Keystoning Ellipsoidal

LDSFX

Member
Ok heres my situation: Im the TD at my church (We have an ETC Unison/Source 4 Jr Zoom based system) and I have a special that highlights the cross on the main wall of the chancel. I use GAM pattern 256 with a hard focus to get a perfect cut on the cross. However, due to the location of our main front light (Box Booms) their is a major keystone that occours to the gobo. The only way to avoid this is to take the gobo way out of focus, but when I do this I lose the sharp definition that I want. When I discussed my problem to a friend LD of mine, he told me that there is a ellipsoidal that corrects keystoning by allowing rotation of the main lenses inside the housing. I have never heard of such a thing and sounds as if he is getting confused with some type of camera. Can anyone verify this for me? If not, is there a way to jerry-rig the lenses of a Source 4 Zoom (25-50) to accomodate this? (If this is not possible, my next idea is to start making cross gobos that keysotne the opposite direction).
 

propmonkey

Well-Known Member
you said you had source 4 jr zooms? if so the barrel doesnt rotate only the pro models.
 

LDSFX

Member
Sorry for the confusion: We use Source 4 Jr Zooms for all lighting EXCEPT the cross special. For that we use a Source 4 Zoom 25-50 Degree. And by rotate I meant the physical lenses themselves, not the lens assembly.
 

Mayhem

Senior Team Emeritus
Premium Member
Key stoning is caused when the surface (read screen) onto which the image is projected is not parallel to the source (read light).

In other words, the distance from the top of the screen to the source is different than the distance from the bottom of the screen to the source. In most instances, the top of the screen needs to be brought forward (you may have seen this in a classroom). However, this is usually the case because projectors are difficult to adjust. Given that you are talking about a zoom projecting a gobo, you probably have more flexibility with the positioning of the zoom.

I know that you said taking it out of focus fixes this error but then you loose the look that you are after. Have you tried adjusting the position of the fixture? Maybe lower the fixture a little and lift the snout or vice versa.
 

ship

Senior Team Emeritus
Premium Member
Otherwise if you can bubble level and figure out what angle your instrument is hanging at you can custom order a glass gobo to counter act this problem. The pattern supplier should be able to help you with the math otherwise if you post what angle your fixture is at, someone else can do the math of how much to as above said - pre keystone the image.

For simple, I like mayhems, throw it on a side arm idea otherwise.

You are using a donut also, correct?
 

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