I have a question in relation to the Aperture and Shutter priority modes on my camera (Canon 450D).

Can somebody please explain to me why the camera sets a slow shutter speed when using Aperture mode. Correct me if I am incorrect but I feel that setting a slow shutter speed with a aperture of say f5.6 overexposes the photo.

Vise Versa with the Shutter Priorty mode if I set what I feel is the correct shutter speed (based on below formula in the example - could also be wrong). The shutter sets an aperture of say f16, which I feel underexposes the photo.

Example

From my readings, if say I am using a 70 mm lens and hand holding the camera that my shutter speed should be at least 1/150 to avoid motion blur. I think 1/150 is 6 stops.

This would then allow me to move up 6 Stops in Apeture F8.

Would I be better off changing to setting the shutter speed and aperture manually until I buy a Tripod.

Please forgive me if this question does not make sense or if it has been posted previoulsy.

Your assistance is greatly appreciated.

Craig.