Durst 138s - diffusion head

[glerin]

Hi,

has anyone tried this kind of modification on a Durst 138?
It's from this link: https://glennview.com/durst.htm
253745

I would like to use it with a LED light source (COB 50W). With classic condensers, I struggle a bit with dust, mainly the combination of 5x7 negative and Nega 138 with 2 glasses.

Or do you have another idea? I'm used to a color head on a smaller enlarger, unfortunately it's quite expensive for a Durst :/

Thanks!

[Drew Wiley]

it's fairly easy to convert the Durst 138 to any number of diffusion designs. To experiment, you could simply slip translucent white acrylic sheets into the condenser positions.

[Tin Can]

I was given an 8x10 feet oversize moldable

opaque signage NOS

as huge sample

I cut some for members long ago

Keeping the rest

Same as SHELL gasoline big signs


I use layers of good paper to adjust and top with glass

[glerin]

it's fairly easy to convert the Durst 138 to any number of diffusion designs. To experiment, you could simply translucent white acrylic sheets into the condenser positions.

So is it really enough just to diffuse the light in some way?

I wonder why the manufacturers of enlargers have always gone the "complicated" route and not made something like this themselves or offered it as an alternative? Why didn't they just diffuse the light instead of a condenser (or multiple condensers)? Where was the problem? In light sources? It didn't work with a classic light bulb?

[Kino]

So is it really enough just to diffuse the light in some way?

I wonder why the manufacturers of enlargers have always gone the "complicated" route and not made something like this themselves or offered it as an alternative? Why didn't they just diffuse the light instead of a condenser (or multiple condensers)? Where was the problem? In light sources? It didn't work with a classic light bulb?

Count me interested as well; I would like to know this myself!

[malexand]

I converted my Durst to diffusion / LED several years ago. Took a little fiddling with light to diffuser to neg distances to get it even, but very doable and well worth the effort. I used strip LEDs and a sheet of white translucent plexiglass for diffusion, and foamcore for the ‘mixing box’.

[Drew Wiley]

The enlarger manufacturers didn't go the complicated route. Condenser heads were largely supplanted by efficient colorheads, which are diffuse. And black and white only applications were covered by cold lights, likewise diffuse. Now you could add to that list LED array pancake-style tops.

Regarding the acrylic diffuser plastic itself, go with thin "Sign White" Acrylite or Plexiglas instead of regular white; it has more transmission. Sometimes two layers of it, spaced apart a little, will provide more even illumination than a single sheet. But that all depends on the efficiency of your mirror box above, or white box (preferably oversized), along with the angle of incidence of your lens in relation to that light output. Sometime diffusers need to be ground thinner toward the corners than at the center in order to get perfect illumination; but that can be quite a chore. There are certain other tricks too that I won't explain in detail here.

[Torleif]

I am in the process of, or rather trying to, converting my L138 to a diffusor. The solution so far is based on the ´glenview´style using 5mm foam core as reflective material. I have a sheet of opal type of glass in the filter drawer (the vertical drawer downstream the heat shield) and a white sheet of acrylic above the film holder. The light source is a 200W equvalent large LED bulb (Osram Parathom, 24W). The Size of the bulb is similar to the original Durst bulb. As a condenser enlarger the light distribution on the base board is quite even. Have managed around 1/2 stop difference center-corner measured with a light meter. However, the diffusion version has a more pronounced difference, more like 1,5 stop center to corner - at least so far.

[Drew Wiley]

Evenness of illumination on the baseboard is also related to the amount of falloff due to the enlarging lens itself. You should check it at the typical working aperture of the lens, at least one stop down, preferably two, and not wide open. A stop and a half difference center to corners is pretty bad. But how are you metering this? With a spot meter from above in reference to a consistent light gray surface, or by some kind of incident meter sitting on the baseboard itself (which might be subject to cosine errors due to oblique light off-center).

This might improve by using a longer than "normal" focal length lens, along with some kind of double diffusion technique. But it might be necessary to grind a custom diffuser which is thicker at the center than the corners and edges.

An effective diffusion chamber up above typically needs to be quite a bit bigger than the area of negative it's meant to cover. That's hard to do in the relatively cramped original condenser housing of a 138 if you plan on printing full 5X7 inch format.

[Torleif]

Thank you for the feedback. I use working aperture on my 150mm Nikon. However, an incident meter on the baseboard was my approach. I have checked today with a spot meter - the difference is > 2 stop. Completely useless for my use (medium format and 4x5). I also checked on my LPL 7700. There the difference centre - corner is max 0,3 stop.
Another issue is the absorption of light in both foam core and styrofoam in the diffusion chamber - to high in my book. At least the versions I have used. I could opt for a COB LED, but the tests I have run on the Osram bulb I am currently using shows that the contrast filters give similar results as my LPL enlarger. (Stoufer step wedge test).

[Michael R]

Thank you for the feedback. I use working aperture on my 150mm Nikon. However, an incident meter on the baseboard was my approach. I have checked today with a spot meter - the difference is > 2 stop. Completely useless for my use (medium format and 4x5). I also checked on my LPL 7700. There the difference centre - corner is max 0,3 stop.
Another issue is the absorption of light in both foam core and styrofoam in the diffusion chamber - to high in my book. At least the versions I have used. I could opt for a COB LED, but the tests I have run on the Osram bulb I am currently using shows that the contrast filters give similar results as my LPL enlarger. (Stoufer step wedge test).

With diffusion, any sort of direct lighting setup from a localized source is going to have falloff unless the light source is quite far from the negative stage. This is why diffusion heads typically place the bulb outside the mixing box.

Alternatives are to use arrays of LEDs on a panel above the negative stage, or an oversized mixing box with side-firing LEDs.

Or, along the lines of what Drew mentions above about variable thickness diffusers, you could DYI some contour masking to place on your diffuser. This will require work and trial and error, of course.

[Drew Wiley]

Somewhere up on my storage loft I have an old adapter head for the 138 which looks similar to the one you're experimenting with. It was included in a horse trade along with a bunch of other stuff. It is just too cramped to work decently. Foregoing the whole original condenser housing and substituting some kind of oversized pancake head or colorhead makes more sense. Atop one of my 138 chassis I have 6"X 8" polished aluminum mirror box positioned between the negative stage and the colorhead. It is also double-diffused. The lower diffuser is slight ground center to edge, giving me nearly perfectly even illumination (within 1/10th stop) over even 5x7 film, and essentially perfect with 4x5. But even that is still relative to certain focal length lenses.

These kinds of projects can take some patience to perfect. Fomecore or styrofoam liner material is inexpensive and easy to experiment with; but don't expect a hole in one.

Per Michael's tip - I once tried making a printed variable density mask on polyester sheeting. In principle it would have worked, except for the fact that I needed it in relation to color printing, and it's difficult to find a printer ink which is truly neutral density, rather than some kind of tainted off-black.

What I have successfully done instead, in one instance, is to order some very fine stainless steel mesh, and trim and overlap it in such a manner that it constituted a variable scrim, allowing more light in at the corners than the central main portion. This should be positioned reasonably above any lower diffuser, an inch or so away, so that the overall effect will be well blended and invisible on the negative focus plane.

The nuclear option would be to thread onto the lens an appropriate graduated ND center filter. Those can easily handle 1-1/2 stops of difference. But there are two problems : First, you'd need have the lens stopped down to at least f/16 to get the full effect; Second, those filters are expensive.