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Dichroic Filters for Omega F

JackRosa

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Nov 23, 2004
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447
Location
Oklahoma, US
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Does anyone know where I can purchase yellow and magenta dichroic filters for an Omega Super Chromega F head (the 10x10)?

I am getting some strange contrast results with both, the yellow and the magenta filtration (*). I have checked that the mechanics (levers, cams - that move the dichroic in/out) are working fine. I wouldn't think it is due to light-color shift from the bulbs; I know I recently replaced the foam liners, ... the only thing I can think of is that the filters have faded. I had the unit refurbished about 10 years ago. I do not use it professionally - hobby/light use but figure 10 years is 10 years.

Possible that the problem is not the filters?

Thanks.

(*) The problem: Magenta: As I increase Magenta filtration from 80M to 90M then to 100M contrast increases 9as it should). THEN ....after 120M, it drops! Yellow: Same phenomenon with the yellow filter .. less ccontrast with 100Y as compared with 80Y (as it should be) ,.... then, I increase to 120Y or 130Y and ... more contrast instead of less contrast! What?

The above problems are not just qualitative/eyeball observation ... I ran some tests by contact printing a Stouffer's 21-step tablet and used the BTZS software to compute contrast range ... sure enough! .. a problem. I turned the safelight off, just to make sure paper fogging was not a factor. I developed the strips at the same time, to make sure developer was not a factor.
 
Yes seems like you have chip or crack in the filters that is letting in white light. If it is just a crack, you might be able to glue it back, as new filters are expensive. Can you post some pictures of the mechanism and filters in full-on and full-off positions? You are sure the White Light mechanism is fully disengaged and not sticking? How many gray bands did you get contact printing the step wedge with maximum Magenta and maximum Yellow?
 
Last edited by a moderator:
Any chance that it is dirt/dust on the filters? Or that the connection to the dials has slipped, making the indicators incorrect?
 
Do the filters move smoothly of do they jump on the adjusting cam? I assume it's a mechanical adjustment, not electronic.

Surplus shed has a bunch of filters but I didn't look for Dichroic.
 
Thanks IC


IC:
- looked closely at the filters ~ no cracks, no chips;
- will post pics of full on/ full off tomorrow but everything looks to be in order - the filters move in/out smoothly;
- 100% positive WL mechanism is disengaged and not sticking
- gray bands: will find the test strips and reply tomorrow.
 
Thanks John

John Koehrer said:
Do the filters move smoothly of do they jump on the adjusting cam? I assume it's a mechanical adjustment, not electronic.

Surplus shed has a bunch of filters but I didn't look for Dichroic.
Click to expand...

Thanks John. I was assured by a fellow who used to refurbish these heads that the dichroic filters do indeed fade with time (?????) Why do you think he would make such claim?
 
Filters Move Smoothly

John Koehrer said:
Do the filters move smoothly of do they jump on the adjusting cam? I assume it's a mechanical adjustment, not electronic.

Surplus shed has a bunch of filters but I didn't look for Dichroic.
Click to expand...

The filters move in/out smoothly / no jumping.
 
Thanks Matt

MattKing said:
Any chance that it is dirt/dust on the filters? Or that the connection to the dials has slipped, making the indicators incorrect?
Click to expand...

1. Checked that the dials (all) have not slipped - they all show the correct filtration settings;

2. Visual inspection: no apparent dirt/dust but cleaned all the dichroic filters thoroughly and will run a test tomorrow.
 
No apparent dust/dirt

JackRosa said:
1. Checked that the dials (all) have not slipped - they all show the correct filtration settings;

2. Visual inspection: no apparent dirt/dust but cleaned all the dichroic filters thoroughly and will run a test tomorrow.
Click to expand...


Intuition tells me it is not dirt/dust as the filter (prior to the thorough cleaning) did not exhibit evidence of heavy dirt/dust.

What blows my mind is that increasing M filtration increases contrast and then decreases with increasing M filtration! ... after a period of almost no contrast no change whatsoever Same with yellow ... decreased contrast as one dials in more Y filtration only to go 180-degrees the opposite way as one increases Y filtration! Strange.

This problem is becoming more perplexing by the hour.
 
JackRosa said:
Thanks John. I was assured by a fellow who used to refurbish these heads that the dichroic filters do indeed fade with time (?????) Why do you think he would make such claim?
Click to expand...

