DIY Diffusion enlarger - focussing screen?

[Dr Croubie]

So one of these days (years) I'm going to build myself an 8x10 enlarger. Lets face it, the chances of stumbling across one locally are fairly slim, and shipping is always a bee arch to get stuff here. Also, I like making stuff out of things that weren't meant to be used in that way (geez, I wouldn't be an engineer would I?)

So the easiest place to start is just by modifying my 8x10 camera, if I do it well enough I can keep using it as a camera, what I'm thinking of is to make an attachment to hold the film in place inside the focussing screen, using the focussing screen as a diffusor, and attaching a light-source to that. Lens at the other end and bam, horizontal enlarger.

The one thing that I'm not sure of is using the focussing screen. And it's the first problem that I have to solve, because, well, I don't really have a real one in my current 8x10 camera. It's more of a "window glass meets orbital sander" kind of screen (well, the whole camera only cost $550 including shipping, so I'm not complaining). Works well enough for composition if it's dark enough under my cloth (jacket), but I'm pretty sure the unevenness will show up if I use it for a diffusor (especially that close to the film).

So, I want to buy a new focussing screen, and my question is, can I use it for both focussing and diffusor, or should they be separate pieces of glass?
I would really love a Steve Hopf Borosilicate with clipped corners and grid markings for focussing. But clipped corners and grids aren't good for a diffusor.
I'm willing to put up with focussing with filled corners and no grid if it means I can save another $200 buying another piece of glass for a diffusor down the track. Even if I have to go to 'regular' glass instead of borosilicate for the diffusor, I can probably live with that.
Or if I can't use any sort of focussing screen as diffusor, ie if I have to buy a special type of something else (like whatever plastic they make LightBoxes out of), then I may as well get my clipped and gridded Borosilicate now and deal with whatever for the diffusor down the track. Anyone have any advice to give?

 

[joh]

I don't believe that a focusing screen is a good diffusor. A cheap and possible better way could be a LEE or Rosco diffusion foil mounted between glas....

 

[Doremus Scudder]

Graphic used to make a light-source back for the Graphic View cameras so it could be used as an enlarger. I believe it had a piece of opal glass as diffusion material. It slid on the Graflok back, so the ground glass was not in place at all. As mentioned above, the ground glass will likely not diffuse nearly well enough for enlarging purposes.

You could likely build a modern equivalent with an LED light panel of the proper size coupled with a sheet or two of white acrylic for diffusing material. You'd have to mate it to the camera back somehow and come up with a negative carrier... neither insurmountable problems.

There was a thread here (Randy Moe?) about building a light source from LED panels. Search for that for some ideas.

Best,

Doremus

 

[L Gebhardt]

You don't want to use the focus screen as it won't be even enough. I've used white plexiglass in my 5x7 head (details at the site in the footer). I've also heard Makrolon LD is even better, but haven't tried it.

My advice is to build a separate back for your camera that has a negative holder (can just be two sheets of glass) and the light box with the diffusion screen. If you build it right you should be able to quickly convert your camera.

 

[DannL.]

I've used my 5x7 to enlarge as a vertically positioned enlarger. I removed the camera back, set the negative on a DIY negative holder, placed a diffuser above that about 1/2", and the lamp-house over that. It worked quite well. The diffuser is a matte piece of plastic stolen from a scanner light source. Anyhoooo, my point is . . . something you might experiment with using. Some time back I was testing the properties of spraying a sheet of glass with matte varnish. I was surprised at the results. Several sheets like that, when stacked, could potentially make a good diffuser. A single sheet alone would probably make a good DIY focusing screen for a view camera.

 

[Dr Croubie]

Thanks guys, that's what I needed to hear. Looks like I'm getting me a real clipped and gridded focussing screen. I'll deal with a separate diffusor later, thanks for all the suggestions on materials.

