Can dichroic filters be used for additive color process experiments on B&W film?

[medform-norm]

Has anyone ever actually tried (and not just discussed) using dichroic filters (of the additive type) for in-camera color separation on B&W film?

I've nosed around in the archives a bit, but haven't come up with anything conclusive, other than that people have pondered their options.

Every now and again this idea of re-creating the additive color process as used in the beginning of the 20th century pops up. The Bermpohl camera used in-camera glass filters, but we can't afford one of those. Now we're pondering if it's possible to build a three-color filter sliding mechanism placed in front of the lens. We know dichroic filters could be an option, as they are precise in separating specific wavelengths.

Would it possible to use the dichroic filters that can be found in color enlarger heads? How big are these filters on average? Do they come in a glass version or are they all gelatin? Would it be easy to find these second hand?

And what would be the better film to use: ortho or pan films? Opinions seem divided on this issue.

As you see, we know next to nothing about this yet! So please pour out your infos in our receptive mental containers.

TIA, Norm

 

[Lee L]

Astrophotographers do color "separations" with B&W CCD cameras routinely, and often stack the three RGB exposures with an L or luminosity exposure. See: http://sciencecenter.net/hutech/idas/type4.htm for one set of filters used for this. I have no clue what your budget is, but there's a link from that page to a price list with standard camera filter sizes. You can also search for color filter wheels and slides that are used for astrophotography, often with threaded holes for inserting your choice of filters. The slides usually accomodate larger filters than the wheels.

Most color enlargers use subtractive filters. The Phillips PCS-150 used additive filters, but these were too small for your purposes, and constructed to filter light before a mixing box, not for use in the image path, so they may not be the quality needed for optical path work even if large enough.

Why not use a standard Wratten tricolor separation set in either glass or gelatin?

Hope this helps.

Lee

 

[Photo Engineer]

Dichroic filters are CMY, but in-camera separations must be made with RGB filters so that you are looking at subtractive vs additive color systems.

No comparison.

You must use a Wratten 98, 99 and 70 with a pan film to get good in-camera color separations. There is a broader set of filters which could be used which give one more stop in speed at the expense of color purity.

PE

 

[medform-norm]

Dichroic filters are CMY, but in-camera separations must be made with RGB filters so that you are looking at subtractive vs additive color systems.

No comparison.

You must use a Wratten 98, 99 and 70 with a pan film to get good in-camera color separations. There is a broader set of filters which could be used which give one more stop in speed at the expense of color purity.

PE

Lee, PE, thanks for the replies.

Lee, I will look into your tips and do a 'guided-by-lee' google. I may add that I'm looking for large filters to use on large LF lenses. Most filters sold are only 50mm sq. -and filter wheels seem to be made for small mounts.

PE, my sorry excuse for research on dichroic filters led me to believe that there are dichroic filters both for additive and substractive separation. RGB filters was what I was looking for.

Have you used the Wratten set you describe? The one Lee suggests (tri-color separation) are numbers 47, 58 and 25. Is your suggestion an improvement on this based on experience?

 

[Lee L]

Passband graphs for some (but not all) of these filters can be found here:

http://www.geocities.com/thombell/curves.html

You can see some of the differences there. The tricolor set you mentioned (47, 58, 25) has overlap between filters, leading to the color impurities PE mentions. His recommended set is more mutually exclusive and therefore gives purer colors.

Lee

 

[PhotoPete]

I am about to undertake a similar exploration. I am glad I saw this post, as I was about to head down the same CMY path as Norm. If I understand correctly, three frames are exposed, each through an R, G or B filter. They are then each projected through the same color filter as they were shot. Have I got that right?

 

[Photo Engineer]

OTOMH, the only one I can think of is using the 29 for the 70. It is broader in bandwidth, gives about 1 more stop in camera speed, but a corresponding decrease in color purity

PE

 

[Helen B]

A bit of overlap is better than a gap when doing separations from the real world - and there may be no need for such precision as wide band pass interference filters would afford.

There's a fundamental difference between making separations from a full-spectrum subject, like the real world (when you find it, let me know), and doing something with (making separations from, printing, assessing) an image made of three dyes. That can be done with filters that match, or attempt to match, the response of whatever happens next (eg the RGB interference filters used for Status M densitometry) - and there doesn't have to be full coverage of the spectrum.

I'd also suggest starting with polyester filters (from Lee, by a strange coincidence) in the standard tricolour set (25, 58, 47B) rather than dichroic/interference filters. Besides, interference filters become a little pointless at wide angles of incidence.

eg RGB ie Lee

Best,
Helen

 

[Kirk Keyes]

Dichroic filters are CMY

I'm sure PE meant to say "Dichroic filters in enlargers are CMY", as you can get dichroics for most any band pass you wish. Here's the link to Edmund Optics page with both additive and subtractive dichroic filters. Dead Link Removed

But they can be much more expensive than gel or polyester filters like the Wratten or Lee.

