Observation Scope.

[CaptainHaplo]

The difference in fast/slow is not only range, but when using a steam torpedo it gives the torp more time to be spotted - and once spotted more time for the target to efade. However, this issue doesnt raise its head with the electrics as they do not have a "visible" wake.

As for the difference between contact and influence - oh man...
Ok - contact is exactly that - if you dont hit the hull, it doesnt go boom. The influence setting is for keel breaker shots. Its designed to be fired under the target, explode due to magnetic field changes caused by the hunk of metal above it, and break the keel. Problem is - early war they were horrible - ran too deep, prematured really badly, etc. If you dont get a true premature, wait till you set a torp to influence and shallow - they tend to go off a bit before they hit the hull, meaning little or no damage to your target, yet even visually (thru a scope) it looks like an impact. Not to mention - keel damage was NOT implemented in the stock game.

As for the scope - torplexed got it right, although let me add one thing. The scope is larger, so is supposed to have better light transmission so you can "see" better at night supposedly. Either can be used to attack a target. Just know that since the O-scope is bigger, its easier for the enemy to see.

Check out NSM 4.0, keel damage has been added, and it also corrects a few other torp inaccuracies.
Hope this helps clear up the questions - and Welcome Aboard!