That also means I won't have to frig around with mounting the film inside the focussing screen and attaching/removing the light panel to the other side.
Instead, I can go to my preferred method, which is something along the lines of measuring my back (Cambo SCX) and drawing a CAD model, mounting the whatever-diffusor, glass or clips to hold the film (probably glass is better so I can also do 9x12, 4x5, 6x17, and 4x10), and the panel of LEDs all in one piece. Then to convert camera to enlarger all I have to do is to take off bail-arm focussing back and clip in led-diffusion enlarging back.
I know a guy who's built his own milling-machine from scratch, he's gonna get a fair bit of work from me one of these days.

Next step is to investigate the LEDs, either to just buy a light-panel for $2-300 from B+H or a whole reel of LEDs from Element14 and make my own PCB (which I do for a living, or I'd like to). With the former the only frigging around is getting it to mate with my back, with the latter I can do funky things like maybe compensate for at least one cos-order falloff in the enlarging lens by changing the brightness in the center vs edges.
Next step is compare cd/mW, cd/mm², and cd/$ of a whole bunch of LEDs, to see what's the best way to make my light panel. Lots more smaller LEDs will of course give more even illumination, but I have to hand solder the bloody things so I do want to keep it under a few hundred pieces...

 

[wildbill]

Randy Moe on the lf forum is using led panels for his enlargers. Do a search over there or look up Artograph LitePads. Much easier and maybe cheaper than building one. Remember, gg corners are clipped for two reasons. Air needs a place to go when closing/opening the camera and to view the lens aperture for vignetting. If you don't get a gridded gg you could just add a thin plexi panel or diffusion on top and make a neg holder from an old film holder which already fits under the ground glass.

 

[Dr Croubie]

Yeah, I use the clipped corners a lot on my 8x10 (whoever sanded the focussing screen at least clipped the corners properly), probably because the lenses I've got are a lot tighter in coverage. On 4x5 I know that I run out of movements before my SWD runs out of coverage, so I never bother with the corners much.

I was also thinking about using a film-holder and just sliding it under the GG, but I'd have to hack up an old one I don't have, it'd work fine sliding 8x10s under the rail but for smaller I'd need glass or something, and if I'm going to build something I may as well get it all milled in one go anyway.

Meanwhile, I just ran some quick numbers. The cheapest and brightest LED at Element14 is this one, a 5mm through-hole. If I assign a 6x6mm space for each, I get a grid of 41x33=1353 pieces covering 8x10 (I'll probably need a few more around the edges for evenness, and could get more in if I spaced them hexagon-style, but it's good for a ballpark).
With all of them, I get 62,373 Candelas of light, consume 86.6W, and just the LEDs cost $392 (and I have to solder and cut 2706 leads). I'll have to get rid of all that heat too, probably a lot of thermal vias and a heatsink clamped to the PCB (I also know people with $30k Pro-Eng Finite Element Analysis software to calculate that, thankfully).
Of course, that's just the most I can fit in, I may not need all that light so I might be able to get something cheaper or use less and space them out more.

 

[Dr Croubie]

And at the other end of the price scale, there's one from Multicomp that only costs 16c and gives out 4.5-13.5cd each, that's still 6000 - 18000 cd in 92W for $216.

Now the question is how many lumens do I need? Starting from an average 100W Halogen (like in my LPL6700), that's somewhere around 1-2000 lumens on a 6x7cm area. 8x10 might be something like 12x that area, which means I'd need 12-20,000 lumens (I presume?).
But the LPL is too bright for me, most of the time I end up adding neutral density at f/16 to get the times up over 3s, I prefer times in the range of 5-20s to allow for dodging, burning, and manual-timing errors. So maybe 5-10,000 lumens for an 8x10 panel is more than enough?
This guy says he bought a 5500 lumen panel and had to turn it down to 30% to get a sensible 15s. Of course, projecting massive chromes onto the side of my house at night would make for a very good party trick too, so maybe I do want all that power...

 

[RalphLambrecht]

I think, You'vr got the rong idea about what a diffusion enlarger is. it has a diffused light source(as in mixing box), not a diffusion screen.