Making a Custom Focus Screen

[CámaraLiberada]

Hello everyone, I'm excited to be a new member, and look forward to making more camera repair related posts.

I was cleaning my focus screen on my Nikkorex F (keep in mind dirt was etched into this thing), when I noticed the fresnel (matte swirls) was wearing off. Little did I know that this focusing screen was plastic/acrylic, and not glass like many cameras in this era. I have cleaned tons of focusing screens and never had this issue. Now a good 15% of the image space is dark

I looked into many people who had made custom screens for swappable 4x5 and DSLR screens, but never fixed for something like this.

Does anyone know how I can make a replacement or send dimensions to have another one made, since this will be nearly impossible to find a part for?

 

[loccdor]

There are as-is copies of those cameras going for about $25 on eBay. Would that be an option? Welcome to Photrio.

 

[CámaraLiberada]

There are as-is copies of those cameras going for about $25 on eBay. Would that be an option? Welcome to Photrio.

It is an option, but then I run the risk of having another dirty/damaged screen, and I really don't need any more junk laying around.

 

[dynachrome]

It's funny that you mention the Nikkorex F. A year or two ago I was at a camera store and saw a Nikkorex F with a 50/2 lens. It seemed to be working and was $50. When I saw which 50/2 was attached, I bought the camera right away. The lens was a tick mark version of the Nikkor-S. The lens was in good condition. The Nikkor-S would eventually be replaced by the improved Nikkor-H but for collectors, the tick mark lenses are worth much more than what I paid.

 

[CámaraLiberada]

It's funny that you mention the Nikkorex F. A year or two ago I was at a camera store and saw a Nikkorex F with a 50/2 lens. It seemed to be working and was $50. When I saw which 50/2 was attached, I bought the camera right away. The lens was a tick mark version of the Nikkor-S. The lens was in good condition. The Nikkor-S would eventually be replaced by the improved Nikkor-H but for collectors, the tick mark lenses are worth much more than what I paid.

Ha ha that's a great deal. I also got mine pretty cheap, but no lenses with it. It's not my main Nikon camera, but I'd love to have a good screen since the rest of it is in beautiful condition.

 

[250swb]

There's an article here

https://richardhaw.com/2019/01/05/repair-nikkorex-f-part-1/

on how to remove (and clean) a Nikkorex focusing screen, given you still need to find a replacement screen all I can say is good luck.

 

[CámaraLiberada]

There's an article here

https://richardhaw.com/2019/01/05/repair-nikkorex-f-part-1/

on how to remove (and clean) a Nikkorex focusing screen, given you still need to find a replacement screen all I can say is good luck.

That's crazy how the focus screen is tied to a prism height adjustment. Can't even think of where that adjustment would need to be done. Did they design these things with loose tolerances or something?

 

[koraks]

Welcome to Photrio!
I had a look at the page @250swb linked to. It appears that the focus screen itself is a flat piece of acrylic; I think in many modern cameras, the focus screen and the condenser lens are a single part (perhaps even monolithic), which would make it difficult to make a new one. In this case, it seems to me that if you can source a piece of glass of the right thickness, you should be able to make a 'crude but effective' ground glass replacement. However, even at (approximately) the right thickness, the focus screen should be recalibrated (adjusting the factory set calibration screws) to account for any thickness differences.

 

[CámaraLiberada]

Welcome to Photrio!
I had a look at the page @250swb linked to. It appears that the focus screen itself is a flat piece of acrylic; I think in many modern cameras, the focus screen and the condenser lens are a single part (perhaps even monolithic), which would make it difficult to make a new one. In this case, it seems to me that if you can source a piece of glass of the right thickness, you should be able to make a 'crude but effective' ground glass replacement. However, even at (approximately) the right thickness, the focus screen should be recalibrated (adjusting the factory set calibration screws) to account for any thickness differences.

I know that I commented on another reply that the height adjustment is a bad idea, but I'm now realizing that this may be the best camera to try it in since I can adjust that perimeter so easily. What materials would you use to create the ground glass?

 

[reddesert]

I expect that the focus screen is acrylic, most 35mm SLRs have some kind of acrylic fresnel/matte surface combination. Often the condenser lens is separate and may be glass. Something I learned recently in a different context is that acrylic plastic can be attacked by high-concentration alcohol - I had thought that the 75-95% isopropyl or ethyl alcohol that people often for cleaning things was safe for typical plastics, but there are some where it's not, including acrylics. I suppose that most lens cleaning fluid is lower concentration and probably unlikely to damage acrylic. But that may explain some damaged screens I've seen. You may be able to substitute a replacement screen from a different camera if you can get it to fit, and then adjust the height.

