Film-Tech Forum ARCHIVE: Lens turret adjustment

This is topic Lens turret adjustment in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Matthew Taylor (Member # 2310) on 04-15-2005, 02:19 PM:

'fraid it's another question about my Prebost P70 [yawn]!

When projecting in scope there is quite noticable vignetting on the lower corners of the screen. After getting my lamp alignment pretty good, I have come to the conclusion it must be the position of the lens vertically. It's not a scope and backing lens problem with my short throw because the vignetting is there when just projecting through a 100mm lens.

So the question is how do I adjust the vertical positioning of the lens turret?

Posted by Jason Whitlock (Member # 3077) on 04-15-2005, 11:05 PM:

could it be your aperture plate?? wouldn't a vertical alignment problem on the lens result in the entire bottom of the image being darkened instead of just the corners?

Posted by Gordon McLeod (Member # 33) on 04-16-2005, 09:46 AM:

Remove the cover frombehind the apperture block and look at the spot of light on the apperture and ensure that it is even there
before moving the turret around

Posted by Matthew Taylor (Member # 2310) on 04-16-2005, 02:09 PM:

Jason, it could be the aperture plate, the scope is slightly overcut - it's about 18mm high and the gap between the aperture and the bottom of the plate is 5mm whereas the gap at the top is 6mm. Is this too much?

Gordon, I did what you suggested and there is a nice amount of light above and below the aperture at the rear of the block.

Posted by Gordon McLeod (Member # 33) on 04-16-2005, 02:35 PM:

Is the 100mm lens a actual 35mm projection lens or from somewhere else?

Posted by Matthew Taylor (Member # 2310) on 04-16-2005, 03:24 PM:

The 100mm Isco lens was just another lens I used to make sure it wasn't only happening with the 60mm backing lens and scope combo I was using.

Posted by Gordon McLeod (Member # 33) on 04-16-2005, 03:59 PM:

I would sugest that you completely remove the apperture plate and project a loop of 35PA test target and look to see if by moving the framer up and down you can project the full area of 690x825
I think it is most likely lamp related as I don't remember any vertical adjustment on the prevost lens mounts

Posted by Matthew Taylor (Member # 2310) on 04-17-2005, 10:35 AM:

I have done this and with a scope lens fitted I do not get a full scope frame lit up. Without the scope lens I do get a fully frame lit. It seems that it's partly the lamp alignment and mostly the combination of a scope lens couple with 60mm backing lens. I think the lens is too short focal length because if I use my 100mm lens the pic looks great. This is odd, surely some cinemas use a 55mm with scope?

Posted by Richard Fowler (Member # 893) on 04-17-2005, 11:11 AM:

Generally, based on the scope lens used, under 75mm the scope will start to have image / light drop off at the edges. Lenses under 75mm are usually close coupled to the scope with proper mating rings ( providing your existing lenes is made to use them ). The rings bring the prime optic as close as possible to the rear optic element of the scope.

Posted by Matthew Taylor (Member # 2310) on 04-17-2005, 01:41 PM:

My scope lens does screw into the backing lens BUT due to the short throw and the fact the cope lens only pushes so far into the turret, to get focus there is a gap between the lenses. When I push the backing lens so it contacts with the scope, it does seem to remove some, but not all of the vignetting, although it's hard to tell because the aperture is then really out of focus.

Would an integrated 60mm scope lens work better?

Posted by Richard Fowler (Member # 893) on 04-17-2005, 02:14 PM:

Yes, the elements between the prime and the back of the scope need to be closed coupled to the point that the lens element are practically almost touching each other + the integrated set is usually lighter and the front element of the scope generally set up to pass the light without cropping.

Posted by Matthew Taylor (Member # 2310) on 04-17-2005, 02:18 PM:

mmm...the problem with that is getting an integrated anamorphic that focuses down to 7-8 feet. I use an old B&L clone that I modified. It gives a surprisingly sharp picture considering the hacking about I had to do!

Posted by Gilbert Travin (Member # 2138) on 04-18-2005, 01:39 AM:

With your P70 it is better to use an integrated lense+anamorphic system ! (there are too many alignement problems with the separate anamorphoser!)
But your problem of vigneting may be come from your lamphouse that is designed for 16 mm : light rays have too much incidence.
Gordon, what is your opinion ? [Eek!]

Posted by Gordon McLeod (Member # 33) on 04-19-2005, 09:44 AM:

for 35mm there is a beam diffuser that need to be fitted in the front of the lamphouse

Posted by Matthew Taylor (Member # 2310) on 04-19-2005, 10:42 AM:

There is a diffuser fitted. As I said the picture is very evenly illuminated when using a 100mm lens.

Posted by Gilbert Travin (Member # 2138) on 04-19-2005, 11:10 AM:

Matthew

you can find integrated anamorphics here (Strong europe) :
http://www.adde-cinema.com/News/ListeOccasion05012005.pdf

Posted by Matthew Taylor (Member # 2310) on 04-19-2005, 11:53 AM:

Thanks Gilbert but I will have trouble using one of these 'off the shelf' as it were because of my short 7 feet throw. I had to modify my B&L Scope lens for this to work. I really don't fancy spending out 675 Euro on a nice Isco lens and then taking a hacksaw to it!

I may just have to put up with the vignetting.