fresnel lens
 

1 is it possible to convert an open type 800w redhead by fitting a fresnel lens instead of the glass plate?

2 what are the pitfalls if any?

3 where can they be got - suppliers etc?

4 what sizes focal lengths etc?

The focus of any fresnel from soft to hard (flood to spot) depends on the distance from the lens to the lamp. There is an optimum range of distance for every fesnel lens over which one can adjust from spot to flood.

The practicality of what you suggest would depend on being able to adjust the distance of the redhead's lamp from the lens (or, alternatively, the lens from the lamp) over the correct range of distance for the lens you'd be using. If there is no way to adjust easily, I'm not sure it's worth doing. Why are you thinking about this?

You can buy fresnel lenses as replacement parts from most lighting instrument manufacturers. The instrument specs provided would give you an idea as to focal length etc.

my heads take p2/13 dxx lamps and the lampholder moves in and out for spot and flood settings - should this movement be within the focal length of the fresnel?

i want to have the fresnel option without purchasing new heads which are very pricey for the fresnel type - also they are suppossed to be more flattering for direct face illumination

I've never seen an open face lamp retrofitted to fresnel, but it may well be possible if you can figure out how to safely secure it to the lamp so that it doesn't drop out when the head is tilted down.

Lighting a face directly with hard light from either an open face or fresnel light is not often done as it is pretty harsh. If you are looking to make folks look pretty, you will want to use some diffusion, at the very least clipped to the barn doors and preferably on a frame in front of the light. Another option is a Chimera on the light, or bouncing the light into a card (these choices are softer still). Fresnels are great for creating hard shadows in the background, they are much easier to shape with flags and gobos than an open face light, plus they have a more controlled, tight light pattern.

Good luck with your project!

Yes, the movement must be within the focal length of the lens for this purpose.

thanks for all the replys

looks like my scheme is not out of the question, ill investigate with some suppliers and report in the future

found a great site here

http://www.schott.com/uk/english/products/lighting/fresnel_lenses.html

they make fresnels in all diameters and focal lengths - fascinating reading

You might want to also take care that the lamp doesn't overheat. Open face inherently have front face for ventilation. Once you close it up with a lens you're going to lose a lot of heat venting.

On a fresnel there is a reflector behind the lamp, both travel together. In a Red head open face the lamp is moved clser to or away from the reflector, the reflector is stationary. if you look at the contruction of any open face, it's quite different from any fresnel I've ever looked at (Arri, Strand,Lowel,Pepper)

Also, as mentioned above, the cooling is totally different.

thanks for the advice on cooling

My heads are fitted with safety glass and there is no cooling issue, if the safety glass is replaced with a fresnel could you explain how this will affect the cooling?

Just make sure you use a Fresnal rated for the heat otherwise the glass will crack. I would also provide more vent holes since you are going to contain the heat.

You might also want to try using a combination of lenses a front and a back and not mess with the lamp adjustment. I have seen a few slide in assemblys for par lights that do this though I cant remember the link.