An old Kodak Wratten 18a filter

[JMC]

I was recently sent an old Kodak Wratten 18a filter by a friend. It's un-threaded and is described as 'size 370'. Here's what it looks like.

 

 

 

It's very thick - about 4.5mm, and almost as thick as the ring it is mounted in. Not sure if this thickness was normal for 18a, as I've not seen an old one before.

 

Here's the transmission spectra of it.

 

Sharing it here as I think it is an interesting historic item.

[Stefano]

What is its diameter? 370 could be in tens of millimeters, so 37 mm (just guessing). It's an odd size for sure.

[JMC]

Stefano, that was my assumption too, but the outside diameter is about 42mm.

[dabateman]

Its definitely from the UK and should be really old as in 1930s old

UK used Kodisk notation which is hard for me to track down.

 

Here in the US it would be call a Series VI filter.

[Andy Perrin]

Wow, amazing. Would be interesting to see some pics. Looks like it passes a little visible.

[JMC]

Thanks David. I'm also struggling to find out what the '370' refers to.

 

Andy, will need to think how best to mount it for pics, as I need to combine it with a IR blocker as well.

[Fandyus]

You can't just make this post and show us no samples :)

[dabateman]

Jonathan this is it see pages 86, 91 and 92 of this wratten book. Actually pages 48, 49 of the document.

Kodak-WrattenFilters_text.pdf

[Cadmium]

Jonathan, It looks to be in good shape.

I think it is a series style filter, but I think it might be a slightly different diameter than a series VI.

There are a couple series 370 filter rings on eBay.

Two up top:

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=series+filter+370&_sacat=30040&LH_TitleDesc=0&_odkw=series+camera+filter+370&_osacat=30040

You could put it in a step up ring, but given the depth, you would need to find some other way to hold it in the ring. You could get fancy if you find all the right parts, using extension rings, and maybe a tread reversing ring.

[JMC]

Pictures at some pojnt when the the mounting issue is sorted.

 

David, thanks the book is excellent.

 

Steve, there is a scratch on one surface unfortunately, but it doesn't really show in the photos of it, but other than that it is good. Not sure about buying those original mounting parts yet. I might try and use different setup up and down rings. Need to have a think.

[Andrea B.]

Yes please, a photo made with this Wratten ??

 

It almost looks like the peak could be at 370 nm ??

[JMC]

Easier said than done at the moment Andrea, not many flowers at home.

 

I did get this though today (to keep the forum happy of course). Lit with a single studio flash with a quartz tube. Lens - Ultra-achromatic Takumar 85mm. Shot with a Baader U, and the Wratten 18a + Schott s8612 2mm. I had to up to ISO for the Wratten shot due to the filter letting less light through and the flash being maxed out already. Whitebalanced in Darktable.

 

Firstly, camera phone shot to show the flower in visible light.

 

Baader U

 

Wratten 18a + Schott s8612 2mm.

 

I guess there may be more of a difference with a different flower. This one seems to be giving info mainly from the 370nm and upwards based on the colour of the final image.

 

Still need to sort out a proper mount for the Wratten - wedging it between the s8612 and the lens is not the best idea.....

[Andy Perrin]

Yeah, with this flower, it seems there is little difference with the Baader. It’s a gorgeous flower though.

 

Did you use the same WB for both photos?

[colinbm]

Pretty fair comparison Jonathan.

[JMC]

Thanks Col.

 

Andy, for each one I took a white balance image of a PTFE plate using the respective filters. Each image was white balanced with its own white balance shot. They were then processed the same than that. Only difference is ISO - 1250 for the Baader U image, 2000 for the Wratten one. I had the flash maxed out for the Baader U shot, so needed to up the ISO for the Wratten image.

[dabateman]

I am surprised the exposure settings differ by so little.

Based on your spectrum above and using a Flash, I would expect them to look the same. Only much slower exposure time for the 18A.

 

With sunlight, maybe the Baader venus will be slightly more golden, getting closer to the deeper peak max. But maybe not.

[JMC]

David, I expected the exposure with the Wratten to be about half that with the Baader U, this ISO difference is about right I think. The flashes I have are quartz tubes I had made for the Bowens GM500, and the spectral distribution in the UV is not too different to sunlight.