Let’s Say that you are taking photographs. You want to work on them and send them to a friend. How should you send them? This article will give some basic terminology on graphics – photos in this case. I’ll discuss this from a non-technical perspective.
First, we need to know the type of file we should use for photographs if we are sending them over the web. We use a JPG or JPEG file. The file extension is .jpg. We use this because this type of file can be internally compressed and is used for bitmapped images (to be defined later). Most digital cameras now use this type so we don't have to worry about changing it.
There are basically two types of graphic programs. Those that are bitmap based and those that are vector based. The former comprise the image editing and painting programs while the latter refer to the drawing programs. Programs such as Adobe Photoshop and Corel PhotoPaint fall into the image editing, photo editing, and painting category. CorelDraw and Adobe Illustrator are programs used mainly for creating illustrations and logos that can be as small as a 1” by 1” or as large as you can make them. These images will retain their quality while bitmapped images won’t. Let’s see why.
A bitmap image is made up of pixels or we can call them dots arranged on a grid. Simplistically, it could be stated that when one creates an image, one is covering pixels. If we look at the image below, it is obvious that the image using 8 dots per inch will be better than the one using 4 dpi.
Below is a graphic reproduction of how the image is actually formed.
Photographs are bitmapped images. The number of dots per inch, example, dpi, determines the resolution of the image. As an important note: Although dpi is used to talk about image resolution, the term should only be used for printer resolution. The correct term is ppi or pixels per inch, but since dpi is used commonly to describe an image's resolution even though it is incorrect, we will use it. What do we mean? When you send a photo through e-mail, you want it's size to be as small as possible and, yet, have a decent quality. To send images or to post them on a web site, we use a dpi of 72. That is also called screen resolution. This means that there are 72 dots per linear inch. But what if my image from my camera is much bigger? We have to do two things. The first is make it smaller by removing some of the pixels. That is called resampling.
I changed the width of the image from 32" to 5". This, also, changed the number of pixels of the image, but it did NOT change the resolution. The quality of the image is still the same as when you took it. It is just much, much smaller.
But often when we send images, we also need to compress them. This can reduce the quality. Depending on the program, the computer can do this for you. At this point, if your image is not a .jpg image, you want to change it to a jpg. In any graphic program, you just do a "save as a jpg".
Different graphic programs handle this differently. Most will usually give you a choice. A good choice is medium. This is a measurement of compression. It compresses the JPG or JPEG file by a certain amount. This speeds up the time it will take to download because it makes the file size smaller.
Now what happens if your image has a different resolution.? Some digital cameras for high quality images use a resolution of 240 dpi for example. Think in terms of 72 dots per linear inch compared to 240 dots per linear inch. One is obviously denser and, thus, will give a better quality. (As a note: when we print we use for our image resolution 300 dpi although it is acceptable to use 180 dpi or 240 dpi. You should not use 72 dpi for printing because you will lose quality.) You need to change the image's resolution from 240 to 72. Below represents the figures for an image downloaded from a camera.
Here, I changed the resolution from 240 dpi to 72 dpi. Notice how the pixel dimensions have changed. Also, the image went from 22 MB to 2 MB while the document size stayed the same. Our goal was to reduce the physical size of the file. Will the quality of the file look different on the computer screen? Not really, because the screen resolution is 72 dpi. However, you wouldn't want to print this.
If by this point you are confused, that is normal. The concepts of pixel dimensions, file size in Megabytes (MB) for example, resolution and document size can be confusing. The best way is to take an image and depending on the program, make these changes. Then look at image size or a similar title from the top toolbar. You will be able to see how all these dimensions change.
