Film-Tech Forum ARCHIVE: focus best practice

This is topic focus best practice in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Gavin Lewarne (Member # 5397) on 12-18-2012, 03:02 PM:

Simple question......

When setting the default focus for flat and scope, is it best to use a test pattern generated by the projector or some actual, real, dcp content?

I'm currently focusing on the test patterns and in the auditorium its pinsharp, but features look a little bit 'soft'

Thanks!

Posted by Scott Norwood (Member # 30) on 12-18-2012, 03:11 PM:

The test pattern and DCP content should focus on the same plane (unlike RP40 and films printed on color print stock). If the test pattern is correctly focused, then the feature material should also be in focus. If you can see the individual pixels and the space between them when you walk up to the screen, then the focus should be correct.

Lots of movies are just not shot in sharp focus, so the focus can drift from shot to shot from being razor-sharp to slightly soft; is it possible that this is what you are seeing?

Posted by Brad Miller (Member # 2) on 12-18-2012, 03:53 PM:

Always use the test pattern.

Note too if boresight/Scheimfleug is not properly aligned during installation, you may find there is a place on the focus setting to get one or two corners in focus...going further on the focus brings them all in focus (sort of)...going further takes the previously blurry corners into focus and blurs the previously in focus.

The proper solution to the boresight/Scheimfleug is to have your technician properly align the projector. You should be able to take the image out of focus, then bring it into focus and all 4 corners should just start to hit in focus simultaneously. Then you merely center the focus adjustment for best focus.

Do note it is common to have one format properly aligned in respect to this, but not the others. This is due to optics and lens shift. I typically align for perfect focus on scope and then other formats will be just a tiny bit off. (Yes I am being super picky here, but we use binoculars and telescopes for calibration too.) This is important because if you have scope perfect, you can use the depth of field of the lens to find that middle ground for other formats so they are all in focus.

If you put up a scope target pattern and walk up to the screen, you should be able to see the tiny black lines between pixels in all 4 corners of the target.

Posted by Gavin Lewarne (Member # 5397) on 12-18-2012, 04:31 PM:

Thanks both, that reaffirms that's its the features themselves that are nit always razor sharp.

Both scope and flat test patterns are razor sharp in all four corners and centre, and next to the screen I can see each indivual pixel with no "softening" of the edges.

Our installer did a fine job at scheimflueg'ing, we were at it for over an hour and a half and I wouldent let him finish until I was happy. (Steve if your on this forum you did a grand job)

Just wanted to check, thanks for the information :-)

Posted by Pete Naples (Member # 761) on 12-19-2012, 04:10 PM:

He's not Gavin, but I will pass on your sentiments.

Posted by Gavin Lewarne (Member # 5397) on 12-19-2012, 05:45 PM:

thanks pete - tell him i haven't futzed with anything, was just checking it [Smile]

did a test screening of our DCP of the hobbit, was extremely sharp so yeah, digital films not always shot with the same focus either!