How did old movies (and photos) get translucent and transparent effects?

Nowadays it’s easy, load up your fancy editing software, select the thing you want to edit in, change its alpha value (even the method for alpha mixing is pretty simple were you to have to do it from scratch with only a graphics library for some strange reason).
But how did they do it when they used… what do they call it? Starts with an “f”… film! That thing! Editing in something opaque seems easy enough, super-impose two things, with the thing you want on top closer to the light, and shine a light through to another piece of film. Bingo, the new cell on the new reel of film has the original image with the opaque part on top (or maybe I’m off on this too).

But how did they do transparency, cross fades, etc? My guess is that it involves reflection or lenses in some way, but that’s really a WAG based on what little I know about how to manipulate light.

You’re on the right track with your guess. Dissolves were created using Optical Printers as early as the 1920s.

If I understand what you were trying to say, that won’t work. There’s no concept of “opaque” with film, expect maybe completely saturated white.

The simplest, and most obvious way, is double exposure. Shoot something, rewind, shoot something else: you now have a new image that is the sum of the previous. Without any sort of fancy equipment, you can do various effects with double exposure. For instance, shoot something and gradually close the shutter. This gives you a fade out. Now, rewind, shoot something else and gradually open the shutter: you now have a fade in over a fade out, in other words, a cross-dissolve.

Of course, in camera effects require careful planning and are very risky: screw up your second shot and you lose both shots. Optical printers solve this problem.

Double exposure. Expose the film twice.

For the transparent effect, film the same scene twice with the camera on a tripod and the shutter closed one F-Stop to only let in half the light. The background will be in both exposures.

Here’s a still shot of my two children taken 25 years ago with a film camera. The background curtain and couch is the same in both shots. The boy moved to a different position for the 2nd shot.