Any number of problems can cause low back pain, all the way from muscle spasm to cancer and other serious conditions. Your doctor should be able to diagnose the problem after talking to you and doing a physical exam. Chances are that your doctor will order an x-ray or other imaging test. The usual series includes three x-rays one frontal and two from the side. In certain circumstances, oblique views may be added on.
Most institutions reserve the full five-film series for special cases.
The additional radiation is not small, and the additional time and effort generally do not justify the expense. The LS x-rays provide us some information about the degenerative changes that are occurring and can even provide a pretty good look at disc spaces. The real reason to have LS x-rays is to see if all the little vertebrae line up properly. Although the entire LS spine from L1 at its highest to L5 at its lowest may be affected, it is, by far, the lowest part that is affected the most. It is not unusual on a series of LS spine films to find a case where the 5th lumbar vertebra appears to be sliding off the sacrum below it.