Back-Focus issue with Canon KT14x4.4 Lens at DVinfo.net

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Old May 30th, 2011, 11:59 AM   #1
Tourist
 
Join Date: Jan 2011
Location: Dublin, Ireland
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Back-Focus issue with Canon KT14x4.4 Lens

I was wondering if anyone could through some light on an issue I am having with the JVC GY HM750E, namely the supplied Canon KT14x4.4.

When I back focus the lens with the ND filter off (ie interior shot), the image is as expected; crisp and sharp. However once I apply a ND filter to film outside, the back focus needs to adjusted again.

I have completed back focuses using a HD monitor from many different distances from a back focus chart and always the same result.

Any suggestions would be really gratefully accepted!

Many thanks
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Old May 30th, 2011, 02:18 PM   #2
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Re: Back-Focus issue with Canon KT14x4.4 Lens

As ND filters affect exposure, and therefore iris and therefore depth of field, please allow me to ensure you are adjusting back focus correctly.

You ARE back focusing while forcing the iris fully open, correct? Both indoor and out? Use ND, shutter speed and light intensity as required to force the iris open FULLY to ensure the thinnest possible depth of field when back focusing in order to ensure that a wider DOF isn't allowing you to set the back focus to a setting that is then compensated for by the depth of field.

Hope this makes sense.

If you are adjusting flange back properly, sounds like you may have a bad lens.

Oh, and MAKE SURE the macro switch is FULLY locked out. I don't know HOW many times I've seen the macro switch on SLIGHTLY, messing everything up.

Good luck!
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Old May 30th, 2011, 02:33 PM   #3
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Re: Back-Focus issue with Canon KT14x4.4 Lens

Hi Shaun,
I have done what you suggested again at many different lengths from a back focus chart, and to no success. When I do open the back focus locking nut and rotate the adjustment ring, I notice that it feels very rough, as though there is dirt in there somewhere. The camera is only a few days old so hasn't got to go outside to get any dust etc into it. The whole mechanism seems to be plastic so I hope multiple adjusting hasn't in some way damaged it! Thanks again for your help.
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Old May 30th, 2011, 05:12 PM   #4
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Re: Back-Focus issue with Canon KT14x4.4 Lens

The flangeback-screw is plastic, many operators told me that their flangeback-screw is broken, but they put them to strong enough, that`s not cheap.
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