Back focus adjustment on the F3 at DVinfo.net

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Sony XDCAM PMW-F3 CineAlta
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Old March 30th, 2011, 01:42 AM   #1
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Back focus adjustment on the F3

The flange depth of the F3 can be tweaked with an allen key using the adjustment screw by the lens mount. I presume that this distance has been set in the factory. PL mount lenses, being top of the pile are calibrated to a set distance before they go out so that they all back focus correctly regardless of the camera used ie. Arri and Aaton film cameras.

With say a Sony HDCAM using a 2/3 inch sensor, there is a requirement to fine-tune the backfocus at the rear of the zoom lens itself on a regular basis. I've found that hot or cold conditions can affect back focus notably with lenses using softer lens compounds like Flourite glass.

Can I mount a swag of different lenses with a dog's breakfast of mounts; PL, F mount, Nikon, Canon, Voigtlander onto the F3 and be confident that they are all back focussing absolutely spot on? At this level of rez there is no room for doubt - wide angle lenses at T2 are very prone to back focus error as we know.

I'm aware that large sensors are less prone to back focus error, but will this mean that the flange depth on the camera body may have to adjusted for some lenses if they don't find infinity? There are a lot of variables in an video camera body especially when using lens mount adaptors which introduce another point for error.

Last edited by Pieter de Vries; March 30th, 2011 at 03:00 PM.
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Old March 30th, 2011, 02:29 AM   #2
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Re: Back focus adjustment on the F3

Backfocus adjustment is required for zoom lenses. With prime lenses, provided the flange back distance is reasonably accurate then any small error only shows up in a very slight difference in the position of the focus ring relative to the focus marks on the lens. After all, the only thing the focus ring is doing with most primes is altering the distance between the lens and the sensor, ie adjusting the flange back distance.
However with a zoom lens, back focus must be adjusted to match the lens as if it is wrong then focus will alter through the zoom range. So it is only really an issue with parfocal zoom lenses.
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Old March 30th, 2011, 02:32 AM   #3
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Re: Back focus adjustment on the F3

Mount adapters have been used for many years on film cameras. but these have been manufactured to tight tolerances. However, I wouldn't hold my breath over $20 adapters on ebay matching the required standard, so buy quality adapters.

You should test your lenses, so that you know if any have back focus issues and then get that particular lens set up by a lens technician if there's a problem. You should be able to switch your lenses without touching the back focus on the camera.

I expect we'll hear if the F3 mount itself is prone to drifting due to temperature variations etc as people start to use them. I know people use the RED without touching its back focus after it's set at the rental house.
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Old March 30th, 2011, 06:33 AM   #4
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Re: Back focus adjustment on the F3

Hi Alister

it was really a pleasure meeting you last week in Dubai...and the session very helpful.

I just received a old Lomo 200mm lens from a friend and it's a nice lens by what i read.
Problem is it's not PL mount, does a PL to Lomo exist? is it really worth it?

I attached the pictures of the lens, your thoughts are very valuable.
Attached Thumbnails
Back focus adjustment on the F3-lomo-200.jpg   Back focus adjustment on the F3-lomo-200b.jpg  

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Old March 30th, 2011, 07:08 AM   #5
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Re: Back focus adjustment on the F3

PL to OCT-19 doesn't exist, flange depth and diameter are too great on the OCT-19. BUT, F3 to OCT-19 does exist. I can't vouch for the quality though...

Konvas LOMO OCT19 OCT 19 to Sony PMW-F3 F3 adapter | eBay
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Old March 30th, 2011, 11:42 AM   #6
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Re: Back focus adjustment on the F3

Thanks David, seems a good quality product.

But the question remains....is the 200mm Lomo worth it?
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