>>3608905All points you can draw in that triangle mark points of equal exposure when interpreting the triangle's sides as the axes of the three quantities ICO, aperture and shutter speed.
In practice, this triangle is completely useless.
In practice it boils down to this:
1. Changing one of the three settings requires at least one of the other two to be changed as well in case you want to keep the same exposure as before the first setting's change.
2. You have a light meter in your camera (or an extra one with you if your old film camera does not have a light meter because it does not feature auto-exposure). Use it.
3. The scene that you shoot puts subjective constraints on your three settings. E.g. you want to limit movement blur, are in darkness, are in bright light, have unsteady light, want bokeh, etc.
