A pixel is the smallest unit that can be represented or controlled in a digital image. More pixels equal better resolution, resulting in a higher image quality. One megapixel is equal to one million pixels. Generally speaking, print size is determined by how many megapixels you have, which is determined by several different factors, including the camera's sensor, camera settings (resolution and image quality), and print settings (resolution).
Resolution is the primary concern in digital photography. Higher resolution opens up options for larger prints. As an example, the Canon EOS 7D has an 18 megapixel (18 million pixels) 5,184 x 3,456 sensor, which contains enough resolution for a 16 x 24-inch print (at 220 dpi) without any loss in quality.
The EOS 5D Mark II delivers a maximum RAW file size of approximately 26MB, thanks to its 5616 x 3744 pixels full frame sensor. If you are shooting large JPEGs, the approximate maximum file size comes in around 6MB. So, even when coming from the same camera, exact file sizes can vary, based on the file format, subject, ISO speed, picture style, as well as other variables.
Print resolution has a lot to do with viewing distance. For example, a higher resolution (300 dpi or higher) is needed for a 4 x 6-inch photo of a baby than for a 20 x 60-foot billboard (12 dpi or even as low as 9 dpi).
Some software applications like Genuine Fractals allow you to increase image size by using a fractal-based interpolation algorithm – without the loss of sharpness or detail that you see on the sample images.
Another option to increase the file and size is by using RIP (Raster Image Processing) software applications, which expand the capabilities of the standard printer driver. There are RIPs designed for sign printing, offset proofing, screen-printing, and fine art photography. Many applications enable you to perform gallery-quality black and white printing with toning and split toning capabilities. They feature an enhanced spooling system and you can create photo packages for event and wedding photography. When using RIP applications, be sure to verify that they are ICC compliant so you can use your own custom profiles with them.
The following example shows the same exact same image. The first image is saved as a low resolution: 600 x 400 pixels 72 dpi. The next image is the exact same image, but a high resolution version: 5148 x 3456 pixels at 300 dpi. Can you tell the difference?
The high-resolution version has smoother transitions, especially in the shadow areas and in areas where a dark hue needs to be maintained without a sudden shift to black.
These techniques and applications work well, but always keep in mind that no software application can add resolution to an image that does not have sufficient resolution to begin with.
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All images are copyright Eduardo Angel