The focusing screen position has to be quite precise - I'm not sure what the factory tolerance was but likely better than 0.05mm. I looked at Richard Haw's writeup, and I imagine that the focusing screen was inserted and then the frame set down and height factory adjusted, probably with an autocollimator. Then the prism would be set on top. The screen would rarely require service and in those days a shop that did have to service it would likely have the equipment to recalibrate the adjustment. It's the position of the matte lower surface of the screen that really matters, not the prism.

 

[koraks]

What materials would you use to create the ground glass?

Personally, I'd start with a bog-standard normal piece of glass and then critically evaluate its performance. Then if there are problems with this, see if I can fix those problems specifically. This is in the assumption I could easily source a piece of glass of the proper thickness; i.e. virtually the same as the original acrylic fresnel.
For making the new ground glass, I'd use a suitable polishing/grinding powder (I've used valve grounding powder in the past, which worked well). You'll end up with a ground glass instead of microprisms, so I expect the image will be much more dim. And there won't be a split-view section in the center, of course.

 

[reddesert]

I've made a ground glass for a view camera, and for a temporary medium format focusing screen. It's not difficult. I used 600 grit silicon carbide, which I bought at a local tool shop that caters to gem and rock workers/hobbyists. You just need the grit, two pieces of glass (even picture frame glass will do), a bit of water, and some patience. The hardest part for me is cutting glass to size without screwing it up.

You won't get a focus aid like microprisms or split image, but you can make a pretty bright ground glass (no fresnel, so it may be darker in the corners). However, I think it would be easier for 35mm format to find a replacement acrylic focusing screen from a camera that has interchangeable screens (screen only not an attached condenser, eg like a Nikon FE, not like a Nikon F), and trim it to fit if necessary.

 

[CámaraLiberada]

I've made a ground glass for a view camera, and for a temporary medium format focusing screen. It's not difficult. I used 600 grit silicon carbide, which I bought at a local tool shop that caters to gem and rock workers/hobbyists. You just need the grit, two pieces of glass (even picture frame glass will do), a bit of water, and some patience. The hardest part for me is cutting glass to size without screwing it up.

You won't get a focus aid like microprisms or split image, but you can make a pretty bright ground glass (no fresnel, so it may be darker in the corners). However, I think it would be easier for 35mm format to find a replacement acrylic focusing screen from a camera that has interchangeable screens (screen only not an attached condenser, eg like a Nikon FE, not like a Nikon F), and trim it to fit if necessary.

That may be the solution to find a generic focusing screen then adjust the prism height, though I don't know how far that travels. I don't like split focus anyways, so it may be worth it to try and make a simple ground glass.

 

[CámaraLiberada]

Personally, I'd start with a bog-standard normal piece of glass and then critically evaluate its performance. Then if there are problems with this, see if I can fix those problems specifically. This is in the assumption I could easily source a piece of glass of the proper thickness; i.e. virtually the same as the original acrylic fresnel.
For making the new ground glass, I'd use a suitable polishing/grinding powder (I've used valve grounding powder in the past, which worked well). You'll end up with a ground glass instead of microprisms, so I expect the image will be much more dim. And there won't be a split-view section in the center, of course.

Sounds reasonable... I'd almost rather use a real glass because I don't like how frail that original one was.

 

[CámaraLiberada]

I expect that the focus screen is acrylic, most 35mm SLRs have some kind of acrylic fresnel/matte surface combination. Often the condenser lens is separate and may be glass. Something I learned recently in a different context is that acrylic plastic can be attacked by high-concentration alcohol - I had thought that the 75-95% isopropyl or ethyl alcohol that people often for cleaning things was safe for typical plastics, but there are some where it's not, including acrylics. I suppose that most lens cleaning fluid is lower concentration and probably unlikely to damage acrylic. But that may explain some damaged screens I've seen. You may be able to substitute a replacement screen from a different camera if you can get it to fit, and then adjust the height.

The focusing screen position has to be quite precise - I'm not sure what the factory tolerance was but likely better than 0.05mm. I looked at Richard Haw's writeup, and I imagine that the focusing screen was inserted and then the frame set down and height factory adjusted, probably with an autocollimator. Then the prism would be set on top. The screen would rarely require service and in those days a shop that did have to service it would likely have the equipment to recalibrate the adjustment. It's the position of the matte lower surface of the screen that really matters, not the prism.

Yes. I have cleaned TONS of focusing screens with high concentration alcohol and never had an issue with one being ruined. I really thought this one was glass. Though I also cleaned a portion of this one with distilled water and it wore more of it off. I think it just was weak to begin with. Literally have never seen this before.