I discovered along the way a general rule of thumb for setting minimum exposures: the minimum exposure is equivalent to the size focal length you are using. For example, if you are shooting with a 135mm lens, the minimum exposure should be 1/135. I have found that 1/125 works fine with this size lens, but nothing less. Now, this is all a general rule for handheld use; placing the camera on a tripod and using a shutter release cable changes everything.
So, in order to take the pictures below I used a 135mm lens, with a shutter exposure set around 1/250 and a large aperture to make up for the loss of light. 1/250 is pretty fast, and captures motion fairly well. The Pentax has a minimum exposure time of 1/1000 and I have found this to be more than enough for any type of action shot.
First of all, this picture should be compared to a similar one I took a few weeks ago in B&W film. You can see the difference in composition between full color and black and white. I wanted to capture the motion of the water fountain in a freeze frame, instead of a motion blur. If I had used the typical 1/60 exposure, the water fountain would have been blurred together. Instead, I used a 1/250 exposure and got this great picture.
I love this picture. This was taken in front of the Burney Center. Again, I used a 135mm lens, 1/250 exposure, and a large f/5.6 exposure. This captured the motion of the water in a freeze frame. I love water features, and this picture captures that motion beautifully. 