You want to take some pictures with your camera to illustrate your poster? Excellent! This is usually a much better choice than hopping onto the net and grabbing someone else’s images.*
Camera pictures are going to be bitmapped images, made of lots of individual pixels. To ensure your picture looks good when you print it full sized on a poster, it pays to do a little calculation.
Remember to set your camera for the highest resolution possible. You can always lower the resolution of a picture if necessary, but you can never increase the resolution of a picture (notwithstanding what you see on crime shows). A problem here is that cameras often describe their image sizes in megapixels, which describes the total number of pixels in the image. For our purposes, it’s easier to think in terms of the number of pixels along the edges.
My camera gives me a little more than 3,000 pixels along the long edge of the photo at its highest resolution. But I have to be thinking about how many pixels will go into each inch on the poster.
Printers are still largely measuring print resolution in dots per inch (dpi); a dot is essentially a pixel. A old, low end fax machine might be 200 dots per inch; a laser printer might be 1,200 dots per inch. 300 dots per inch is usually considered an acceptable resolution for printing.
If want to keep my picture reasonably sharp, I want to make sure the picture has 300 dot per inch or better on the paper when printed full size.
For my camera, 3,000 pixels / 300 dots per inch = 10 inches. Thus, I don’t want the final size of the picture on my poster to be much more than 10 inches.
This particularly becomes an issue because people will pull images from the net to use on their posters. For instance, people will grab an institutional logo or such from a website and enlarge it. But many net images are only a few hundred pixels wide.
If you have an 800 pixel wide image, and you want to print it at 24 inches across, you have a print resolution of about 30 dpi. It’ll look like rubbish. It’ll still look poor if enlarged to 12 inches.
Will it scale?
* Picture by Potatojunkie on Flickr, and used under a Creative Commons license. Hey, this is a blog, not a poster. Remember, do as I say, not as I do!
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