List of Questions

[icewolf08]

Back to donuts for a second. Here are some crude images.

Without a donut light from anywhere on the lens can and does hit anywhere on stage. So you have stray light that doesn't hit where it is "supposed" to. This is what yields unsharp images.

With a donut you cut down on the extraneous light and only allow the light to go where it is supposed to go. Consider that any point on an object emits light in all directions, but to get a sharp image you only want the light ray that comes from that point, through any optics that hits the same point in the image on the stage. The ray that comes out at a slightly different angle will focus through the optics differently and not hit the same point on the image on stage, thus making it not as sharp.

 

[JChenault]

The ray that comes out at a slightly different angle will focus through the optics differently and not hit the same point on the image on stage, thus making it not as sharp.

So by that logic there is nothing magic about having the donut be in the center of the lens. IE if my donut has a three inch diameter opening, it can be in the center, or on the side.

I think I need to make some off center donuts as an experiment.

 

[Van]

Alex ! Nice demo !

 

[icewolf08]

So by that logic there is nothing magic about having the donut be in the center of the lens. IE if my donut has a three inch diameter opening, it can be in the center, or on the side.

I think I need to make some off center donuts as an experiment.

I believe that it actually has to be in the center of the lens due to the way lenses work. Theoretically the light that you want should be focused towards the center since the lens is going to refract all the light towards center.

 

[church]

the donut needs to be approximately centred.

The donut works differently to an iris. The iris and for that matter a gobo are placed at a focal point which is why you can focus the image or get a hard edge on the iris. The donut is placed after the optics and is not at a focal point so while it can remove stray light you cannot focus the light to achieve a hard edge of the donut.

If GOBO means Goes Before the Optics then the Donut is really a GOAO

LOL

 

[icewolf08]

the donut needs to be approximately centred.

The donut works differently to an iris. The iris and for that matter a gobo are placed at a focal point which is why you can focus the image or get a hard edge on the iris. The donut is placed after the optics and is not at a focal point so while it can remove stray light you cannot focus the light to achieve a hard edge of the donut.

If GOBO means Goes Before the Optics then the Donut is really a GOAO

LOL

Just for the record, we were comparing Donuts to the iris in a photographic lens in how it affects depth of field and sharpness of image.

 

[Kelite]

In projecting super small text within a black and white glass gobo, a particul user was having halo issues around the edges and throughout the text. This isn't unexpected, as extraneous light likes to play too. I cut a window that was the same shape and 10% larger in size than the text within a piece of black foil. This customized donut was placed within the color filter slots of the ERS. Granted, the amount of light escaping the fixture to illuminate the projection surface was minimized by 20-30% but the small text was absolutely beautiful.

The diagram was indeed very helpful Icewolf, thank you for sharing-