Gel Blurring Gobo

[Ekij]

I'm lighting a production of Joseph at the minute and for the Jail scene I have a jail bars gobo but I want to put it behind a blue Gel (L132) in order to maintain a dark 'cell like' feel for the scene.

 

Whenever I add the Gel the focus of the Gobo is shot and it becomes difficult to see the bars anymore.

 

Is there any way to avoid this?

[J Pearce]

Have you got a donut in the lantern?

 

I have only recently had the opportunity to A/B with donut and no donut and it does make quite a difference, particularly, I found, when gels are in use.

[Ekij]

Have you got a donut in the lantern?

 

I don't know what a donut is (though I'll look into it) so almost certainly not.

 

Thanks for the suggestion!

[J Pearce]

Imagine a gel frame but with a much smaller circular hole, about half the diameter of the usual hole. It slots in a spare gel frame runner (or could possibly squeeze in the same runner as a gel frame) and it tightens up the focus.

 

Found a picture.

 

http://i2.photobucket.com/albums/y21/bangadrum/donut.jpg

[Ekij]

Imagine a gel frame but with a much smaller circular hole...

Found a picture...

Excellent, thanks!

Looks like I'm getting out the sheet aluminium and the craft knife tonight :D

I'll let you know how it works out in tomorrows show ...

 

:P Edited quote of entire previous post (with picture). Really not necessary ;)

[Guest lightnix]

I use donuts a lot and they do go some way to cleaning up the edges of gobos, albeit at the expense of intensity. You can also make them out of black wrap and experiment with holes of varying diameters. Presumably you've checked the alignment of the lamp (if the lantern allows).

[the kid]

Just how does a Donut work ?

[Stu]

What lantern are you using to do this gobo? L132 is quite a saturated blue so if your using a small 500w profile it might not have the required guts to cut through the gel and still give a visible image on stage. Perhaps think of a lighter blue gel (something like 161 or 079 etc) just to suggest blue.

 

Also play with the peaking of the lamp, you should find a knob to do so on the back. If you play with this you'll notice the centre of the beam getting brighter than the edge at one end, or the whole gobo being the same brightness across the image at the other end...

 

Also if your using a wide angle profile then donuts don't seem to work as well - especially one with less than great optics.

 

HTH

Stu

 

PS. As to how they work, something like a pinhole camera I suppose. Couldn't say for sure!

[paulears]

How does a donut work?

 

In optics, the best analogy is to a camera with F numbers on the lens. Depth of field is the thing here. Each lens tries to converge the light paths into the same place - the focal point. As the light passes through the thickest point of the lens in the middle and right out to the edges, the focal point gets a little blurred - the angle of the glass to the source changes as we move from the centre out. The light gets refracted as it travels through the glass, and these differences in angle prevent the gobo image from being truly sharp. The donut is the lighting version of the aperture ring on a photographic lens, mimicking the function of the iris. By preventing the light travelling through the very edges (the worst part for refraction) the image sharpens up, but gets dimmer. Even a thin gel creates more disturbance to the light paths - so donuts are pretty important for coloured gobos. Cheaper lanterns also have cheaper lenses which make gobo projection worse. Proper optics, as in condenser types help no end too. The comparison between things like Strand Optique and Cantatas is a good example. Optiques have an optics path that allows nice crisp gobos, while the cantata is pretty crude - nice (or not) colours around the edges.

 

Hope that was what you wanted

[bruce]

A simple test you can do, which exhibits the same principle. Doesn't really work if you have 20/20 vision though - but if you are short or long sighted it's effective.

 

Press your thumb and forefinger together - both hands. Now press your right thumb & forefinger agains your left. Gosh - this is really difficult to describe.

 

Now, where the four fingers meet, there will be a tiny squarish gap in the middle, about 1mm across. (alternatively, make a 1mm hole in some card!)

 

Now, with one eye, look at a distant object. Then, look at it through the tiny hole. Unless you have really good eyesight, the image through the hole will be considerably sharper.

 

This explains why many people find it easier to read far-away text - eg on a screen - when they half-close their eyes...

[andy_s]

What lantern are you using to do this gobo? L132 is quite a saturated blue so if your using a small 500w profile it might not have the required guts to cut through the gel and still give a visible image on stage. Perhaps think of a lighter blue gel (something like 161 or 079 etc) just to suggest blue.

Alternatively (and dependant on you having enough lanterns and circuits) use a reversed jail bars gobo (where the bars are made of light not shadow) in open white and a blue wash from other luminaires to give you the effect, then you could use a pretty saturated colour for the wash. You could justify this as added value, as your blue wash becomes available for other scenes as well.

[Ekij]

Alternatively (and dependant on you having enough lanterns and circuits) use a reversed jail bars gobo (where the bars are made of light not shadow)

I had thought of this, however I assume the only way to suspend the blacked out spaces between the cut out bars is to make the gobo out of glass.

The gobo I have is aluminium bars with air spaces.

Unfortunately as the show only runs last night, tonight and tomorrow night there is no time to buy what you suggest and it's beyond my abilities to manufacture it myself.

 

I'm also short of channels on the dimmer and desk (but then aren't we always :P )

[andy_s]

however I assume the only way to suspend the blacked out spaces between the cut out bars is to make the gobo out of glass.

 

 

I'm also short of channels on the dimmer and desk (but then aren't we always :P )

 

No - I was thinking of a negative or reverse gobo - I don't know if one is commercially available without looking at the catalogues, but there are certainly positive and negative leaves gobos, and the same principle would apply although you might have to make it yourself. The positive version is narrow strips of aluminium making shadows representing the prison bars with wide spaces representing the gaps between the prison bars, and the negative would be wide strips of aluminium making shadow representing the space between the prison bars, and the narrow gaps representing the prison bars themselves with light. With a simple shape like this it's easy, with more complex designs, you would have "tagging" issues, and a glass gobo would be the best solution.

[Ekij]

The gobo I have has vertical and horozontal bars.

While a vertical only could be done in negative the vertical and horozontal could not (without a transparent support medium such as glass).

I'll try the donut tonight and if it doesn't help enough I might try the negative gobo (vertical bars only) tomorrow night.

[Ekij]

The lantern is an acclaim profile and I had difficulty getting the gobo focused in sharp without the gel in the first place, it appears that the lantern hasn't been cleaned since it was installed and it's very rough to adjust.

I added the donut tonight and dropped from L132 (Medium blue 8% transmission) to L165 (daylight blue 20% transmission). I had a quick look before the show when some of the house lights were on and I thought it would be OK but there was too much drop in intensity and; while sharper; the gobo wasn't visible enough (but it was better than last night).

Unfortunately the only lighter blue I have is L061 (Mist Blue 62% transmission) which is not saturated enough for the desired effect.

I think I'll try doubling up on L061 for the final night tomorrow.