Probably will need to boost the ISO setting for action at 300mm f/6.3
and that'll be manual focus.

My 2¢:

Take your meter readings off of the green grass and use that for all
shots...re-adjusting as lighting conditions change.  This will prevent your
meter from being fooled by white uniforms or dark uniforms/skin.  Green
grass makes for an excellent middle-tone, and is great for metering things
like weddings and ball games--where attire is often extreme whites and
darks.  If you're shooting plays at the bases, you'll want to stay pointed
at the base area rather than tracking the runner.  Otherwise you'll get
motion blur around the first baseman (and everything else in the frame)
except for the runner...which makes for a rather weird rendition.
Usually -better/more natureal to have the stationary player (the first
baseman) still and sharp, leaving any motion blur to the moving
runner...unless you are simply focusing ONLY on the runner's movement, etc.
Just in case you hav REALLY close seats (like just of of first base) where
you could use fill flash from a powerful shoe mount...make sure you switch
to second curtain synch...so any ghosting shows up BEHIND the action, rather
than weirdly showing up in front of the runner, etc.

If you're shooting the batter, think about where his best movement will be
when his position is most interesting.  Are you after his stance?  His
swing?  Ball-off-the-bat?  Ball travel?  Do you want to capture the entire
scene (batter, catcher, ump and ball travel)?  --Of course if you can't get
close enough to frame tightly, none of these will matter so much...but its
worth thinking about.  Pitcher shots can be interesting with all the various
arm movements and release angles.  Many pitchers look quite contorted when
their arm movement is frozen.

Remember that you can crop later when trying to get quick action.
This helps when you're trying to capture a play in the field, since it's
very tricky to be fully zoomed on a playeras the ball gets there.

Your telephoto zooms at a very slow f6.3.  This means you'll have to really
crank up the ISO, or you'll never get the shutter speeds you need (under the
lights).  -Better to have a little high-ISO noise in a sharp picture than to
have excessive motion blur or camera-movement blur.  Try to shoot at 1/250th
or higher if you want to freeze the action (that isn't going to freeze the
ball...)

If you're taking shots of the pitcher, focus on where he'll be when he's in
his motion toward the plate, and then stop AF if your camera lets you do
that via a button, etc. and frame with some space in front of him so you
catch ball movement on the release, rather than have him end up throwing his
arm right into the hard edge of your picture.

Remember that shots don't have to be close telephotos to tell the story.
Sometimes giving some context (umpire and fielder) to the center of
attention (batter) is a good thing.
Here's an example in a shot I grabbed of Tony Gwynn's final big league hit
(Qualcomm)...right toward his favorite 5.5 hole:
http://www.pbase.com/markuson/image/50566039/original

The best news is...with digital, you'll have an immediate indication of how
its going--unlike film.

Where are your seats at Petco?
There are a lot of good angles from the seats in the new ballpark compared
with Qualcomm...where everything seemed so far away.

I'll likely be there the next night (Saturday)...

-Mark²

Do take a monopod if you can...  f6.3 is a bit slow if light is poor.
Pump up the ISO to 400 or even 800.
Use manual exposure - meter off something in scene like grass, flesh or
other mid tones... this will stop player's white outfits from over-exposing.
Check histogram...
(Shoot RAW if you use this workflow).
Look for 'peak of action' - a sports trigger finger needs practice ( the
difference between a killer shot and an 'almost spot on' is timing).
Shade light is much nicer than direct sunlight...
Get close to the action - even a 300mm needs some help.
Enjoy...  :-)

Guy

<snipped>

So how did it go?
Any pictures you can post?