What does it all mean?
Many printers will state that they require all images at 300dpi (or dots per inch). Infact, the dpi value of an image file is pretty meaningless really. For example, when shooting images with a digital camera, the camera’s internal EXIF setting denotes what the dpi of the file will be, but this doesn’t mean anything, it is just a value that is included with the properties of the image file. That’s all. You can change it to any number you like when editing the image properties, it doesn’t matter. It only matters if you resize the image by changing the dpi.
The most important figure is the actual pixel dimensions of a saved file. Any image that is to be printed as a giclee ideally needs to have a dpi (dots per inch) size of no less than 300dpi at the intended print size. Although, we can sometimes get away with 240dpi or lower.
To explain this a little clearer, you may of heard before that screen images need to be 72dpi and print images need to be 300dpi and that’s the way it should be…end of discussion. Well, this isn’t always the case. The value of 72dpi is commonly stated as that’s the maximum number of pixels a computer monitor could display over a square inch, so anything higher is pointless and likewise, a printer could only print to a maximum of 300 dots per square inch of paper. However, it is relative to the actual pixel dimensions and quality of the image invloved.
You’ve done this before, downloaded an image from the web, used it in a document, printed the document only to find the image is really pixelated and choppy. Images used on the web are generally (or atleast should be) only the exact size that they need to be. Why have an image that would fill the screen if it’s only to be used as a small thumbnail! It’s a drain on server space and web page loading times. This is why most web images are small (in pixel size), say, 150 x 150 pixels for example. It really doesn’t matter what dpi value the image has…it’s 150×150 pixels. It will always be 150×150 pixels. Your screen will display the image 150 pixels across and 150 pixels high. Images can be resized on screen (responsive web pages for example), but the image should never be resized larger than it actually is. This is why the image looks great on screen but has a really poor print quality when printed at a larger size, because it was only ever intended to be viewed on a screen at its exact size. This is why many online image retailers only display low res versions of an image and keep the high res images in a seperate location.
To find out what the same 150pixels sqaure image would be like when printed at 300dpi, we divide the pixel size by our required dpi. So, if we wish to have a dpi of 300, we would simply calculate 150pixels divided by 300dpi equals 0.5inches. We now know that this image will be 300dpi at a half inch. Tiny! If we ignored this and printed the same image at 30 inches square for example, we could calculate the resulting dpi by dividing the print size by the pixel size. 30inches divided by 150pixels equals 5dpi. This means that there will be only 5 dots of ink on every inch of paper. that’s a really, really bad print. It’s like spreading a small amount of butter over a huge piece of toast!