Fresnel condensor?

[svlindbe]

Condensors for LF enlargers are big, heavy and expensive. Fresnel lenses are used for many applications to cut size, weight and costs. I have never seen Fresnel condensors in enlargers, for good reasons, I presume.

What are those reasons?

Svein Lindberg

[Tiziano]

When I use a fresnel lens in studio, I notice that the light circle does not have a uniform distribution.

[Jim Michael]

Also at some point in enlarger evolution cold lights with diffuse illumination became popular for B&W and mixing chambers for color, making condenser technology somewhat obsolete.

[Nathan Potter]

Heat is not friendly to plastic Fresnels - depending on the lamp type of course. As pointed out they are superfluous for uniform cold heads. For condenser enlargers I think they could be useful in a back to back configuration as long as the optical arrangement is such that complete defocus of the Fresnel occurs at the image plane. Heat absorbing glass may be required between light source and Fresnel. Im not familiar with any published designs.

Nate Potter, Austin TX.

[Jason Greenberg Motamedi]

I have two big studio spotlights which have been adapted to strobe use--a Colortran Fresnel and a Strand profile (a focusing double condenser). Comparing them it is clear that the profile produces much a more evenly dispersed light. The advantage of the Fresnel is that it is half the size and weight of the profile. So, it makes sense that huge studio lights (or light houses) have Fresnel lenses, since precision is less important than size and weight. On the other hand, precision and even light is much more important with an enlarger, and size is of secondary concern.

[Drew Wiley]

Conventional focussing fresnels are indeed hard to make practical for conventional enlarger usage. However, there are specialized types called linear array fresnels which are very using for achieving even illumination in mixing boxes.

[Maris Rusis]

An approach I have used in the past is to make condenser lenses out of Fresnels.

Big glass condensers are way too difficult to fabricate but I figured any lens that converges light should do, even a Fresnel lens. The "whole page magnifiers" I find at my local bargain store for $2 can be cut with scissors to fit an enlarger head. My last lamp-house conversion (from 4x5 to 6cmx7cm) needed a stack of three Fresnels. The optical quality is terrible but, hey, these are only condensers and a sheet of frosted glass smooths out all optical sins before the light gets to the negative. So far, so good. Nothing has melted or caught fire.