If you look on the website of any manufacturer of dichroic filters, they all state that they do not fade. Why would they make such a claim?

I suspect that you have another problem which has not become apparent to you yet.


Steve.
 
Got it

Steve Smith said:
If you look on the website of any manufacturer of dichroic filters, they all state that they do not fade. Why would they make such a claim?

I suspect that you have another problem which has not become apparent to you yet.


Steve.
Click to expand...

Got it Steve. Will do further digging to get to the the bottom of the problem
 
Steve - dichroic filters do not fade per se, but being composed of vac deposition surface coatings subject to considerable heat, these can
partially spall off over the long haul, so in effect act as if they have faded. Some higher voltage color mural heads needed filter replacement every six months! I once had a unit where the reflectors were made of parabolic ground quartz, because no suitable glass would take the heat. The electric bill for that damn machine and its cooling fan was higher than my whole house and shop combined! I got rid of it and designed a far cooler 8x10 head of my own. Most Omega F heads are pretty old by now, so the issue could be simply mechanical, needing a slight tune-up, or dirty filters (which can be gently cleaned like a lens), or actual partial failure of the coatings, or discoloration of the diffusion chamber. Replacement filters brand-specific might or might not be available; but custom ones can always be obtained from someone like Focal Point or Hoya.
 
Replacement dichroic filters for the Omega color heads are currently available from B&H in NYC at $65 each. The Super Chromega Dichroic F head has 4 lamps and Cyan, Yellow, and Magenta filters in front of each lamp for a total of 12 filters.

Even if you only changed the Yellow and Magenta filters you’d be replacing 8 filters @ $65 each for a total of $520.

http://www.bhphotovideo.com/c/searc...&N=0&InitialSearch=yes&sts=ma&Top+Nav-Search=

A much cheaper alternative for variable-contrast B&W printing is to leave the built-in filters at zero and use the excellent Ilford MG Filters below the lens. It’s fairly simple to make a filter holder for these filters to fit the Omega F lens mount (most of these have a filter-holder mounting tab).

The 6” x 6” size should cover any lens you’d be likely to use on the Omega F.

http://www.bhphotovideo.com/c/product/114880-REG/Ilford_1762640_Multigrade_Filter_Set.html
 
Thanks Drew


1. Thank you Drew for sharing this insight with us. My dichroic filters were replaced in 2004 by Focal Point. Visual inspection does not show apparent discoloration, but I would doubt that the naked eye can appreciate such fading.

2. I cleaned the filters last night (with a lens-cleaning kit) and will re-run my tests today or tomorrow (and report findings). I did not"eyeball" too much dust or dirt, but cleaned the dichroic filters gently and thoroughly nonetheless and will re-run the tests.

3. At the suggestion of an APUG member whose technical expertise I respect, I placed a color analyzer under the light and changed Magenta filtration. The meter's needle moves with all the changes in M filtration. I paid special attention to the Magenta range where I detected the crazy contrast change/no change.

4. The Chromega F has four (4) 250-watt bulbs and it does get HOT! The fan sucks up hot air but I know it does get HOT inside.

5. I checked the mechanical pars over and over. The cams, the cam followers, the White Light lever ... everything is working smoothly (mechanically).

6. I replaced the diffusion chamber liners less than a year ago. I inspected it and it looks bright white.
 
How Long?


Drew: How long, in your opinion, will new dichroic filters last before they show signs of "fading" and start affecting contrast?
 
Thanks again Ian

- - - - -

Thanks agin Ian. I am seriously considering you suggestion. Just want to make sure that I understand the issue with the dichroic filters ... I just need to know.

I do have a set of Ilford filters - the exact same ones your url directs one to. THANKS for this info.

I do agree ... the dichroic route is an expensive proposition, especially if I am staring at having to replace them again in just a few months or even couple of years. I am wondering how long they actually last before they show problems.

a) My Omega F is mounted on a FOTAR chassis . . . I do not see a filter-holder mounting tab anywhere.

b) my most-used lens, a 300mm Rodenstock is quite large. How did you make the filter holders to hold the 6x6 filters - not just the filter holders themselves (I can make or buy) but how do you hold the filter holders under the enlarging lens?

Thanks in advance.
 
