What's wrong? Last week I went to the zoo and ended up with a bunch of overexposed

[PhotoAmateur]

What's wrong? Last week I went to the zoo and set my camera on shutter
priority and ended up with a bunch of overexposed pictures. Some of the
pictures couldn't be printed, because the negatives were too light I'm
using a Canon EOS Elan IIE. I tried experimenting with the shutter priority
setting using a 75-300mm telephoto zoom and a 2x extender and 800 ASA film.
I was shooting in an inside jungle using a tripod and zooming in to the max.
So I figured with 600mm, I'd set the shutter speed to 1/700/sec. I think
the camera set the aperture to 5.6. There is a good deal of light through
the translucent ceiling. I used 800 ASA film to see if I could get sharper
pictures with the long focal length and movement of the animals. I had some
problems getting sharp pictures with 400 ASA, but at least the pictures were
exposed right under the auto setting. Maybe I should try 400 ASA and
aperture priority? Is it the film speed? Someone please help, because I want
to go back and take some more photographs soon.

[Brad]

I'm a bit confused. You say you ended up with a "bunch of overexposed
pictures." Do you mean that the prints are overexposed (white)? If so,
white prints would come from dark negatives, not light negatives as you
say you had.
Light negatives (transparent) would produce dark prints and would be an
indication of underexposure.

So, are you prints black or white? And are your negatives black or
transparent?

Please answer these questions and then maybe I can help.

[PhotoAmateur]

The prints are much lighter than normal. I went to a one hour photo lab and
the girl wrote on the envelope, "some of the pictures wouldn't print,
because the negatives are too light", I just looked at the negatives and
some of them are very light. So I guess I must have took some photographs
that were both over and under exposed? I used a Canon EOS Elan IIe with 800
ASA film, Canon 75-300 zoom and Tamron 2x tele extender. I set the camera
on shutter priority and set the shutter speed to 1/700 sec, because I was
zooming into the max(600 mm). I believe the camera set the aperture to 5.6.
I also used some other modes, auto and sports, but used aperture priority
mostly for an experiment. Some of the photographs turned out fine, probably
due to auto maybe, but most were lighter than normal. What did I do?
thanks

[alistair]

If your negs are light/thin they are underexposed. You said the camera set an aperture of f5.6 along with your shutter speed of 1/700th. That would not be possible. The max aperture at the long end of your zoom is probably f5.6, add to this your 2x converter and you lose 2 stops making the effective max aperture f11. Your camera may have attempted f5.6 but failing that would just set the largest value available.
Badly underexposed colour negatives will give you pale, low contrast prints with washed out colours.
cheers
alistair