Is it possible to install a filter (or remove one) in a Noblex camera without wasting/exposing film?

Question

Noblex rotating-lens panoramic cameras have the lens inside a drum, which rotates to take a photo, recording a ~130° angle of view on film. Because the lens is inside this rotating drum, there is a unique way of installing filters, whereby the lens drum is manually rotated to access the lens, and the filter is inserted through the drum opening and magnetically attached onto the lens inside. Here is an illustration from the user manual:

But since there is no actual shutter inside these cameras, it seems that during this process, light can penetrate through the lens to the film chamber, ruining/wasting an exposure. The camera instruction manual says, "If you turn the drum counter-clockwise here, the film is exposed and you lose one picture", but it seems that even if you turn the drum clockwise, as per the instructions, you're still going to lose an exposure.

Is there any way to install a filter without exposing the film?

Answer 1

I've owned a Noblex 135 S for almost 20 years, and I just learned in the past few days how to do this...

There's a key step in the German-language user manual, which was omitted from the English-language manual for some reason:
"Stellen Sie die Kamera in Normallage; mit dem Objektiv leicht geneigt nach vorn."
"Put the camera in the normal [upright] position, with the lens slightly tilted forward."

Listen carefully when you tilt the camera forward – you can hear something falling into place inside the camera – this is a cover for the slit in the drum, so that when you rotate the drum to access the lens, no light penetrates through to the film chamber, and you can thereby install/remove filters without wasting any film or ruining any exposures. If you perform this process with the back of the camera open – without film loaded of course! – you can actually see the cover in question, doing its job of blocking the slit. Compare the following two photos showing the uncovered slit, and with the cover in place.

Answer 2

As with other film cameras, placing the camera in a dark bag or other environment with no light and performing the operation by touch is always an option.