Diffusing a condenser - help me understand what's going on...

[M Carter]

Troublesome neg I've been fighting with, realized a dodge mask would be just the thing. However, my enlarger's a 67C condenser.

So, I ended up with the configuration below... good heavens, a dodge mask is pretty magical, just the thing, and even on 6x7 I can make very smooth gradients with a pencil and some rubbing/erasing.

But... I'm getting some extreme falloff - some corners are going almost pure white.

I didn't have any opal glass, but plenty of frosted dura-lar mylar, same stuff that's sold in masking kits. So my "stack" uses an extra sheet of mylar to get enough separation/softness to make the mask seamless.

With my sekonic meter, I tested the 6x7 area (empty neg holder in the enlarger) and found I have about a half-stop of falloff (Nikkor 5.6 85mm lens) when setup as the stock condenser enlarger. When I do my custom-glass-negative mess, it's bizarre... bottom edge and top left corner go almost white, like a shadow is being cast??

I added a sheet of diffusion under the bottom condenser (tried it in several places, filter drawer and under the heat absorber). Same results.

Added a mask to the top of the stack as well... still getting this extreme falloff. Any ideas as to what's going on? See the diagram - I'm not using the negative carrier at all, the glass is the same size as the full 6x7 carrier and masked to 6x7 with black heavy paper.

 

[ic-racer]

bottom edge and top left corner go almost white, like a shadow is being cast??

How are you centering the negative to the light source and lens?

 

[M Carter]

How are you centering the negative to the light source and lens?

I placed the neg carrier in the enlarger and traced the position on the baseboard, and made sure my 6x7 masks hit the same spot.

The glass I cut is the same size as the carrier, and the masks are taped in place - so even though the glass doesn't have the slots that align with the pins in the negative stage, it's within a few millimeters of the correct position.

I've already modeled a glass-holding carrier for 3D printing (the beseler 67 series doesn't have a glass carrier available) but I want to figure out what's causing this falloff before I commit.

I was wondering if the edges of the glass are scattering light, but seems that would give me problems with dark areas, not falloff? Or the fact that my neg carrier is essentially a big sheet of glass with negs and masks just sitting on it? If I have a carrier printed with a frame for the glass at just slightly bigger than neg size, that would address that by enclosing the glass in black. If the "well" for the glass is the same thickness of the glass, I'd still have plenty of margins for taping masks that could be hinged up and down. The neg itself would be taped to the bottom of this carrier idea.

Also, the fact I'm using clear glass vs. frosted or opal? But the glass is sandwiched top and bottom with mylar, so it's essentially frosted glass.

 

[L Gebhardt]

I tried all sorts of things to diffuse my condenser enlarger, to no avail. I couldn't avoid getting really strong falloff. I found thicker diffusion plexiglass worked better than the thinner stuff. I imagine you would get better results with a diffusion product like Makrolon LD compared to frosted mylar.

Even with thick plexiglass I needed to make a mask above the diffusion material to even out the light. I printed a gradient on transparency material and it worked ok. However the plexiglass and mask combined to make the exposure times so long it wasn't practical to print even medium sized prints. My solution was to build an LED head for my Durst enlarger since a diffusion head was hard to find and expensive. You might be best served buying a new head for your enlarger.

 

[ParkerSmithPhoto]

I tried all sorts of things to diffuse my condenser enlarger, to no avail. I couldn't avoid getting really strong falloff. I found thicker diffusion plexiglass worked better than the thinner stuff. I imagine you would get better results with a diffusion product like Makrolon LD compared to frosted mylar.

Even with thick plexiglass I needed to make a mask above the diffusion material to even out the light. I printed a gradient on transparency material and it worked ok. However the plexiglass and mask combined to make the exposure times so long it wasn't practical to print even medium sized prints. My solution was to build an LED head for my Durst enlarger since a diffusion head was hard to find and expensive. You might be best served buying a new head for your enlarger.

Larry,

I would be very interested to see some of your designs in detail. I am having the same problems as the OP, only with the 4x5 head. I simply cannot get an even light through my condensers, no matter how much diffusion I add in however many places. It's truly maddening. This huge head will not evenly cover a 6x6 neg!!

P

 

[L Gebhardt]

Larry,

I would be very interested to see some of your designs in detail. I am having the same problems as the OP, only with the 4x5 head. I simply cannot get an even light through my condensers, no matter how much diffusion I add in however many places. It's truly maddening. This huge head will not evenly cover a 6x6 neg!!

P

Here's what I built: http://www.trippingthroughthedark.com/category/equipment/led-head/. There's also a thread on the large format forum that was started to discuss building one. It's kind of stalled, but I've been doing some more research into improving the design. I should be posting a few ideas in the next week or two once I've had a chance to test things out.

I know there is (or was) at least one person selling a pretty neat looking setup. It uses a recent (needs Blue Tooth 4) iPhone or iPad to control the head. The site is http://www.modernenlargerlamps.com, but it seems to be down at the moment. I hope he hasn't folded shop.

 

[L Gebhardt]

Also, if you are ok using contrast filters, either under the lens or above the negative, you can build a much simpler version with some white LEDs and a simple plug in constant current power supply. Should run around $100 in parts and be very bright. Again it's a do it yourself project, so some comfort with basic electrical wiring and metal work would be helpful.

 

[Thomas Bertilsson]

Anybody tried milk glass?

 

[M Carter]

I should probably do a test with diffusion in the enlarger but just with a standard 6x7 neg in the regular holder, see if that eliminates things - some of these problems could be caused by that stack of stuff on the neg stage.