Kirk - www.keyesphoto.com

 

[Struan Gray]

Norm, have you seen Henri Gaud's article at Galerie Photo?

http://www.galerie-photo.com/test-trichromie.html

He has an update on his latest experiments using MF in the discussion forum:

http://www.galerie-photo.info/forum/read.php?f=1&i=1186&t=1186.


This might be a use for those sets of four radionar lenses on one board and accompanying septum/baffle that Sinar used to sell for passport photos. Or one of the Cambo passport cameras. Both sell for little money these days. You'll have to worry about parallax, but not movement-induced colour fringes. R,G,B and a luminance 6x7-ish images on a single sheet of 4x5.

 

[Lee L]

I'd also suggest starting with polyester filters (from Lee, by a strange coincidence) in the standard tricolour set (25, 58, 47B) rather than dichroic/interference filters. Besides, interference filters become a little pointless at wide angles of incidence.

eg RGB ie Lee

Best,
Helen

The Hutech IDAS astrophoto filters I linked to are interference filters, and are typically used for very narrow fields of view; less than a couple of degrees. The distance between the interference films changes as light travels more obliquely through the filter, as when using a wider angle lens. So the characteristic passband, and therefore the color balance, shifts between center and perimeter of the field of view as the field of view widens. I can't tell you what the practical limits are for an acceptable angle of view through these filters, but if you place them right in front of your eye and look at a wide field, the shift from center to edge is readily visible. I can't run a practical test either because the set I use in an observatory is 1.25 inches in diameter for telescope use, and too small for any of my lenses.

Disclaimer: I have no association whatsoever with Lee filters, but if they want to cut me a check occasionally, I'd use their filters.

Lee

 

[medform-norm]

Norm, have you seen Henri Gaud's article at Galerie Photo?

Struan,
I did read his article a month ago and his promising results is what kept me on the road. But I hadn't seen the new stuff he did. Wow! They are looking better and better. Perhaps I should contact him in my best schoolbook French, to find out more.

So tell me, are you also on the verge of doing additive color processes, that you are keeping an eye on him? It's good to hear that there is a recurring interest in the technique, like from PhotoPete. Maybe it's time we all did a little workshop in France, over the summerholidays - doing still lives with wine and fruits and bread and flowers to get that color rendition going. Is that enough real world for you, Helen?

I'll have to experiment with all the color filter combinations mentioned. Good, will give me something to do instead of twiddling my thumbs and squandering my time here or on eBay

 

[Struan Gray]

My winter project is to finally get my 12x15 camera into working order. So far this has meant buying a bunch of lenses on eBay

I am generally interested in 'primitive' colour processes, partly to use on the 12x15 beast once it's working, partly because I can't see mainstream manufacturers making ULF sheet film at prices I can afford, but mostly because I simply find them interesting from a technical and aesthetic standpoint. The mixture of muted colour and detailed B+W is something that I find naturally appealing, and which I think is appropriate for the sorts of image I see myself making with ULF.

My wife's great uncle was one of the revivers of fine art book making between the wars. He made the subscribers' edition of T.E. Lawrence's "Seven Pillars of Wisdom." He also lost lots of money trying to make and sell four colour gum prints. That gives me pause for thought.

 

[srs5694]

Dichroic filters are CMY

I'm sure PE meant to say "Dichroic filters in enlargers are CMY", as you can get dichroics for most any band pass you wish.

Even the statement that dichroic filters in enlargers are CMY is a bit too strong. I've got a Philips Tri-One PCS130 with a PCS150 color controller. This is an additive color enlarger with red, green, and blue light sources. According to the manual, it uses dichroic filters:

The Tri-One system uses permanent, narrow-band, dichroic filters with unique color characteristics.

That said, Tri-One enlargers are rather odd beasts; the vast majority of color enlargers do use CMY filters.

 

[Photo Engineer]

Kirk is right, of course. You can get dichroic filters with any bandpass but the common ones for enlargers are CMY and you need RGB for in-camera use. The OP did specify additive, so I should have been more explicit.

Helen has a point also, but I hesitate to recommend narrow vs wide RGB additive filters. They both have merit in making original separations from the real world.

Narrow pass filters tend to give more vivid color with more separation, while broad ones tend to give more pastel rendition with higher camera speed. You would have to experiment to determine which suited your process better. This very fact here was an oft repeated argument back about 75 years ago in the early years of color photography with tri-color cameras, IIRC.

PE