A Use for Old Slide Mounts
Back in the age of film photography, 35mm slides were everywhere. Whether they were in stacked in yellow Kodak boxes or something else, most photographers had lots of slides. Slides are a lot scarcer now that digital has taken over though. But if you have an old slide mount lying around still, go dig it out. You can use it to help you improve your photographic vision.
First, take the slide out of the mount since for this tip you'll need only the mount not the slide itself. Use one of those old slides that you used to think was really good but now that you've been shooting for a few years and have gotten much better you can't imagine why you took that photo in the first place. You know you have them; we all do. It's OK to admit it. Remove the film from the mount and throw it away. Plastic mounts open more easily, but even the cardboard mounts can be peeled open and then re-glued shut if need be. Even if you don't have any old slides you can cut your own mount out of cardboard without too much work, or just ask your photographer friends — I'm betting someone has one they can spare.
Now hold the empty mount with your outstretched arm and look at life through it. Since the aspect ratio of the slide mount is the same as both the 35mm film frame that came out of it as well as the digital images captured by newer cameras, what you see through the frame could be captured as a photograph if you had a camera with the right lens. Hold it far away from your eye and you can only see a small area through it. As you move it closer you can see more and more, just as if you were zooming out with your imaginary camera and lens. The distance from your eye needed to frame a subject will give you a rough indication of the focal length needed to capture what you see framed with your slide mount.
To help you learn to see photographically, take this slide mount with you even when you aren't carrying your camera. If you see something interesting, look through the mount to play with how best to frame a composition of what you see. Don't just move the slide mount around though, move your feet too to so you can experiment not only with focal length but also with perspective. Have some fun with it.
You can sort of do the same thing by making a frame with your hands by touching the index finger of each hand to the thumb of the other and various other methods. I've found it much harder though to keep the shape of this sort of finger-frame correct and the relationship between the length of your outstretched arms and the equivalent focal length would be less clear cut.
A slide mount is small enough to carry in your pocket all the time so you pull it out whenever the mood strikes you. Before you know it you'll be seeing possible photographs everywhere.