Film-Tech Forum ARCHIVE: 180 mm lens problem.

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Posted by Ken McFall (Member # 845) on 02-16-2009, 10:52 AM:

I've got a problem with a cinema, single screen, that uses 180 mm backing lens 97' throw. The picture is AWFUL even with a 2k lamp.

When I first visited the cinema it was clear the lamp house was not installed very well. Off centre and pointing upwards!!
I've done all of the line up again raised the lamp house table etc etc and the light is as good as it's going to be.

What is curious is that starting from 1:85 the light is acceptable. As you go longer the light deteriorates and a Black Hole appears in the middle of the light as you get to 1:33 and then Scope. Looking closer the Black hole is the end cap of the lamp and you can just see the anode cable and the cooling tube. It a cinemeccanica 16h lamp house. The lamp house has never performed well since it's install.

The anamorphic is a gold isco and is in excellent condition.

I've confirmed that it can only be the backing lens as when I use the 1:66 lens, a gold isco, with the anamorph the picture while too large is easily twice as bright!

Have not taken any measurements but to give you some idea the rear element of the 180mm lens is in line with the last clamping ring on an FP20 manual lens mount. Add on about 6" of lens tube with the anamorph screwed on! It's some length and the furthest away from the film I've ever seen.

Sorry I've been able to provided any more detailed info at the moment.

Regards.

Posted by Steve Guttag (Member # 268) on 02-16-2009, 11:47 AM:

What 180mm lens? You didn't specify that. I know that ISCO's Kiptar is not rated to work with Scope as a prime lens.

Steve

Posted by Ken McFall (Member # 845) on 02-16-2009, 11:58 AM:

Hi Steve I was hoping either you or Gordon would chip in.

It's an old Kiptar 62.5 mm dia screwed into an adaptor to 70.6 mm dia. In good condition but clearly not right for this situation.

I was not aware that this type of lens was not rated for use with scope.

The cinema has 2 Kiptar lenses, scope and 1:33, both exhibit the same Black Hole problem.

Can you explain why they are not suitable. I'm not up on optical theory so some information would be appreciated.

Regards.

Posted by Marin Zorica (Member # 1716) on 02-16-2009, 01:22 PM:

hi Ken,

I do have one changeover booth with 2500W vertical bulbs, they use 180mm Kiptar and ISCO ULTRA-STAR HD anamorphic on one projector, and SANKOR Adapter B on another, well ISCO is much better, but there are NO problems on black spot or so on either projector, or on either scope or flat. Kiptar's are as with you with 62,5 to 70,6 adapter and anamoprhic is attached directly to kiptar via adapter ring.

Maybe is problems lamp to reflector distance are not as it should be with that lamphouse and then distance lamp to aperture?

This Kiptar's are purchased before two year or so, and there are only 180m lenses ISCO still offers, as they said until they have them on stock!

Posted by Ken McFall (Member # 845) on 02-16-2009, 02:00 PM:

Hi Marin,

I'ce double checked the alignment and got the correct specs from Sound Associates, the dealer in the UK.

The problem only appears with the Kiptar lenses and the light is smooth with the Gold Isco's.

I'm interested in what Steve had to say and hope he can elaborate further.

This is very strange and in all my years doing this type of work I've never seen anything like it. If the lamp house was out of line then it would show up on all ratios, to a lesser or greater degree depending on ratio, but it would be there.

So for now I'll just have to wait and see if anyone can come up with an explanation or see if I can get a different type of lens to try.

Regards.

Posted by Gordon McLeod (Member # 33) on 02-16-2009, 02:17 PM:

years ago I had problems with a line of Ziess lens that only produced a flat field of light when there was a small field flatning lens inserted directly behind the apperture plate
There is a lot of interaction between projector lens and the lamphouse optics not just in the issue of there F speed but also in there ability of resolve the cone of light
John Pytlak once told me of atheatre that had a very sharp image with there hilux lens and peerless carbon arcs but the same lens would not focus evenly hen they installed a ORC lamphouse on one of the two machines in the booth and the lens in question if inserted into the second machine that was left carbon was sharp
The thought was the ball of light from carbons being larger the optics had been optimized for the more scattered cone of light

Posted by Steve Guttag (Member # 268) on 02-16-2009, 02:18 PM:

I was looking on the back of one of my old slide rules by ISCO...It lists the 155-220mm Kiptars as only okay for "widescreen" but Anamorphic 35mm or anything with a Magnacom as being a "NO" go.

As for the 180mm focal length...with a 62.5mm barrel lens, at best you have an f/3 lens! So some of your light is gone right there.

