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How to mark a zoom lens for pulling focus


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#1 James Malamatinas

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Posted 14 June 2011 - 07:04 PM

Hey, got a shoot in the morning and we are working on the canon 5d with the 24-105mm zoom lens. I will be pulling focus without a follow focus so have to mark up the barrel.

My preferred method is wrapping some tape around the lens and then putting set distances on the tape. This would however require a new bit of tape for the various focal lengths between 24 and 105 since the focus marks would for one focal length would not be accurate for all the others. Is there a more sensible efficient way of doing this, I have googled but cannot find anything so feel I be may be missing the point completely?!

Any advice in the next 6 hours would be much appreciated.

Thanks,
James
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#2 Chris Millar

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Posted 14 June 2011 - 07:12 PM

This would however require a new bit of tape for the various focal lengths between 24 and 105 since the focus marks would for one focal length would not be accurate for all the others.


I don't know the lens in question but why would this be the case for any normal zoom ?
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#3 James Malamatinas

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Posted 14 June 2011 - 07:49 PM

Sorry, I was getting confused, to ask it slightly differently. What advice would you recommend for marking the tape/barrel to avoid getting too many marks on the tape and having to redo the tape so often. Unfortunatley my distance judging is not good enough yet to use ft marks alone! Also, does the fact that the lens extends not affect the tape marks or, because it is from the focal plane it does not matter?

The focus on the lens also has no end points, what is the easiest way of matching the marks on the tape back up with the lens?

Cheers for the advice, im still very much learning as you can tell.
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#4 Chris Millar

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Posted 14 June 2011 - 07:58 PM

Also, does the fact that the lens extends not affect the tape marks or, because it is from the focal plane it does not matter?

Doesn't matter


The focus on the lens also has no end points

One end point will be infinity - the other you could find by putting it at that end point then moving an object until its in focus, then measuring from the focal plane (the circle with the vertical line though it scribed on the camera) to that object.



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#5 Chris Keth

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Posted 14 June 2011 - 09:15 PM

Sorry to tell you, but with that lens you'll have to get marks by eye at the focal length of the shot and pull strictly by those marks. You'll probably just have to redo them for each setup. The distance marks mean nothing and are too close together and the lens isn't even totally parfocal.
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#6 Matt Kelly

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Posted 14 June 2011 - 10:53 PM

Ditto to what Chris said. My best advice would be to not stress too much over it. It is what it is, but be sure that whoever you're working with is aware that proper marks are impossible with an SLR lens like that. If they expect you to truly do the job of pulling focus, than motion picture lenses with proper marks are necessary. I wouldn't even consider the canon zeiss primes "motion picture" as they also lack the markings for critical focus. A remote focus would be a good request for the future if you're ever forced to pull on still lenses. (it can help lengthen the short throw, and helps repeating marks) But normally that kind of job won't be willing to dish out money for a bartech :P. Good luck though and welcome to 5D hell! :D
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#7 Jonathan Bowerbank

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Posted 17 June 2011 - 12:35 AM

Those lenses are also notoriously out of whack back focus wise when you're zooming around. So don't zoom in to get critical focus, if you do, when you zoom back out to the desired focal length your marks will be off and worthless.

1/2" paper tape around the barrel, and using a black stabilo/grease pencil or wet erase pen so you can erase your marks for each setup, and you'll be good.
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#8 James Malamatinas

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Posted 28 June 2011 - 04:45 PM

I just wanted to chime in and say thanks to everyone that gave advice. The shoot is over now and it was very, very useful - I definitely managed to do a better job than without the advice.

Just to confirm a few things that I experienced:

  • The distance marks on the lens were accurate BUT they were few and far between, I think it was 3, 5 and 10 feet which meant that I ended up having to make more marks on the tape than I would have liked.
  • There wasn't time to mark up tape all the time so I simply used eye marks as suggested.
  • To help with marking the tape I; made sure I marked close focus so that I could always reset the lens back to this for each shot. The focus on the lens has no end stops so you simply have to rotate the focus ring until your close focus mark matches up - remember to rotate the correct way!
  • To try and distinguish between marks with more clarity, and to avoid having to change tape so often I used various coloured pens for different set-ups (where necessary).
  • I did however have lots of strips of tape already cut so that I could change the cluttered tape when needed.
  • Wrapping several bits of tape on the barrel at once can save time, once the top tape is too cluttered simply peel it off and there is another below.
  • On this lens the f-stop was f4.0 so depth of field wasn't as much of a nightmare as it could have been but, as some have said the focus mechanism on the lens is not precise compared to cine lens so to make minor adjustments can be pretty difficult.
  • One side-effect of doing it is that you can end up shaking the camera because you are always touching the lens. You can try and only bring your hand to the focus ring when an adjustment is made but this can invariably cause a bigger shake when your hand comes back into contact with the lens.

Overall, it was a good learning experience given that I'm pretty new to focus pulling and it definitely help me develop my distance judging.
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#9 Jonathan Bowerbank

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Posted 01 July 2011 - 12:07 PM

Sounds like you did fine James :)

Good practice, yes, but hopefully we'll have to work like this less and less as the DSLR world gets more cine friendly.
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#10 Matt Kelly

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Posted 01 July 2011 - 01:48 PM

I unfortunately had to do another one of these jobs recently with a 5D nearly wide open at all times. I'm still not used to Canon primes so I tried some marks at first. The 100mm macro we had was impossible. A mark on the lens gets you maybe +/- 3 inch accuracy, which is useless for wide open stuff. I found that there was actually play in the lens itself between then outer barrel and the inner magnet driven focus barrel. That lens became a game of making marks for muscle memory of whatever the particulr shot was, and then keeping a good squinty look at the monitor to make sure it was good and adjust. Basically a game of pulling off monitor, since I couldnt trust marks. The 85mm was "OK" for marks, but for critical again I found myself monitor pulling. And the 200mm was maybe the best of them just because it has a larger spread. Every lens was untrustable for marks that day though, but I got a lot of fun/entertaining monitor pulling practice, which I have to say turned out pretty good despite the backwards nature of it.

I'm glad to hear you didn't get completely stressed out over it though. That's good :)
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Technodolly

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Aerial Filmworks

The Slider

Abel Cine

Wooden Camera

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