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Muscle memory/menthal map of a lens

pull focusac lenses marks on lens focal lenght

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#1 davide sorasio

davide sorasio
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Posted 18 June 2015 - 01:42 PM

Hello everybody, I've been thinking about it for a little time and I couldn't find an answer so I hope somebody could maybe give me some helpful tips.

I know that the marks on a lens are put following a logarithmical pattern ( if I'm not wrong it means that the closer the more I have to spin the wheel). What I'd really like to know is how good ACs can remember by heart when it becomes necessary on the lens to start pulling faster etc etc. What I wanna know is if different lenses/manufactureres kind of follow the same patter (for example if after 8 foot marks become more distant between each others when you pull closer on a given focal lenght lens).

And if not how ACs train their muscle memory on lenses they have never used.

Thank you so much everybody!


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#2 Satsuki Murashige

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Posted 18 June 2015 - 08:45 PM

You are right that all manual focus lenses have increasingly longer focus throws as you approach close focus. This seems to be an inherent property of optics. Hopefully someone who knows more about the subject can explain why this is the case. Mechanically, these are the consequences.

Most lens manufacturers have traditionally used a simple threaded insert (a helicoid) with a fixed pitch to attach the focusing elements and to move them back and forth. This means that the spacing between the focus marks is predetermined by the size of the lens barrel and cannot be changed. So if all the lenses in a set are the same size and have the same design, then the focus throw should be similar but never exactly the same. Lenses of different sizes and designs will all be different. Large zoom lenses of the 70's and 80's had much bigger barrels than primes from the same era and thus had more widely spaced marks, making focus pulling a little easier.

In the 90's, Cooke began using cams instead of helicoids for their focusing elements in the S4s. This meant that lens manufacturers could now adjust the spacing of the focus marks, increasing throw to the far distances and shortening throw to close focus. They could also add more precise and widely spaced marks without using huge lens barrels. In the recently designed Leica Summilux-C lenses, the focus throw from infinity to 6' is identical for all lenses in the set.

In current practice, matching the correct distance mark on the lens to the scene is physically easier with modern lenses. But that is offset by higher resolution optics, camera sensors, and displays, so the tolerance for missed focus is much smaller than before. Not to mention the disappearance of camera rehearsals and marks for actors. On the whole, the 1st AC's job has become much harder in current times. Many ACs now prefer to use a remote follow focus system which in some cases (Preston FIZ 3, Arri WCU4) allow you to map the focus marks of many different lenses to the same pre-marked focus ring. So then the focus throw is exactly the same for every lens, and you can rely on muscle memory and your eye to judge distance.

So to answer your question, yes most lenses are similar enough that an experienced focus puller can guesstimate the focus throw of an unfamiliar lens by feel. Sort of like how a good basketball player will make more shots on a non-regulation court with a non-regulation ball than the average person. But really, for precise work you have to rely on muscle memory which can only be developed by practice over time with 'regulation' equipment. Which explains why so many ACs now prefer to use their own wireless system. It adds a measure of uniformity and familiarity to the feel of the lenses, which in turn makes their work more consistent.
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