Graphics

Digital pictures are composed of "pixels" which are displayed at a certain "resolution". A pixel is a single "unit" of color. Resolution is how many of these pixels are compressed into a given area.

A Megapixel simply means "one million pixels" and is used when describing digital camera capability. I am often asked, "If I buy a digital camera, how many megapixels should it be?" Some digital cameras have 5 megapixels or larger. Without getting too technical, the larger the megapixel capacity, the larger the picture which can be printed from that digital file. Here's how big:

As you can see, 2 MP is typically sufficient for printing great quality 5 x 7 photos. The larger megapixel sizes are handy for cropping out a section of a very large picture, and still having a nice size picture. For our web page, I resize the pictures to a 4" x 6" size, which, measured in pixels, is 378 pixels by 504 pixels, with a resolution of 72.

The size in pixels (not in inches) determines how big on the screen you want your image to appear. If your monitor is set to a resolution of 1024 x 768, then an image at that size will fill the screen if set to 72 ppi (pixels per inch). Since image size (dimensions) determines file size (the number of bytes, k), mg) you shouldn't make the image any larger than necessary for sharing photos through email or online. The magic number for screen formats—TV, computer monitor, PDA or cell phone—is 72 ppi. If you are scanning a photograph, you may use a resolution of 300 ppi. 96 ppi is also a common size. Some digit cameras allow you to set a resolution, or might have a "web setting." High resolutions are used for printing sharp pictures, but are not used for email or web pages.

When in doubt, always use a higher resolution. You can always shrink a picture with satisfactory results, but once a picture is reduced to 72 ppi, it cannot be enlarged without losing quality. This is demonstrated below.

72 ppi

This is the actual size of the picture, about 1.5 by 2.25 inches. At 72 ppi, what you see is what you get.

The file size of this picture is only 4.78 K and it measures 112 x 157 pixels.

150 ppi

150 ppi

At 150 ppi, the picture is about twice as big. The file size is 13.9 K and it measures 234 x 337 pixels.

300 ppi

At 300 ppi, the picture seems very big! While the picture is actually only 1 1/2 x 2 1/4 inches, it DISPLAYS much larger. The file size is 47 k and it measures 468 x 654 pixels.

Notice that each time, the ppi was (almost) doubled. 72 to 150 to 300. The pixel widths went from 112 to 234 to 337. HOWEVER!!! Notice how the file size changes -- 4.78 k to 13.9 k to 47 k. MORE than doubles from 72 to 150, but the file size at 300 K is about 10 times larger than at 72K! Luckily, this was a very small picture to start!

Remember, you can always start large and go small. But here is what happens to the picture, if you start small and go big:
If I take the smallest one, the 72 ppi, and enlarge it four times-- it becomes "pixelated". You can see the individual pixels. The 72 pixels per inch have to be stretched to fit the larger area. This picture, at 448 x 628 pixels, is about the same size as the 300 ppi picture above, but with only about 1/4 of the pixels. Big difference. If ever you did any photo editing on a picture, use the largest resolution available to edit, and resize thereafter.

Session 3: Graphics Files: Size Matters

Some terminology: Pixels and Resolution

72 ppi

150 ppi

150 ppi

300 ppi

Session 3: Graphics Files: Size Matters

Some terminology: Pixels and Resolution

72 ppi

150 ppi 150 ppi

300 ppi

150 ppi

150 ppi