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Getting pictures in focus with a DSLR (digital SLR) in a nutshell.

Higher-end cameras have a number demarcated by the letter "F" that one can set to change the size of the opening (called the iris) that lets light onto the sensor. High numbers make the iris smaller. Optical physics aside, the higher the number, the more of the picture will be in focus and the more light you will need. Likewise, the higher the shutter speed, the more will be frozen in time and the more light you will need as well. So, if you want to capture something that moves fast and have it completely in focus, you need a ton of light, a high F number and a high shutter speed number. Note that a particular lens only supports a particular range of F numbers (called F-stops). You don't have a huge amount of light usually. So you have to find ways to compromise and/or cheat. One way to do this is to use a flash, but that can scare butterflies. Another way is to set the camera to do more processing. This number is demarcated by "ISO". My SLR goes from 200 ISO to 1600 ISO, if I recall correctly. New ones go to 12500! The higher the number, the less light you need, and the more you rely on the camera processor and the quality of the light sensor. Thus to get a nervous butterfly on a cloudy evening, you need to higher-end camera so that you can set the shutter speed, F-stop, and ISO numbers really high, and still get a photo that's not grainy.
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