Mid-roll Film Change in Canon EOS Cameras
With many Canon EOS film cameras (at least since 1990s) it is possible to rewind unfinished film, replace it with another one, and later switch back and continue shooting where you left off. This way you can use films of different type (slide, negative, black and white) or speed (100, 400, 800) exactly at the time you need them, without having to think too long in advance about what to load in the camera.
Why is it possible?
The newer EOS cameras use infrared light to position the film at the beginning of the roll as well as for each frame. Certain number of sprocket holes (the small holes at the edge of film) are counted after film is loaded, and then 8 holes for each next frame. If you rewind the film in the middle of a roll, and later re-load it, it will be positioned precisely in the same way as before, and the frames will overlap exactly.
In contrast, many older cameras use mechanical sprockets, and the initial position of the mechanics is not precisely determined when you load film. Therefore, while mid-roll film exchanges are still possible, you will need to skip a frame and tell your photofinisher that frames may be mis-aligned.
Which camera do I need?
For most convenience, you will need one which uses infrared film counting and has leader-out rewind capability. The EOS 50/50E/ElanII/ElanIIE, the EOS 3, as well as the EOS5/A2/A2E have both, and many others probably do as well (but I have not checked). The EOS 300/Rebel 2000 and the EOS 3000 can do mid-roll rewind but not leave leader out. The EOS 1N has a sprocket wheel to count the film.
Of older EOS bodies, the EOS 100/Elan, as well as EOS 1000N/Rebel II, EOS 500/Rebel X/XS, and EOS 500N/Rebel G have infrared film counters, but no leader-out rewind capability.
What should I pay attention to?
When you replace film next time, have a look into the inside of the camera, where you normally have film. Near the upper right corner of the picture frame, there is a small hole or two. This is the infrared film counter. Whenever you load film, make sure that the film is not covering these small holes as you close the back of the camera (just as shown on the drawing). With regular pre-loaded films by most manufacturers, this is never a problem.
Always have a waterproof marker with you for writing the number of shot frames on the film canister. These numbers are very easy to forget in a hurry.
If you have a camera which does not support leader-out rewind, get a film-leader retrieval tool. This is a $10 or cheaper piece of metal or plastic which you will need to re-load your film. Ask at one-hour developing labs, they might have them or know where to get one. Even if your camera leaves film leader out, having such tool for emergencies is a good idea.
How do you actually do it?
Here comes the cookbook recipe:
If your camera has leader-out capability (normally a custom function), activate it. Note that custom functions take effect only in the P, Tv, Av, M, DEP modes. In the green and PIC modes the film is always wound completely into the cartridge.
Mark down (or memorize well) the number of frames already shot on the roll.
Make sure the camera is in P, Tv, Av, M, or DEP mode.
Press the mid-roll rewind button. Wait for film to rewind.
Remove film from camera. Write the number of exposed frames onto the cartridge immediately.
If your camera does not have the leader-out function: retrieve film leader using the retrieval tool described earlier.
Load new film. If that film had been also unloaded mid-roll, memorize the number of frames shot on this roll (you wrote it down on the cartridge last time).
Turn the camera to M, set the fastest shutter speed and smallest aperture, turn autofocus off, and put lens cap on. If you have a date back, make sure it is set not to print dates onto the film.
While holding a hand on the eyepiece to keep stray light from getting in, press the shutter button until the correct number of frames for the new roll is displayed in the LCD panel.
Turn AF on, set correct exposure, and continue shooting.
If you want to be very, very sure, you may skip a frame, but during about a hundred midroll changes, sometimes 3-4 times for the same roll, I have never had any overlapping frames or uneven spacing.
One final recommendation: mark the film canisters with fully exposed (finished) film clearly as such. One easy trick is to manually wind the film leader into the cartridge.
An alternative approach
If your camera does not have the leader-out rewind function, you may watch the film counter as it counts toward zero while rewinding. After it has reached 5 or so, listen very carefully for a small click which tells that the film leader has come off the take-up spool. At that moment open the back of the camera quickly. The rewind stops and the film leader is out of the cartridge. You may want to practice this with a roll of spoiled or low-value film first!
Thanks to Yuri Gindin (yuri [at] xpert.com) for this suggestion.
While my camera functions as described above, yours might not. Test the procedure with your actual film and camera first, and skip a frame when re-loading. I will not take any responsibility for spoiled film, broken cameras or other damage.