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measuring focus from front of lens


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#1 Alex Wuijts

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Posted 26 November 2007 - 03:46 PM

Hi,
With digital formats, is focus always measured from the imaging sensor to the subject, or are there exceptions to this, like measuring from the front of the lens?
I've heard this a couple of times now and I don't understand the logic behind it.
Thank you,
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#2 Chris Keth

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Posted 26 November 2007 - 04:16 PM

You always measure from the sensor. If you measure from the front of the lens you could easily be a foot off with a zoom.
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#3 Daniel Madsen

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Posted 26 November 2007 - 04:21 PM

I seem to remember under some circumstances you measure from the front nodal point. This could be for close up photography. I'll get back to you.
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#4 Brad Grimmett

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Posted 26 November 2007 - 05:26 PM

If memory serves, it depends on the lens. I think some video lenses should be measured from the front element, and some from the sensor. I think this is mostly just an issue with zooms, and not an issue with primes. If unsure, check with the rental house and/or manufacturer.
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#5 Brian Drysdale

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Posted 26 November 2007 - 05:27 PM

The focus marks on ENG style lenses are from the front of the lens, the focus marks on cine style lenses are from the plane on the camera.

Edited by Brian Drysdale, 26 November 2007 - 05:28 PM.

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#6 Michael Nash

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Posted 26 November 2007 - 05:47 PM

The focus marks on ENG style lenses are from the front of the lens, the focus marks on cine style lenses are from the plane on the camera.


Correct. It depends on the LENS. This is a point of confusion for some people crossing over from the film world.
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#7 Chris Keth

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Posted 26 November 2007 - 07:08 PM

Well, thanks guys. I had no clue. I've never used that type of lens. Do they give you a tape-hook or a marking?
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#8 Alex Wuijts

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Posted 27 November 2007 - 02:12 AM

Thanks for answering everybody. I wonder too about the marking.
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#9 Ben Rowsell

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Posted 27 November 2007 - 03:48 AM

Correct. It depends on the LENS. This is a point of confusion for some people crossing over from the film world.


......what he said.

If in any doubt, put something that you can focus on at a particuar distance from the focal point of the camera (measure the distance by tape, make sure it is a distance that is marked on the lens, such as 6 foot), then zoom in and do an eye sharp. Look at the distance marking on the lens. If it matches the actual distance that the object is by tape, then the lens is calibrated to the film plane, if not its probably the front of the lens (check that in the same way)

As mentioned, ENG lense are usually from the front, cine from the sensor. A common giveaway on ENG lenses is if they have a green strip around the front of the lens. Also most eng zooms have an attached zoom servo, most cine zooms do not.

As for tape hooks, on video cameras there is usually a attachement for the shoulder strap within 1/2 inch or so from the focal plane that you can hook the end of your tape onto. If you're good enough to pick 1/2 inch by eye you probably wouldn't be needing to ask about this :-)

If you intend to be a AC dont bother using the front of the lens as a point of reference, it will disadvantage your distance perception by eye for future shoots. In this case you either need to remark the lens (or datum mark), or use eye sharps if you have to.
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#10 Michael Nash

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Posted 27 November 2007 - 05:45 PM

Well, thanks guys. I had no clue. I've never used that type of lens. Do they give you a tape-hook or a marking?


ENG lenses are really designed to be focused by the operator, using very fine finger-adjustments (small degree of rotation) on the focus ring. Distances aren't very well marked on the barrel, and are very close together.
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