I have just loaded in a 35mm film to my camera (basic Canon point and shoot) but the back accidentally opened. I haven't taken any pictures yet, is the rest of the film okay because it made that noise it makes when you first put in a roll of film.
On many semi-automated film cameras, you tuck the film end into the take-up spool, then when you close the door it auto-winds a couple of frames - enough to make sure all the exposed film is safely out of the way.
If you then opened the door at this point, you will have spoiled about the same amount of fresh film. When you close it, it will wind that on to the take-up reel.
The rest of the film should now be safe - though you shouldn't rely on the numbers any more, there will be fewer frames left than the camera may anticipate.
There are generally three places 35mm film sits inside a camera:
- Film which is inside the film cassette.
- The film directly across the back of the camera; this is where it gets exposed to the light from the lens when you take a photo.
- Film which is spooled onto a spindle at the other side of the camera from the cassette.
When you open the back of the camera:
- The film in the cassette is protected by the design of the cassette
- All film across the back of the camera will be completely exposed and unusable
- Film on the spindle will have been exposed to some light; in some cases images may be recoverable, but best not get your hopes up
In most cameras, what happens when you load the film is:
- The first few frames of film are exposed to light while you load it, so they are unusable.
- The camera (or the user, on a hand-wound camera) pulls these frames onto the spindle (i.e. from position 2 to position 3), pulling fresh film from the cassette into place for the first exposure.
- Then, after each exposure, this repeats: the camera (or the user) winds the exposed frame onto the spindle (position 2), and a fresh frame from the cassette into position 2.
- Once the whole film is exposed, the camera (or the user) winds the entire film into the cassette, protecting it until it is processed.
In this case, the film exposed to light when you open the camera is the film you've already exposed - photos you'd taken up to that point might be gone forever, but luckily in this case you hadn't taken any yet. Film still in the cassette is safe to use, but the part where the next frame was going to be exposed (in position 2) is now useless. Take a couple of "nothing" shots to wind fresh film out of the cassette, then carry on with the rest of the film. Note that the counter might reset when the back opens, so the film might run out sooner than you expect.
In a few cameras, the direction is reversed: when the film is loaded, the whole film is wound into position 3, and then with each frame, it's pulled back across to position 2, exposed, and stowed into the cassette. This is relatively rare, but is most common among late film era compact point and shoot cameras that automatically wind the film with a motor, and would probably be visible when you opened the camera.
In these cameras, the film exposed to light when you open the camera is the film that's not been used yet. Any photos already taken are safely in the cassette, but the remainder of the film should not be trusted. In your case, this is actually the worse scenario - you don't have any photos safe, but will probably want to throw away the film, rather than risk poor quality or useless exposures.
It will depend upon the model of the camera.
In some cameras (as Tetsujin has answered) the film winds forward. The film is left inside the cassette until a frame about to be exposed and then the exposed frame is wound onto the open spindle.
In other cameras the film winds backwards. The film is pre-wound onto the open spindle, and then as each frame is exposed it is wound inside the cassette.
The simplest way to answer your question may be to open the camera again. How much film is wound onto the open spindle ? does it fill the space available or is there only a short length of film tucked around the spindle ?
This question has already been answered, but I'll give another angle... Film that is exposed to light is ruined. Film that hasn't been exposed to light is still usable for photos. (Film inside the cassette is protected from light.) There isn't really much more to it than that.
Take a look also at this other question on the site here:
My film camera’s exposures count went back to 1?
Nobody has mentioned: just because the back opened doesn't mean ANY film is impacted.
In addition to understanding what film might have been exposed, the other question is: how much light and what kind of film is it?
"Slow" (low ISO) film, such as 25 or 50, is much slower to react to light. Might have no issues.
Black and White film, exposed briefly to red light, may not have any issues.
If it was night time, perhaps no issues.
In general, it's worth developing the film rather than assuming it is worthless.
At the extreme, we never saw my in-law's wedding photos. They sat in the glove box of their car during a very hot summer as they traveled on their honeymoon. Yes heat, not so much light. But the principle is similar.
We developed the film. Yes, the photos were all washed-out orange. YET -- a quick color correction in modern photo tools, and everything became perfectly usable. Every single photo.