Philadelphia Photographer

Question: Does DPI matter? 

Answer: Simply put, only for printing, and can be changed at will to suit your needs.

DPI refers to the physical size of an image when it is reproduced as a real physical entity, for example printed onto paper, or displayed on a monitor. A digitally stored image has no inherent physical dimensions, measured in inches or centimetres. Some digital file formats record a DPI value, which is to be used when printing the image. This number lets the printer know the intended size of the image, or in the case of scanned images, the size of the original scanned object. For example, a bitmap image may measure 1000×1000 pixels, a resolution of one megapixel. If it is labeled as 250 DPI, that is an instruction to the printer to print it at a size of 4×4 inches. Changing the DPI to 100 in an image editing program would tell the printer to print it at a size of 10×10 inches. However, changing the DPI value would not change the size of the image in pixels which would still be 1000×1000. An image may also be resampled to change the number of pixels and therefore the size or resolution of the image, but this is quite different from simply setting a new DPI for the file.

A very good analogy for DPI is this;  If you asked someone "How far is it from New York City to Philadelphia?" and they gave you an answer of "Oh, about 55 miles per hour."  That is not the answer you expected, but is semi relevant since it might tell you how fast you can travel.  What really matters is the distance in this analogy, or the number of miles.  The answer you were looking for is "About 95 miles." 

What tells you the quality or size of the image is the number of pixels, or pixel dimension, of the final file.  Typically our files are approximately 4000 to 5000 pixels on the long side.  This means if the photo is vertical, it will have a pixel count from top to bottom of about 5000 pixels.  This is a great size and will fulfill just about any need you may have.  This file would, however, be way to large to post to your website.  It would just take way to long for someone to download.  You would then have Visual Technology resize the file to anywhere from 200 - 1000 pixels on the long side, depending on how large of an image preview you would like.  If it is just a small thumbnail image then it should be about 200 pixels, where as a detail photo of your product should be about 600 or 800 pixels on the long side.

If you would like to read more about DPI, please refer to this article http://en.wikipedia.org/wiki/Dots_per_inch