I think that the filter thread on the 300mm Rodagon is 86mm. One solution is to make a box-like filter holder similar to the holder in the Ilford MG kit for smaller lenses. It can be made of card stock, folded as necessary and glued at any joints you need to make. Obtain an 86mm to 95mm or an 86mm to 105mm step-up ring.

Cut a hole in the top plate of the holder just big enough to admit the 86mm male thread and screw in the ring through the hole in the top plate of the holder and into the filter threads of the lens. The larger diameter of the step-up ring will support the holder under the lens. Make the box deep enough vertically so that it’s easy to insert/remove the filters. The opening in the bottom of the holder allows the filtered light to pass through to the enlarging paper and supports the filter.

If you use some ingenuity, there are many ways to fashion a simple filter holder for under-the-lens filtering and a variety of ways to secure the holder to the lens, lens board or some other convenient mounting location on the enlarger.

You might consider using something like the following altered as necessary to fit your Fotar and scaled to accept whatever filter size you find practical to use:

http://store.khbphotografix.com/Under-lens-Filter-Holder-for-Omega-D5-and-D6-Enlargers.html

The following shows the Omega F lens mount for the 300mm Rodagon. The forked tab hanging down is for mounting a below-the-lens filter holder. Possilby you might make something like this as a mounting point for a filter holder on your Fotar enlarger.

http://www.bhphotovideo.com/c/product/39439-REG/Omega_421181_1_1_4_Cone_Lens_Mount.html
 
I'm of the opinion that using sheets of VC filter material is kinda pain-in-the-butt method of working compared to simply setting a colorhead. And even at zero setting, UV is still going to gradually affect the filters and the definition of "white" light. But actual coating spalling would seem to take quite awhile - maybe several decades - unless the colorhead is not properly cooling. It's important to have more than an internal fan when four bulbs are involved. You need to vent the heat outside with an inline or pull duct fan, because all that heat rises and basically surrounds the colorhead unless you have an exceptionally high ceiling. What is also useful to know is that the intake air circulating in the colorhead might need to be filtered. Darkroom are supposed to be clean; but even in relatively clean ones, little bits of whatever over time can carbonize on the hot glass filters and gradually tarnish them. I recently cleaned a set, and it made a difference.
 
JackRosa said:
How did you make the filter holders to hold the 6x6 filters - not just the filter holders themselves (I can make or buy) but how do you hold the filter holders under the enlarging lens?

Thanks in advance.
Click to expand...
You need a pretty big filter holder for the 300mm Rodenstock. I made this holder for uncut 6x6in Ilford filters for use while I was looking for a dichroic head. After finding a head, I no longer use the filter holder. It so happens there is a bolt on the end of the Durst red filter holder to keep the red filter from falling off. I removed the chrome finger-bolt (and made a little keeper hole for it) and attached my filter holder to the bottom of the stalk with a longer bolt.


 

Thanks again Drew.

1. The color head has a pull duct fan (no internal fan) and sucks the hot air out;
2. I filter the air coming in, although I must admit I've been doing this only for the past couple of years. This head sucked unfiltered air in for 8 years (I had the dichroic filters replaced and the entire head cleaned/refurbished about 10 years ago)

I cleaned the dichroic filters carefully but thoroughly with a lens-cleaning kit. I intend to run additional tests to see if cleaning the filters made a difference.
 
Most Clever


Clever! It gives me ideas! Thanks.

Naturally, my preferred route would be to be able to continue to use my color head and take advantage of the dichroic filtration. I need to get to the bottom of this problem!
 
Thanks (again) Ian

Thanks again Ian. All very useful information. I appreciate it. My Fotar set-up as attachment

 

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UPDATE

Hello to my fellow APUG members and my thanks to all for sharing your insights with me and for providing very valuable advice. Special thanks to IC, who was kind enough to take time to PM me numerous times to guide me through a systematic trouble-shooting process. Thanks to his guidance, and to the advice I received from all of you, I was able to systematically dissect the problem.

I cleaned the dichroic filters and used a color analyzer to assess whether increases in Magenta or Yellow filtration were "seen" by the analyzer. These changes were indeed seen. I also looked at every possible source of light leaks and moved the 21-step tablet to the center of the contact frame. I re-ran the contrast tests at various dichroic filtration settings for both, yellow and magenta.

I am delighted to report that the tests reflect that everything looks OK with the Omega F dichroic filters.

Thanks again to all. You have been most helpful.

Jack
 
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