jjagar, your new best friend is named "Shutter Speed Priority". here is a small tutorial to get you to snappin some fun pics.
Turn the a640 setting to TV (that is Shutter Speed Priority)
Set ISO to 200 and leave it there. you will prolly never have to change it.
with the right left arrow buttons on your circle control you can select the shutter speed. Set the SP to 1/25 or 1/50
If you have a tripod it will help a great deal.
If your snapping a photo of a coral, hold the center of the lense on the coral and press snap button half way. This will set your auto focus and youll be able to tell if you have enuff light.
Now that you are ready to take a photo of the said coral, center the coral in the lense and snap the photo. If you cant get a good auto focus, place a white sheet of paper in front of the lense and press button half way.
Keep in mind that everything in the background will be somewhat blurred. Thats ok. The focus is on the said coral.
If you are snapping pics of fish, you have to ask yourself, where am i gonna take a pic of him....back of tank, front of tank etc.
Pic a spot where you think he might be and set your auto focus to it, such as a rock or coral, or white sheet of paper.
SInce timing is everything, wait till he gets in position, and with a normal swimming motion and SP of 1/25 to 1/100 you will get a good clear photo.
Hope this helps! BTW the canon A640 is a kick *** camera that i love playing with.
