Mystery: What is this box of filters for?

pdeeh · Aug 25, 2015

Here we have something that I turned up from a long-dead photographer's darkroom and I've been meaning for ages to ask about it.

It is a set of four threaded filters in a fitted velvet-lined box, and very solidly made too boot.

The four filters are marked, respectively, "Alpha", "Beta", "Gamma" and "K2".

The threads seem to be a touch over 46mm O/D - they won't screw onto any of my 46mm stuff. No idea what the tpi is as I don't have a gauge nor any intention to buy one.

There isn't a single mark on any of them or on the box to indicate a manufacturer, but my guess (given the other stuff I got as part of the same lot) is that they will be early-mid 20th Century and made in the UK

"K2" is rather obvious, but why "Alpha", "Beta", "Gamma" ?

Are these part of an LF set of gear or darkroom gear?

David Allen · Aug 25, 2015

From the depths of my (admittedly cluttered) memory, I have a vague recollection that these were used by an old friend of my father's who was in to Astrophotography. All four were intended to make a specific impact on the absorption of varying wavelengths of Ultraviolet Light.

Bests,

David.
www.dsallen.de

AgX · Aug 25, 2015

Interesting also that at least one is a gelatin filter which should indicate that the absororbtion spectrum is somewhat out of the ordinary.

AgX · Aug 25, 2015

In case of astrophotography filters,

Alpha should be red

Beta should be green

Gamma should be blue

pentaxuser · Aug 25, 2015

If the holders are marked but the filters not and these marks on the filter boxes do not correspond to AgX's colours could it be that they were housed in containers that do not relate to the filters?

pentaxuser

pdeeh · Aug 25, 2015

AgX && pentaxuser - I don't understand what you are saying or why.

These aren't filter holders that take drop-in filters. They are one-piece filters.

None of them are gelatin, they are all glass, and quite thick glass by the look of it.

The case is clearly constructed to take the filters.

AgX · Aug 25, 2015

At least the Gamma one looks as if the locking screw has been fiddled with.

pdeeh said:
These aren't filter holders that take drop-in filters. They are one-piece filters.

None of them are gelatin, they are all glass, and quite thick glass by the look of it.

The case is clearly constructed to take the filters.

-) Nobody spoke of drop in filters. But of exchanging one actual filter by another (by losening a retaining screw and then a retaining ring; at least that seems to be the construction, I never saw such holder(filter-ring) before).
Whether it would make sense to exchange filters/filter-rings remains the question of course... Maybe the used filters were in clip-on rings and screw-on rings were needed. Speculative, I admit.

-) One filter shows edge marks as one would expect from beginning glass seperation in a glass sandwich containing a gelatin foil. Thus my hint at gelatin filter(s).

railwayman3 · Aug 25, 2015

Most modern telescope eyepieces take 30mm screw filters for visual observations and use the Wratten filter designations. There is a less-used larger eyepiece which takes 48mm screw filters....could this be the size of yours? (I also have a vague recollection from somewhere of seeing the Alpha, Beta, Gamma designations being used for astronomical filters, but can't track anything definitive at the moment).

pdeeh · Aug 25, 2015

I spent a fruitless google on astro myself ... I think fruitless mostly because almost every hit is regarding digital astrophotography and these are from tens of decades before digital was even a twinkle.
Definitely not 48mm, as best as my cheapo Screwfix caliper can tell it's just a tad over 46mm (46.3?), so presumably an Imperial size, but my eyesight isn't good enough even with glasses to read the inch engravings, and I can't find my loupe.

Sigh.

I hate hate hate getting older ...

David Allen · Aug 25, 2015

AgX said:
In case of astrophotography filters,

Alpha should be red

Beta should be green

Gamma should be blue

In this instance, absolutely false. Red, Green, Blue are the primary colours and this has nothing to do with the purpose of these filters. Despite how they may look to our eyes, their function is completely related to their specific impact on the absorption of varying wavelengths of Ultraviolet Light.

After all, as I have often posted on this forum, THE most useful filter for landscape photography is a Wratten 12 (Minus Blue) - because it has similar effect to a red filter without the over the top contrast increase and a filter factor of 2 - yet it looks pretty pale yellow to the naked eye.

