jonmikal said:
the 25-400 refers to ISO/ASA
Yup, you set this dial to the ISO of the film you are using.
As far as how how shutter speed and aperture (size od lens opening) affect your photos...that is the basics of exposure. I suggest doing some reading and then come here to ask any specific questions you might have.
Basically, shutter speed is how long the shutter is open. Obviously, the longer it's open, the more light gets in. Also, the longer the shutter is open, the more change of getting a blurry image due to the motion of either the subject or the camera. When holding the camera in your hands, you want to keep the shutter speed number at about or higher than the length of your lens. So for a 50mm lens, don't set you shutter speed below 1/60. This is to avoid blur from camera shake. If you use a tripod...this does not apply.
The aperture also affects exposure by controlling how much light gets it. It's represented in F numbers....the smaller the number the bigger the opening. If the opening is bigger, it obviously lets in more light...so when you combine the size of the opening and the duration the shutter is open...that is your exposure value. Therefore, you can get the same exposure if you adjust both the shutter and aperture. If you open the aperture, you have to make the shutter faster and visa versa.
Aperture also controls DOF (depth of field). This is the range in which objects are in focus, front to back. If the aperture is bigger, the DOF is smaller (shallow). If you close down the aperture...more of the scene will be in focus.
Are you confused yet?