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02-21-2021, 09:55 AM   #1
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Blacking? Flecking? what's the black stuff called on the inside of a lens?

I'm having a bit of a vocabulary crisis based on something I need for a lens that is no longer a lens...

In a recently acquired pile of junk selection of loved vintage gear, I found a Focal 2x teleconverter with A contacts.

It had an easily removable group of lens elements, and once I remove the Ricoh pin (both of them), it will hopefully become a nice extension tube.

My vocabulary crisis is on the black stuff I should use to cover the shiny threads where those lens elements used to be.

Blacking? Flecking? I don't recall... but there's something I remember I should be using that's a little better for the purpose than the simple flat black enamel I was originally thinking to use.

I don't think it's the fuzzy stuff (flecking?), like on the inside of a telescope tube, as that might pile up on the threads and block the optical path...

Any ideas? Assistance on the vocabulary issue and/or the technical one are both appreciated

Thanks,

-Eric

02-21-2021, 10:01 AM   #2
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It is likely black flecks of paint used to absorb light that is reflected off surfaces outside the image circle of the lens

It probably won’t be noticeable in images but if a lot has come off you may experience a loss of contrast due to internal reflections and light pollution
02-21-2021, 10:24 AM   #3
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In this case, there is a ~1cm long series of very shiny aluminum threads in the middle where the optics used to be.

I can't imagine that wouldn't cause a contrast problem at the very least...

-Eric
02-21-2021, 10:36 AM - 1 Like   #4
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Hello,

In scale modeling we use "flocking" to simulate carpeting. It is a very fine powder and it comes in different colors.
This is a 1/25 scale interior tub:


But I wouldn't use this in a lens. Any flat black paint would do better as this will shed dust all the time.

*EDIT* I'd use acrylic flat black paint. Make sure it is not glossy. A sharpie is usually glossier (higher risk of reflections) but could work in a pinch.

Thanks,

02-21-2021, 10:57 AM - 1 Like   #5
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Flocking, as ismaelg said.

Used to use it in telescopes to achieve the same goal of reducing reflections in the tubes. There are very black acrylic paints that do better job for lenses, again mirroring what ismaelg said.
02-21-2021, 01:07 PM - 2 Likes   #6
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Flocking... got it. I knew it was one of the round vowels

Thank you both.

Now for some paint...

-Eric
02-21-2021, 02:54 PM   #7
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Flocking in a telescope is more normally done with a felt that doesnt shed fibres. Its like a very fine but fuzzy felt.

The best non reflective paint is blackboard paint. It dries flat and very black, its used for a lot of astronomy optics as a reflection killer. Warning, if use you it then you need to stir it for a geologic age, it takes forever to mix, if you dont it dries shiney.
02-21-2021, 03:21 PM   #8
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Wondering it a piece of matte black paper (construction paper?) would work, and be easier to apply,
Gently roll it into a tube and slide it in so it expands against and covers the threads. A dot or two of CA glue to keep to in place?
Or maybe black masking tape?Similar idea and application
Not sure of all the geometry you are working with, so maybe too many fiddly parts to fit around.

02-21-2021, 04:41 PM   #9
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For my pre-owned zoom lenses, I obtained several screw-in metal lens hoods - which had a design of rings around the inside, presumably to prevent reflections. These, however, were finished with a slightly glossy black paint, which I covered, as suggested, with matt black acrylic paint, which did the job well.
02-21-2021, 06:52 PM   #10
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Not sure if gun bluing would work??
Google Image Result for http://nebula.wsimg.com/ec1f28cdda0cffb074830c9e5de097a8?AccessKeyId=9A3068F5050C127C9FA2&disposition=0&alloworigin=1
02-21-2021, 09:28 PM   #11
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Paint it with inexpensive yet durable matte black paint. Other options will cost more in materials than a used extension tube is worth. You can get that at any hardware store; just make sure it's good on metals.

See below if you want to experiment with other materials and please share your results here.

Flocking paper is used inside telescope tubes. As you already noted, it will narrow the tube diameter, although I don't expect it will cause vignetting. It's available with or without adhesive backing. Here's one source of flocking paper, but even their smallest size is way more than you need Light Absorbing Black-Out Material | Edmund Optics

Amateur telescope makers are interested in paints called Black 2.0 and 3.0. Those paints are designed for art projects and aren't very durable. Yes, the inside of a tube is protected, but I'd still worry about the paint flaking as your aluminum threads flex with temperature.
Discussion at New flocking material/Paint - ATM, Optics and DIY Forum - Cloudy Nights
Product at The blackest acrylic paint in the world - Black 3.0 ? Culture Hustle USA
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