
COST-EFFECTIVE PHOTOGRAPHY
IMAGE INTERPOLATION (resizing an image)
DOES EVERYTHING NEED TO HAVE AN ADOBE SLANT?
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