Film-Tech Forum ARCHIVE: Left/right lens shift

This is topic Left/right lens shift in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Edgar Prass (Member # 7558) on 10-31-2014, 07:36 AM:

Yesterday our Barco 20C arrived and was installed just 0,6m off-axis and that was already the limit for horizontal lens shift (max 15%). That actually surprised me a lot. Our smaller auditorium (with 5,6m screen) has a non-DCP projector and projection window about 1,5m off-axis (53%). The questions is, are there any lenses/smaller DCP projectors with wide horizontal lens shift range?

Posted by Dave Macaulay (Member # 813) on 10-31-2014, 11:17 AM:

With 15% lens shift, that's a 4M screen I think. With a 20C?
These projectors have a lot of lens shift available. I don't see how a .6M offset could not be within the shift range. Is the lens centred on the imager, and the projector dead square to your screen brfore trying shift? Does the image shift left and right from centre the same distance with the projector dead square to the screen?

Posted by Marco Giustini (Member # 4544) on 10-31-2014, 03:40 PM:

it really depends on the projector/lens and on how much vertical shift and zoom you are using. The software will limit the horizontal shift if you are already at the bottom of the lens - to avoid cropping the beam.

The 20C should have plenty of lens shift though. Are you sure your projector is straight? The wall is a good starting point to align a machine but eventually the picture is your target.

To quickly check, put a white pattern and adjust the horizontal shift so the pattern is in the middle of the lens. Now look at the screen, if you say the projector is 0.6m off centre, your white pattern should be roughly 0.6m off centre. If it's waay off... well, the machine is probably misaligned.

Posted by Gavin Lewarne (Member # 5397) on 10-31-2014, 03:44 PM:

I know its a different model, but my barco 12c here has so much lens shift I can project almost completely off the screen in every direction

Posted by Mark Gulbrandsen (Member # 72) on 10-31-2014, 04:56 PM:

Note that the amount of available lens shift in any dorection is also tied to a given lens focal length. Some lenses can go further off axis without cropping that others can.

Mark

Posted by Steve Guttag (Member # 268) on 11-01-2014, 06:08 AM:

Mark hit it. Which lens you are using will change your lens shift range GREATLY. I forget which it is...the 1.6-2.5 or the 1.95-3.2...has very little lens shift range compared to others. You'll also find that the smaller the imager (S2K) the more range you'll typically get...with the 4K chips typically the most limited. In fact, there are some 4K specific lenses and aside from MTF improvements, the inlet pupil is larger allow for more shift range.

Posted by Mark Gulbrandsen (Member # 72) on 11-01-2014, 11:17 AM:

The longer the focal length the less the available lens shift there is....

Mark

Posted by Carsten Kurz (Member # 5396) on 11-02-2014, 02:07 PM:

I recently installed an NC900C S2k on a very small screen (3m flat, 3,80m scope).

The machine had to be offset about 0.7m over a 10.5m distance. I wasn't able to center the image with the 2.09-3.9:1 lens.
You would think that the smaller imager projectors should be able to do huge lens shifts - but when you look at these tiny lenses, it seems clear that the lens is really the limiting factor here. I actually though that the lens mount was defective.

Then someone pointed me to a datasheet that gave max H/V shift ranges for the NEC and the accessory lenses - it barely has ANY H-shift per datasheet - 0.11 H-Shift, 0.5 V-Shift.

NC900C datasheet

Now that number is probably given for simplicity reasons, as it varies betweens lenses and focal lengths as others have pointed out.

As a matter of fact, what I was able to achieve on site was even slightly more than the datasheet quotes - yet not enough. However, it clearly is a software-imposed limit to prevent vignetting or uneven screen brightness, etc., as the lens and motor have more physical space to move.

When doing a lens mount initialize/center, you can see the lens is driven around in a circle and the largest H-Shift is possible with the lens centered vertically, and vice versa. In this installation, however, the V-Shift actually was already near zero.

That was a bit disappointing. I know various inexpenive home cinema projectors with low-cost lenses, yet they have H-Shift ranges of 0.5 and V-Shift of 1.

- Carsten

Posted by Mark Gulbrandsen (Member # 72) on 11-02-2014, 07:23 PM:

There is also an indicator on the lens setup tab on DCC that will show you if you have gone outside the available lens shit area. It is a red warning that pops up. You can't miss it. I have also seen lens memory loses occur when lens shift was at or near maximum but that was supposed to be taken care of in an update that was released earlier this year. I had one NC-2000 that was way off C/L and we had it shifted pretty far over but not to it's limits and it constantly lost lens memory. I re-set the projector centering and down tilt the old fashioned way and it has never lost the settings since. I am going to make another attempt at keystone correction and off set next time I go back so we'll see if the firmware update has fixed the issue or not. This is the only NC-2000 out of a couple hundred that has ever lost it's settings.

Mark

Posted by Carsten Kurz (Member # 5396) on 11-02-2014, 08:32 PM:

On the NC900C, DCC will only indicate that the chosen shift can not be stored ('out of range'). So you can go beyond a maximum shift a few steps, but can not store this position. You have to fine-shift a few steps trying, trying to save after each step until saving is possible.

- Carsten