Focus problem when using follow focus

[alexweb]

Hi !

I'm a beginner in shooting video and i ve just purchased a follow focus (Fotga DP500III) witch i use with my Canon EOS 70D & EF-S 18-135mm IS STM lens. I've set it up on my rig, i've set up the gear belt on the focus ring of my lens, for now all is ok. This follow focus has 2 hard stops, so I wanted to test the focus between 2 objects by adjusting the focus of object 1 and by positioning the hard stop A, and the focus of the object 2 and by positioning the hard stop B (all that in MF of course).This works fine, except that by doing several back and forth between the two points I lose the focus of the 2 objects (which were well settled initially) and the more I do the more and more the defocus increases ! This does not come from stops (which could have loosened), the follow focus seems accurate (apparently) and the gear belt doesn't slide over the focus ring of the lens...
So I do not understand why I lose the setting of my focus as the back and forth with the follow focus?
Does this come from my lens and its way of making the point with the ring in manual?
Should i have a Cinema lens so that there is no such problem?
Thanks for your help.
AW.

[kvbarkley]

I am guessing that this is because this lens is a focus-by-wire lens. There is no mechanical connection between the lens and the focus mechanism. You don't need a cinema lens, but you need one with a mechanical linkage.

[Waddizzle · Accepted solution]

I think the lens you are using is a "focus by wire" lens.  There is no mechanical connection between the focus ring and the focusing motors or focusing lens elements.  Your exposure metering system needs to be active for the lens to be manually focused.  The STM lenses are designed to allow the camera to auto-focus the lens, but not necessarily for manual user control.

I think a cinema lens would make a big difference.  Be prepared to buy primes, though.  Rokinon makes a pretty good lineup of budget priced cinema prime lenses.  Cinema zooms can be quite costly.   One benefit of cinema lenses, unlike your 18-135, is that you can use the same follow focus without having to re-adjust for a new lens.  The Rokinon DS series of lenses are made with identical focus ring gearing on several different primes.

You could use most any EF lens with your follow focus.  Just avoid the STM lenses, that's all.

[alexweb]

Thanks for your answer and your detailed explanation & advices !
Rokinon could be a right choice...

And what about EF 100mm f2.8 Macro L is usm, for exemple (I love this lens ) ?

[Waddizzle]

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@alexweb wrote:

Thanks for your answer and your detailed explanation & advices !
Rokinon could be a right choice...

And what about EF 100mm f2.8 Macro L is usm, for exemple (I love this lens ) ?

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What about it?  Yup, it is a great lens.  Never heard of anyone using it to shoot video.  It doesn't have a fast AF system.  But, when it does lock focus, it is very sharp.  Most of the time when I use it, I am manually focusing the lens in Live View at 10x.

[msrtyn]

I am a novice at using a telephoto lens. I recently bought a second hand canon eos40d body and linked it with a Canon EF 75-300mm lens.  I want to take pics of aircraft,  today I was taking a pic of a plane coming towards me from take off. I had it focussed at about 300ft high but when I depressed the shutter halfway i lost focus as though nothing was there. What am I doing wrong? Something simple I hope.

[kvbarkley]

I suggest that you use one-shot AF and the center fous point. Put the center focus point over the plane and press halfway down. Does that work better? It has worked well for me:

[msrtyn]

 Nice pic, I will try thanks,

Colin