Sunday, October 26, 2008

Fitting a K3 Focus Screen to a D70/D70s


Over the summer I modified an Nikon K3 focusing screen (meant for an FM3A body) to fit my D70s. This post describes how I modified the focus screen, and how I like the result. According to the Internets, the K3 screen is the same as the normal Katz Eyes screen so you can have the same screen for $30. This project was based on this discussion.

This procedure should work for many different DX format DSLRs such as the Nikon D40, D50, D60, D70, D80, D90, D100, D200, and D300. The only difference might be the dimensions of the screen. You can remove your existing screen and measure it to get the proper dimensions.

Edit: according to comments from Dennis below, the D40, D50, D60, and D70 all share the same screen dimensions. For other cameras, you should measure your original screen before you cut the new one. If you do measure them, please add a comment below with the dimensions; I and other camera hackers would appreciate it!

1 x Nikon K3 focus screen from
1 x Roll of Scotch tape

- Utility Knife
- Calipers
- Pliers
- Steel Ruler
- 6-Inch Bastard File


Here are the target dimensions of the D70s focus screen. These figures are not to scale, and the prisms should be pointing at you, out of your computer monitor:

Here's the dimensions of the K3 focusing screen:

Here is the amount to cut off the K3 screen to get the D70s dimensions (close enough).

Cutting the Screen

Stick a layer of scotch tape on both sides of the focus screen. This should help you avoid scratching the focus screen.

Repeat for each of the three sides with big cuts:

Set your calipers to the amount of screen you want to cut off the current side and lock using the caliper's set screw. Slide the calipers along the side of the screen, scoring the tape. Place a ruler against the screen, and score with the utility knife (make two passes). Flip the screen over, and score the opposite side with the knife and ruler. Press the screen against your table, and grab the portion you're trying to cut off with the pliers; snap it off.

For the forth side, score and snap off the tab that says K3. Use a file to remove the .022".

Clean up the edges using the file, and sand paper.

Remove the tape. If there's tape residue, take the sticky side of a fresh piece of tape, stick it lightly to the residue, and pull it off. This should pull off the residue.

Remove the Existing Screen

Take a junky mini-screwdriver and file a notch in the tip. Turn your camera upside down and find the wire bail near the focus screen, by the lens mount.

Use the screw driver with the notch to press the bail towards the film plane and unhook the wire bail. Gently push the bail away from the focus screen.

Let the screen fall out of the camera. You might find a shim like the brass shim pictured. Keep it because it sets the focus screen in the same focal distance as the film plane. You will reuse this shim if your camera has one.

Rotate the screen so that the split prisms stick out towards the top of the camera (towards the penta-prism). Rotate the screen so that the split prisms are closer to the film plane. Drop the new screen into place. Close the wire bail.

Finally, test the focusing screen. Here's my setup: I focused on one hash mark on the ruler using the split prism and exposed a frame. Then I examined the frame, and the focus screen is dead on.


I've probably taken a few hundred exposures with the new focus screen. It is much easier to focus with the new screen, but it's not as good as my old FM. The viewfinder is smaller, and darker, but the K3 screen helps.

The K3 also shows the correct depth of field which is great. When I press the DOF preview button, I can actually trust what is in the viewfinder.

One thing is weird about this K3 screen compared to my FM and EM bodies is that the images in the split prisms are out of focus when the subject is out of focus. In my old cameras, the images in the split prisms is always in focus. I prefer the way the old screens work because it's hard to see what's going on in the split prism when it goes out of focus.

In the end, it was worth $30 and the hour or two I spent messing cutting the screen and installing it!


Dennis Dietz said...

This is fantastic, thanks. I have been considering doing this for my D40 and I have on question for you. When does the K3 start to blackout? I have several f/5.6 lenses that I use a lot and many screens start to blackout at that aperture. I've read that the K3 starts at around f/8 but I'd love to be sure before I spend the money.

You can email me also if that is more convenient for you.

Alex said...

