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Old April 20th, 2006, 05:13 PM   #1
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Question about Follow Focus Gears, the 1:1 ratio

from what i see on most follow focus's is the handle/wheel, (the wheel with the whiteboard attached) that is linked by a shaft which goes into a 90 degree gear box. The shaft that comes out of the box is attached to a smaller sized toothed gear (usually plastic) which interlocks to the SLR lens plastic toothed gear which is about as big as the diametter of the lens. and then you turn your wheel as it simultaniously turns the 35mm lens. since the slr lens gear(helicopter gear in this case) is large compared the the smaller gear that is on the 90 degree box shaft. How is the gear ratio still 1:1. wouldnt you need 2 gears the same size to get a 1:1 ratio.

or is there another gear involved somewhere that i missed. (note, i know that on professional FF's they have an adjustable extension peice that is pulley driven, that way you can swivel your gear back and forth for differnt size lenses. But i assume that pulley driven extender is 1:1 in itself. so i left it out of my question. to keep things simple)
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Old April 22nd, 2006, 09:13 PM   #2
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Ah, no one knows? i know alot of people do.

here, i made a picture to demonstrate what i mean...

http://i11.photobucket.com/albums/a1...cetti/foff.jpg

thank you guys and gals for everything
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Old April 22nd, 2006, 11:55 PM   #3
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From one I have designed, the 1:1 ratio is in the gear box. The outside wheels should be larger if you want it to be half-turns instead of turning all the way around...
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Old April 23rd, 2006, 01:32 AM   #4
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I haven't shopped for a follow focus so I'm not well-informed about anyone's claims of a 1:1 ratio, nor am I in a position to say why anybody selling them would talk about it. It does seem to refer to the ratio between the 90-degree gears, though.

However, I can tell you a 1:1 ratio of drive gear (FF) to driven gear (mounted on the lens) would not be desirable. It would be harder to turn the knob and would be harder to get focus just where you want it. Gear reduction can be a good thing!

Anybody who has ridden a 2-stroke motorcycle with an extremely short throttle twist knows what I am talking about!
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Old April 23rd, 2006, 01:40 AM   #5
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Bill, I think i get what you mean, you mean set the gear ratio so that when you spin the knob, its moving the lens a bit faster than you spin the knob. or is the the otherway around, where the lens will spin slower than your hand spinning the knob.

because to do the former, you would a Bigger gear incontact with the lens gear, that would create the super fast whip turns.

But... the 1:1 ratio i think refers to the overal Follow Focus. Meaning how much you spin the knob, is how much its spinning the focus ring. Just flipped 90 degree, but it should be just like your with your hand over the lens spinning it, just at a different angle. That way, you keep the desired 1:1 movement. and the only way im trying to figure out thats possible is if you use 2 gears the same size, (or you have a gear box that can fix it so you can use a smaller gear)
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Old April 30th, 2006, 04:30 AM   #6
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I think that it's more of an advantege to get the actual lens barrel moving slower than the control knob. This will give a much more accurate finding of focus.
It's what you get anyway with a normal 1:1 bevel gear and a small output gear mated to the much larger ring around the lens.


Robin
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Old April 30th, 2006, 09:23 PM   #7
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Robin, you are correct. is this how all follow focus's are made then?
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Old May 1st, 2006, 03:34 AM   #8
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Forrest,
Yes, as far as I know...

Robin
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Old May 1st, 2006, 09:03 AM   #9
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Anyone have a parts list for a DIY Follow Focus?

Also has anyone tried using a belt-drive system?
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Old May 30th, 2006, 06:06 AM   #10
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Hy guys. Sorry to bother.
I read this thread and a week ago I tought of a different kinf of aproach of the follow focus. This will not include a 90 degree gearbox, just two gears. One big gear on the lens and another one witch name I dont know in english, but I did a drawing.

This is only a drawing. I didn't tested yet. I have to build it first. If somebody has any idea if it will work well please let me know.
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Old April 20th, 2007, 02:48 AM   #11
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I'm learning about follow focus, and came across this thread.

At NAB I had a look at several commercial units (chroziel, arri, and some others) most were 1:1 gear boxes, one was a 1:2. If you are using the FF for a 35mm SLR lens, you want a smaller diameter drive gear, as turning the 35mm lens 1/2 way around is the whole focus range.

If you are using it with the stock lens on a fixed-lens video camera (like Panasonic DVX or HVX) you'll want the FF to be geared much higher- the lens is a very large diameter, and it goes nearly 360 degrees (or more?) to go through the full focus range.

At the Able Cine Tech booth I tried an HVX with a 1:1 FF on it, and it was hilarious- I can't imagine what the point is, since I had to turn the FF knob about 4 revolutions to rack focus- really not a good option.

-tom
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