Your hypothesis is correct.

• If the exposure is short enough, vibrations caused by the mirror will be irrelevant.
• If the exposure is long enough, the vibrations from the mirror will decay soon enough to have little impact on the exposure in total.

For a specific camera, the Pentacon Six, a relatively heavy medium format SLR, this document states (in German) that vibrations from the mirror are e.g. only an issue with the shutter speeds 1/15 and 1/30 seconds. With 1/8 (and longer) or 1/60 (and shorter), mirror induced vibrations should not be a problem.

These are however numbers for a specific camera and it will be very difficult to give a general advice, since the vibrations' amplitude, frequency and decay curve will depend on several variables like e.g:

• The weight of the mirror will matter. A heavy mirror will cause more vibrations than a light mirror.

• The speed of the mirror will matter. The faster the mirror moves, the more vibrations will be caused.

• The weight of the camera (and everything attached to it) will matter. A heavy camera will vibrate less than a light camera. This means that even if a longer lens and a narrower field of view in general makes the exposure more prone to shake- or vibration induced blur, the extra weight of the longer lens and hence the higher inertia, may improve the behaviour.

• Using a tripod may or may not help. The increased weight of the overall system may help (see last point), but on the other hand, many tripods are not overly stable and especially if the camera is attached to a long, extended centre column, the impulse from the mirror may cause the column to swing more and for a longer time, than if you tried to hold the camera steady in your hand.

So if you really need to know when you have to use the mirror lockup: Setup your camera in a specific configuration (with lens and tripod) and make test shots. Be aware though, that you will have to do this for any configuration combination you are going to use.