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Old October 7th, 2008, 10:13 PM   #1
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Question about Backfocus

Hi all --

So I did the backfocus thing on my HD100 (following the description in the Fujinon manual and using Siemens charts about 10 feet away) and everything is in focus regardless if I zoom in or out. However, the infinity is off now. It is overcompensating for infinity and I have to pull back the focus quit a bit to bring it in focus.

What do you suggest that I should do?

Thanks!
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Old October 7th, 2008, 11:21 PM   #2
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Originally Posted by Stefan Immler View Post
Hi all --

So I did the backfocus thing on my HD100 (following the description in the Fujinon manual and using Siemens charts about 10 feet away) and everything is in focus regardless if I zoom in or out. However, the infinity is off now. It is overcompensating for infinity and I have to pull back the focus quit a bit to bring it in focus.

What do you suggest that I should do?

Thanks!
What did you use to set the backfocus? I've only been able to set it with an HD monitor. You using the VF or eyepiece? If so, try a monitor.
Also, what are you using to focus on? I use a 17x22 spiral test thingy printed out with a really good printer, or at least what passes for a good printer at Kinkos.
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Old October 7th, 2008, 11:28 PM   #3
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I used a 24-inch HD monitor. The test chart was a standard Siemens star chart.
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Old October 7th, 2008, 11:30 PM   #4
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My personal preference would be to go outdoors and look for a large distinctively sharp object about a mile away. Set your lens witness mark on the barrel to inifinity. Zoom in and adjust your backfocus until it is sharp. Zoom back to see if it remains sharp.

That hopefully puts you in the ballpark to do finer trims against a Siemens chart indoors.

Your best result will be to light your Siemens chart so that your lens aperture (iris) has to be wide-open, ie., its lowest "f" or "t" number. You can use your in-camera ND filters to keep the exposure under control.

Preferably do not use a fast shutter to control the light when making the backfocus adjustment but control the lighting environment itself.

The reason for wide-open aperture is that your lens depth-of-field is at its shallowest and will give you a more accurate adjustment.

You may find it helpful to mark your outdoors infinity adjustment on the lens body alongside the backfocus lever with a chinagraph pencil or a thin wedge of masking tape, then adjust either side of that reference and separately mark the sharp focus points for zoomed-in and zoomed-out. Somewhere near to centre of those marks will be where it needs to be.

Marks may not be all that helpful as the adjustment is really quite subtle and less than the thickness of a chinagraph mark if everything else is healthy and undamaged.

If the Fujinon standard lens for the JVC has a macro adjustment, also make sure your macro adjustment is selected off before you adjust backfocus. You will know if it is off if you cannot move the macro lever without pulling the little knob outwards and making sure it has clicked back into its rest position.

On most ENG style lenses like yours, if there is a macro adjustment, this uses the same helicoid movement as the backfocus and they interact.

Last edited by Bob Hart; October 7th, 2008 at 11:37 PM. Reason: errors
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Old October 8th, 2008, 12:21 PM   #5
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All lenses for JVC HD series are ENG lenses so yes, there is a macro. Keep in mind that if you do a bunch of macro work you can very easily knock the adjustment off a little without knowing. Not sure if you are doing that or not.

Also, extreme changes in temperature can effect it.

As Bob said, keeping the iris open all the way while making your adjustment is critical.
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