Film-Tech Forum ARCHIVE: Used Lens Values

This is topic Used Lens Values in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Chris Erwin (Member # 176) on 03-01-2018, 12:54 PM:

Quick question to pose to all of those that know more than I. (read:everyone) [Smile]

I have a Barco DP100 that I am finding lenses for. If looking at the lens, I see 116.5mm 2.5/xx.x-xx.x. The xx's are values of the range of the lens.

Now, please tell me how to read this lens correctly to make sure I am selecting the correct lens. How does this equate to focal length?

All the best,
Chris

Posted by Stephan Shelley (Member # 8587) on 03-01-2018, 01:15 PM:

The x'x are a zoom ratio. To calculate what you need:
How to Calculate the Magnification of Primary Lens
SCOPE projection:

Primary lens
magnification =
Length of projection (L)divided by Screen width (W) ÷Anamorphic lens magnification
Use 1x for anamorphic lens magnification if this lens is not in use.
VISTA (FLAT)/HDTV projection:

Primary lens
magnification =
Length of projection (L) dived by Screen width (W) × (2048 ÷ Number of pixels per horizontal line)
* Number of pixels per horizontal line: 1998 for VISTA(FLAT); 1920 for HDTV
* Select a lens that meets the magnification requirement for both SCOPE and
VISTA(FLAT)/HDTV screen types.
Example of Primary Lens Calculation
If the projection distance (L) = 32 m, the screen width (W) = 16 m and the anamorphic
lens is not used:

SCOPE projection =
32m / 16m÷1
= 2.0x
VISTA (FLAT) projection =
32m / 16m×(2048÷1998)
= 1.95x

HDTV projection =
32m / 16m×(2048÷1920)
= 1.88x

Posted by Carsten Kurz (Member # 5396) on 03-01-2018, 05:01 PM:

Chris - please quote a full set of specs/numbers for a typical lens you see. These numbers can have different meanings. Typically in the DLP cinema world, they are throw ratio ranges, and it is very easy to compute the proper range with throw ratios.

E.g. on this page, under 'prime lenses', you will see throw ratios given:

https://www.barco.com/en/product/d-cine-premiere-dp100

e.g. 'M25 1.25 - 1.45:1' means that this zoom lens can vary it's throw ratio from 1.25:1 to 1.45:1

Throw ratio means the ratio of the distance of the projector to screen width.

As a simple example, having a screen of 10m width in a distance of 20m to the projector needs a throw of 2:1. If you find a zoom lens with a throw ratio range of 1.8:1 to 2.2:1 - that would be a perfect fit, as the needed throw ratio lies within the throw ratio range of that lens.

At 30m, that would be 3:1

Depending on your screen type (constant height or constant width), you need to calculate a suitable range for at least flat and scope width. So, if you want to find a suitable lens, you need to measure your distance to the screen, and the image/screen width for both flat and scope. Both ratios need to fit into the lens throw ratio range. Without having measured these, you can not choose a suitable lens.

- Carsten

Posted by Chris Erwin (Member # 176) on 03-01-2018, 06:35 PM:

Thanks for the education on this. The lens needed is a 3.0 to 4.3 (a drive in theatre with a 230ft. throw) The issue I am having is identification of a lens with the numbers given in a photo. In other words, the mm range to focal length such as 3.0 to 4.3.

Posted by Carsten Kurz (Member # 5396) on 03-01-2018, 07:54 PM:

The 3.0-4.3:1 Barco lens is R9806840 - Google R9806840 finds:

http://www.film-tech.com/ubb/f2/t004854.html

There was an ebay auction for that lens with a photo showing the same numbers:

http://www.ebay.co.uk/itm/Lens-Lens-Christie-CP-BARCO-DP-3-0-4-3-1-cinedlp-Case-m25-r9806840-/111828005237?nma=true&si=grVFiJ4OEJOYJuhVHqbrkmue%252Fuk%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

- Carsten

Posted by Stephan Shelley (Member # 8587) on 03-01-2018, 08:26 PM:

The one on Ebay there is defiantly a lens for a Christie. Not sure about the early series 1 Barcos but typically they use different mounting systems.

Posted by Chris Erwin (Member # 176) on 03-01-2018, 09:49 PM:

Thanks guys! That Barco R number was a big help. If I take the mm and divide by 28.28 I get the focal length. I wonder if that holds true across the board....

Posted by Dave Macaulay (Member # 813) on 03-01-2018, 10:48 PM:

I've only seen a DP100 once and not closely, so I don't know how that lens mounts. I believe the DP1500/2000 and DP3000 have lens mounts pretty much identical to the C and B series, respectively.
I don't know where you came up with the "take the mm and divide by 28.28" idea, I thought the mm specs are the focal length???
There are several DMD chip sizes and a given FL lens will have different throw ratios for each: Barco B series lenses have two ratio range specs - one for the 2K chip and one for the larger 4K chip. The other 2K-only Barco cinema projectors all use a single chip size - the C and small-chip S/E lenses each have one throw ratio range spec.
I have heard of making a lens mount adapter and using a non-OEM lens for digital... no details though and as it was in Korea, information is hard to come by.
Calculating image size isn't really difficult once you know the focal length and imager size.
The back focus distance will be your main problem... I don't know what that is with a DLP system: is the image plane at the face of the prism or (more likely) at the imager plane? A film lens is likely to have too short a back focus distance if it has to focus at the imager plane because of the complicated light path inside the prism, the DMD chips are some way back from the prism face.
Regardless, there are used lenses for old digital projectors - not a huge number but S1 systems are being replaced. The ones typical for "normal" cinemas come up now and then. Oddball lenses (ie long throw drive-in lenses) ... good luck, I haven't seen a used one show up yet.

Posted by Carsten Kurz (Member # 5396) on 03-02-2018, 04:27 AM:

It doesn't matter wether that is a Christie mount, as that auction has ended anyway - I quoted it to reference a picture that confirms the lens parameters given in the link above. I wouldn't consider a 3.0-4.3 lens a 'long' lens, it is pretty common.

- Carsten

Posted by Mark Gulbrandsen (Member # 72) on 03-02-2018, 11:17 AM:

A series 1 Christie Lens will fit a DP-100 with some effort. I've done it before...

Mark