Cleaning focusing screens

[MattKing]

You can mold and shape and grind plastic in many, many ways - in some cases in ways that cannot be used with glass.
Some plastics are very high quality.
IIRC there are also lenses that have both plastic and glass elements.
Plastics can also be lighter than glass.

 

[Fin]

Just a point of clarification, all Nikon F cameras (F, F2, F3, etc) have glass focusing screens.

Not quite...

The lens part is glass, but the actual focussing screen itself is acrylic.

I've just tried cleaning an old scratched and battered one using IPA and dish soap/distilled water. The IPA seemed to leave marks/streaks both on the glass and the focussing screen despite several attempts.

Soaking it in a film pot with warm water and soap, then rinsing in distilled water got it clean. Then I polished the glass and used an air duster on the screen. That seemed to get it as clean as I've ever seen this one!

 

[Pentode]

Thank you all for the excellent information and opinions! In particular, the info shared by cooltouch and fin looks to be extremely useful.

I've just tried cleaning an old scratched and battered one using IPA and dish soap/distilled water.

I assume you mean isopropyl alcohol and not India pale ale!

I've just tried cleaning an old scratched and battered one using IPA and dish soap/distilled water. The IPA seemed to leave marks/streaks both on the glass and the focussing screen despite several attempts.

Soaking it in a film pot with warm water and soap, then rinsing in distilled water got it clean. Then I polished the glass and used an air duster on the screen. That seemed to get it as clean as I've ever seen this one!

Like yours, my screens are framed in anodized aluminum with flat-head fasteners. I have not attempted to take them apart yet as I was waiting to see what people's responses were here. Once I take one apart I'll be able to determine whether the fresnel is simply placed against the plano-convex screen and held in place by the frame or whether it is bonded somehow. If they are separate, then everything will be easier and I'll be able to clean each piece on it's own without having to worry about trapping crud in between.

What I may do is try removing the goo from the boxes before attempting removal from the screens. The boxes are plastic - most likely polycarbonate - and are just as gooey as the screens are. If I can clean the boxes without harming them it may give me clues as to which method will not harm the screens. I don't know why this didn't occur to me earlier.

Everyone's input has been very helpful. Hopefully I'll have time this weekend to do some experimenting.

 

[benjiboy]

Just to add to your clarification. Canon F-1 screens (both new and old) are also glass. They're two-piece though . . . or at least the old ones are, so they should probably be dismantled, if possible.

http://www.mir.com.my/rb/photography/hardwares/classics/canonf1n/metering/index.htm
Canon F1 screens aren't "all Glass" only the fresnal lens at the top is glass the opaque material at the bottom is plastic. I don't recommend you try and dismantle F1 focusing screens or attempt to wash them under the tap because they contain beam splitters to divert the light to the metering cell C.d.S in the original F1 and F1n or the S.P.C. cell in the New F1 that is directly opposite to it the rear of the mirror box this applies to the older and the new F1 models.

 

[cooltouch]

Did it need to be, or is there such a thing as Optical Grade Plastic.?

I can't resist commenting here -- probably the last pair of glass eyeglasses I wore were made back in the 80s. Been plastic ever since. And I reckon eyeglasses use optical grade plastic.

 

[CMoore]

I can't resist commenting here -- probably the last pair of glass eyeglasses I wore were made back in the 80s. Been plastic ever since. And I reckon eyeglasses use optical grade plastic.

Yeah...Wow.!
I never realized just how far "Plastics" have come.
Looks like Mr. McGuire was right all along.......

 

[Vincent Peri]

Not quite...

The lens part is glass, but the actual focussing screen itself is acrylic.

Thanks for the correction. I was basing my statement on the MIR website... See:

http://www.mir.com.my/rb/photography/hardwares/classics/nikonf3ver2/screens/index.htm

where it says

"Unlike focusing screens for some of the mid-price Nikon bodies that are made of acrilan, which is prone to damage as well as hard to clean, focusing screens for the F-series bodies since the Nikon F in 1959 are a solid block of glass with a protective metal frame."

Just to be sure, I took one of my old surplus Nikon F3 "K" screens and checked. The top is certainly glass, but the underside is definitely plastic. I took a knife to it and easily scratched the bottom of the screen.

Live and learn LOL.

 

[Pentode]

I took a knife to it and easily scratched the bottom of the screen.

Yikes!!

 

[Vincent Peri]

Yikes!!

It was only a couple of millimeters long... hardly noticeable