Someone explain gear ratios to me

[M Joel W]

This is simple I'm sure, but I want to make sure I get it right:

The Arri MFF-1 has a 1:2 gear ratio. And the Arri FF-5 has a 2:1 gear ratio.

Do I have to rotate four times more on the MFF-1? This would make sense because the focus throw on still lenses is often much less than on cinema lenses.

Or do I have it backwards?

Now let's say I replace the 43 tooth gear on the MFF-1 with a 64 tooth gear. 

Taking the same lens, and assuming the FF-5 still has the original gear, would I have to pull 2.7 further on the MFF-1?

Thanks!

 

[Selinica Harbinger]

1:2 means for one rotation of the input, the output gear will turn twice. 2:1 means for two turns of the input, the output will turn once.

So to get a single turn of the output from each, with a 1:2 ratio you'd turn the input half a turn, and on the 2:1 you'd turn the input two turns. So it is right about a 4x difference.

To calculate the ratio from the gear change, just use the input teeth/output teeth and reduce the fraction. So if the stock MFF has a 43 tooth gear, that is the gear driving the lens if I'm looking at the product chart correctly.

 

You actually can't compare the two yet without knowing the drive gear to lens gear ratio since you don't have your output gear yet (the lens portion). So your system would be more like A:B (the follow focus itself) and then B:C (the follow focus to lens) which would give the final A:C ratio and every other bit of information you'd be able to derive from there.

The focus throw would then be an angle limit and you'd need to back calculate that through the final turn ratio to find how much the follow focus moves for a full focus throw, and then you could start to make comparisons about how far each follow focus knob has to turn.

With just the follow focus, the output gear is 43T, meaning your input would be 21.5T. Half of a tooth isn't possible so it means there must be another gear at least, but it'll work. With a swap of 43->64T you'd change the follow focus from 1:2 to 43:128 or fairly close to 1:3. That's still not considering the 64T to lens ratio though.