Now if you get the newer Schneider 180mm Cinelux, you won't have the problem of getting a good image...you still have some light loss as it is an f/2.5 but likely you will need it stopped down compared to other format for a more even balance. I think it is only offered in a 70.6mm lens barrel but I could be mistaken.

Another choice is to use a 90mm EF lens with a reverse anamorphic. Before Schneider came out with the 160,170 and 180mm, this was the preferred method of those long focal lengths in a 70.6mm barrel. If 101.6mm is possible, then go for a 7" lens with suitable 4" anamorph...you'l have gobbs of light!

Steve

Posted by Ian Parfrey (Member # 5122) on 02-16-2009, 02:21 PM:

Hi Ken.
Three ideas,
1. Has those lenses ever been cracked open before? I had a lens once that was "repaired" by a Fiddler in the Booth and the rear element was inverted. Needless to say the lens was junked after that.
2. Do those lenses have a longer back-focal length than usual? I vaguely recall some lenses were made with altered back focus, reasons why I cant recall.
3. Should the lamphouse have a condensor/spreader lens in the ***(Danger- "Mark-ism" ahead!)*** schnood or nosecone? I dont know how that would affect 1.85 and not 'scope though.

Good luck
Ian

Posted by Ken McFall (Member # 845) on 02-16-2009, 04:33 PM:

Steve thanks again for your help. It's something I can present to the cinema to confirm the lens itself is causing the problem May even get them to put their hands into their pocket and buy a new lens..... HEAVEN FORBID!!

70.6 barrel is not a problem as I don't like using adaptors anyway and it'll screw directly onto the anamorph.

As far as I know, and let's face it you seldom get an honest answere from some booth staff, the lens has never been 'worked' on. I doubt that's the problem but worth checking anyway...

I'll get to work checking out prices and availability and see what I can come up with.

Will look forward to being able to post on this again when it's been resolved.

Regards.

Posted by Louis Bornwasser (Member # 3063) on 02-16-2009, 07:27 PM:

Steve mentioned the 90mm with a reverse anamorph. I really LIKE that idea! (do it all the time) Louis

Posted by Brian Guckian (Member # 1678) on 02-21-2009, 03:39 PM:

Ken, as a matter of interest, do you think they could ever do common height for 'Scope in that cinema? (Talking theoretically, of course).

Posted by Ken McFall (Member # 845) on 02-22-2009, 12:40 PM:

Hi Brian,

they are already doing common height. So you can imagine the size of the other ratios is small.

There is very little scope for increasing the width of the scope picture due to the procenium arch. But even then it would only reduce the lens by about 10, down to 170mm. Given the situation it would not be to any real advantage.

However picture sizes are not not so bad in the auditorium as it looks from the projection room.

I'm toying with the idea that you can 'reverse' a normal anamorphic and get good results. However I will admit the jury is out until I try it.

The biggest problem is that the FP20 dose not use a separate anamorphic mount as in the old days.

It's all down to cost. One price I've heard of for an 180mm lens is about £1600 which is WAY out of the budget for this cinema.

I'll experiment with a few ideas and see what I can come up with.

Regards.

Posted by Marin Zorica (Member # 1716) on 02-22-2009, 06:09 PM:

I remember this cinema here payed around 700Eur for each 180mm isco kiptar, two years ago.

Posted by Brian Guckian (Member # 1678) on 02-25-2009, 12:13 PM:

Thanks Ken, I also think the reverse anamorphic route is a great way to go; I saw an installation in Liverpool once that used it very effectively.

Posted by Ken McFall (Member # 845) on 02-26-2009, 04:13 PM:

Hi Brian,

I agree with the reverse anamorphic idea but it's down to cost....

Yeah we've all been there... a client wants the problem sorted but cannot afford to it. This cinema is a trust and they have to get the best for every pound spent. So we have to try and cover every avenue before we give an ultimatum that so and so is the only way to go.

It's all about trust and a good relationship with the customer. They were stung badly before with the purchase of the current lamp house. Fortunately I have done a bit of work and have a good relationship so they are able to trust what they are being advised. Not quite the experience of the past from what I have been told.

We will get there but it will not be the 'in' and 'out' job I would have preferred.

I go into hospital for a pretty major operation in a weeks time so this will delay progress some what.

Regards.

Posted by Brian Guckian (Member # 1678) on 02-27-2009, 06:35 PM:

As a matter of interest, how much are reverse anamorphics - roughly speaking?

(Good luck with your operation Ken)