Bests,

David.
www.dsallen.de

pentaxuser · Aug 25, 2015

pdeeh said:
AgX && pentaxuser - I don't understand what you are saying or why.

These aren't filter holders that take drop-in filters. They are one-piece filters.

None of them are gelatin, they are all glass, and quite thick glass by the look of it.

The case is clearly constructed to take the filters.

I was simply floating a possible explanation based on AgX's statement and the fact that only the filter boxes are marked. It wasn't clear to me why the filter boxes were clearly constructed to take the filters in that I had thought that there may be filters which fit boxes that weren't specifically made for them.

I may be wrong and clearly you think so but I had thought that what I had said and the reasons for saying what I did was clear.

I thought I was brain-storming but you may have thought I had a storm in my brain

pentaxuser

AgX · Aug 25, 2015

David Allen said:
In this instance, absolutely false. Red, Green, Blue are the primary colours and this has nothing to do with the purpose of these filters. Despite how they may look to our eyes, their function is completely related to their specific impact on the absorption of varying wavelengths of Ultraviolet Light.

Not false at all:

There are such Alpha, Beta, Gamma designations in astrophotography and they relate to specific spectral emmissions of the H-Atom. And these are located in the three primary regions. As are the specific (small-band) filters used today.

And it was you who came up with that astrophotography idea...

And except for the far left one they do not look like UV-filters either.

pentaxuser · Aug 25, 2015

pdeeh, I have since had it explained to me that what I was looking at was the filter edges and not a separate clear box which were marked. It looked to me like clear boxes similar to that used to house Hoya filters.

I wish you luck in getting to an answer

pentaxuser

McFortner · Aug 25, 2015

They look like filters to screw into eyepieces for telescopes for visual observing. Measure them and if they are 0.925 or 1.25 inches across, they are for eyepieces.

pdeeh · Aug 25, 2015

pentaxuser said:
pdeeh, I have since had it explained to me that what I was looking at was the filter edges and not a separate clear box which were marked. It looked to me like clear boxes similar to that used to house Hoya filters.

Ha! No wonder I had no idea what you meant

McFortner said:
They look like filters to screw into eyepieces for telescopes for visual observing. Measure them and if they are 0.925 or 1.25 inches across, they are for eyepieces.

Well, as I've already stated, the filter threads have a diameter of a little over 46mm, which is approximately Two and Three-Fifths of an inch, so I think that disposes of that suggestion

paul_c5x4 · Aug 25, 2015

pdeeh said:
Well, as I've already stated, the filter threads have a diameter of a little over 46mm, which is approximately Two and Three-Fifths of an inch

Your conversion is slightly out. 25.4mm per inch, so 46mm works out to be 1.811" or (approx) 1 13/16"

RobC · Aug 25, 2015

have you tested them with a Geiger counter?

jeffreythree · Aug 25, 2015

I don't know what their purpose is, but I bet it is 46.5mm since there are step up rings from that size and you say it is just larger than 46mm.

Maybe they are for microphotography to photograph dyed samples under UV light or something like that.

David Allen · Aug 26, 2015

And except for the far left one they do not look like UV-filters either.

The fact that they do not look like what we photographer's casually refer to as UV filters does not eliminate the idea that they are designed to remove specific spectrums of light in the UV range.

As I mentioned before, a Wratten 12 (Minus Blue) filter both looks visually lighter and has a lower filter factor than a Wratten 15 (Deep Yellow) filter but has a far more dramatic effect on monochrome film.

Bests,

David.
www.dsallen.de

pdeeh · Aug 26, 2015

paul_c5x4 said:
Your conversion is slightly out. 25.4mm per inch, so 46mm works out to be 1.811" or (approx) 1 13/16"

Hence also my use of the word "approximately" !
(I was suggesting the actual size rather than offering a conversion of 46mm )

pdeeh · Sep 7, 2015

I posted this over at LFPF too here and it seems that these are not particularly exotic.

They appear to be fairly straightforward correction filters for ordinary film photography, and Alpha, Beta and Gamma turn out to have been Ilford nomenclature for pale yellow, pale yellow-green and yellow-green respectively.

AgX · Sep 7, 2015

As usually we tend to overcomplicate things...

Mystery: What is this box of filters for?

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