As for the masking stage - I really like using a diffusion sheet between the neg and the glass (vs. just using ground glass, milk glass, etc), with the dodge mask on top of the glass. Since the diffusion under the glass doesn't need to be registered, I can slide it out and get a much clearer view of the neg to make my mask, and then return the diffusion to soften the mask.

Beseler did make a diffusion head for the 67 series, I'm almost positive. May be something to keep an eye out for on the used market. Not psyched to go enlarger shopping for a while, if I do it'll be to get a 4x5.

Still, it just seems odd to me that I get 1/2 stop or less falloff with condensers, but diffusing the condensed output goes haywire. Doesn't make sense physics-wise to me...

 

[L Gebhardt]

Anybody tried milk glass?

Yep, about the same results as 1/8 diffusion plexiglass. So more even than 1/16" plexiglass, but not even enough.

For a point of reference my Durst 138S with 5x7 negative carrier, 240mm top and 240mm bottom condensers and a 180mm lens has .35 stops of falloff from center to corner at f/8. When I use milk glass or 1/8" diffusion plexiglass it was a full stop of falloff. On top of that the brightness was a full 5 stops less bright on the base board. The thinner 1/16" plexiglass diffuser is about half a stop brighter, but slightly more uneven center to corners.

I imagine on a smaller enlarger the falloff won't be as bad simply because the diffusion material seems to have a certain amount (distance) of spread. So putting a sheet above a 35mm negative may be fairly even, though it will still rob you of a lot of light.

Diffusion enlargers usually have very powerful lamps, since they are so inefficient, compared to condenser versions.

 

[MartinP]

To make a quick and easy diffused light-source, have a quick google for homebuilt 5x7" enlarger light-source at the largeformat photography forum. Basically there is a brand of high-power and high-evenness light box which does the job directly, no messing about. It costs a few dollars more but takes a few months less to produce a working and consistent system. A paper mask on the 'bright' surface will reduce the size of the light box surface to suit your negatives and masks.

Edit: A link is here.

 

[L Gebhardt]

Still, it just seems odd to me that I get 1/2 stop or less falloff with condensers, but diffusing the condensed output goes haywire. Doesn't make sense physics-wise to me...

Intuitively I agree with you, but experience says we are missing something. My guess is it has to do with the condensers focusing the light at the aperture of the lens and not at the negative stage. Maybe it's the angle of the light as it strikes the diffuser (no that doesn't sound right to me either, but it's all I can think of).

If you go for a 4x5 I can recommend trying to find one with an Ilford 500 head. I printed with one for many years before buying a 5x7 camera and enlarger.

 

[MattKing]

The Beseler Dichro colour heads for the 67 series enlargers will give you nice, diffused light, in whatever colour you desire. They are great for variable contrast black and white printing as well.

There have been a few different versions, and some of them require a matching power supply, so you need to be careful if you are buying used.

There are also more recent variable contrast heads, designed strictly for black and white printing. They too are diffused. I haven't seen much in the way of selection for them on the used market.

It doesn't surprise me that you are having problems with coverage. A condenser enlarger focuses and directs the light. If you add diffusion, it throws that focus and direction off.

 

[wiltw]

Have you considered that Physics might be at work?

 

[M Carter]

The Beseler Dichro colour heads for the 67 series enlargers will give you nice, diffused light, in whatever colour you desire. They are great for variable contrast black and white printing as well.

There have been a few different versions, and some of them require a matching power supply, so you need to be careful if you are buying used.

There are also more recent variable contrast heads, designed strictly for black and white printing. They too are diffused. I haven't seen much in the way of selection for them on the used market.

It doesn't surprise me that you are having problems with coverage. A condenser enlarger focuses and directs the light. If you add diffusion, it throws that focus and direction off.

I'd love to get one of the VC heads - they're a great concept, never seen one used - they're $400 new and still available. The Dichro heads show up more often, and I'd guess the modern all-in-one is probably the one to look for?

And yes, this issue all has to do with how the photons are being directed around. Wonder what happens if you just pull all the condensers and diffuse the heck out of it? I've done so many shoots where you point a fresnel, par, or even open-faced light at a diffusion panel and get very soft, even light out the other side. The physics here are a mystery...

 

[ParkerSmithPhoto]

Wonder what happens if you just pull all the condensers and diffuse the heck out of it? I've done so many shoots where you point a fresnel, par, or even open-faced light at a diffusion panel and get very soft, even light out the other side. The physics here are a mystery...

Well, since I've been experimenting with the same idea for the better part of two weekends, I can help you with that.

And the answer is, basically nothing, you still get uneven light unless the light is directly on top of the diffusion, which is directly above the negative. I bought a four inch LED light with a flat top and used that with the condensers, without them, with and without diffusing material in every possible combination and location, and the only thing that begins to work is if you have negative, then diffusion, then the already relatively soft light source, all within the space of about an inch.

Now that four inch LED will not even cover a 6x6 neg, for reasons that become obvious if you lay a neg across the top (it needs to have a diagonal much larger than the neg), but you can move the light and measure and see that, given a large enough light source, you can get your corners down to as little as -0.1 stop.

So for that, you need something exactly like... wait for it... a cold light. Which I've owned in the past and which drove me nuts due to the variability of the output.

I feel like I need to draw a circle on the wall and pound my head against it. Scanning and printing has a lot of advantages, but if you leave that ink jet printer sitting for more than a few days you'll find it clogged, and add to that the ink and paper costs are as much as if not more that silver printing.

I really made a mistake selling my LPL. It was the most friendly, trouble-free enlarger I've ever owned. Like a Mac, it just worked, elegantly; this condenser head is like trying to run Windows 95. Ugh.