It really depends most on where your eye is. If your eye is directly behind the eyepiece, the blackout isn't really noticeable until f/11. However, if you move your eye to the side, then I'll see one prism black out even at f/3.5.

Dennis said...

Alex, thanks!

I just finished this with a used K3 screen I picked up for $13. Followed your instructions, along with several other sources and the Katz Eye instructions for the actual replacement. It works like a charm. Many thanks for providing the impetus to do this myself.

Note to all D40/60 users. The screen dimensions are exactly the same as Alex has provided for the D70. All D40/50/60/70 screens are the same size.

Thanks again,

Alex said...

Hi Dennis, I'm glad you found the article helpful! Where did you find a K3 for $13? I'm actually looking for a K2; I suspect it won't have that weird effect where the prisms go out of focus.

Simon said...

Alex, I fitted my D40 with a K3 last night, following your instructions fairly closely. I didn't want to risk damaging the microprisms, so I taped a couple of shims either side before leaning down on the ruler to do the marking and cutting.

It's a used screen with a couple of scuffs and marks, plus a brand new scratch where I dropped it, but it's still a huge improvement over the standard screen. Many thanks. :)

Alex said...

You're welcome, Simon! Another idea is to take a ruler (or something else with a straight edge) with a hole in it, and line up the hole with the split prism.

pjgoswami said...

I am Pranab from Guwahati, Assam, India. I am having a Nikon D5000.

I have gone through your guidance. It is very much useful, thanks for the article.

In our place camera related accessory are very rare. So I managed some focusing screen from old camera (35mm film camera) repairing shop. But the dimension for the Nikon D5000 what will be required is not known to me. So if anybody know the exact dimension please guide me, otherwise I will keep the split screen in the middle from all the side and go for make it in that size.

Thanking you.

Alex said...

Hi pjgoswami, You can remove your existing focus screen and measure it. You should be able to find the center by looking at the ridges of the Fresnel lens that are molded into the focus screen.

pjgoswami said...

Thanks for the reply. I will try it tomorrow in day light. If there any optically indication then please guide me, because in this field I am very much new.

Thanking you again.

Alex said...

After you pull out the focus screen, one side will be smooth and one side will have ridges. If you look closely at the ridges, you should see that they make concentric circles. The center of the viewfinder will be the center of the concentric circles. When you cut the new focus screen, just make the split prism end up in the same location as the center of the concentric circles.

If you're off by a little bit, it won't matter that much; you'll just have a split prism that's off center in the viewfinder.

pjgoswami said...

Thanks, I will inform you after doing the work & also send the dimension for help other.

pjgoswami said...

The total size of the screen is 26mmX18mm. The center point I observe as per the guidance and measured with a STEEL RULER (forgot to take the Vernier caliper)in the middle of the screen.


top to middle = 9mm
middle to bottom = 9mm
left side to middle = 13mm
middle to right side = 13mm

Can I go for the cutting the focusing screen with this data.

Alex said...

Sure, give it a shot!

pjgoswami said...

Thanks Alex.

At last, I have re engineered it and found to be ok. But one accident occurred during the cutting, the screen get cracked in one side(due to my mistake). So I glued it and cut down to the size. After installing it found that it getting focused little before the object(may be due to the glue).

Is it having keep any up or down side reference when put it to the prism. Because I found one side is little bit rough then other side (Middle of the Circle). And kept it away from the pentaprism base.

I got old Pantax film camera focus screen, with in few days I will get some other make but not sure which will be.

Alex said...

Hi, I believe the rough side with the ridges should be closer to the prism.
If you flip your screen over, so that the rough side is up, your front focus problem should probably go away.

pjgoswami said...

Dear Mr. Alexander S. Yip,

Thanks for the guidance, and it is working now. But I left one work that test it with a faster lens i.e f/1.4 which is with one of my senior colleague. After the test I will inform you.

If you permit I want to mail in your mail address which is you publish in your website.

Alex said...

You can email me, but the snail mail address is out of date.

Bertino Araújo said...

I have a Nikon D5000 as pjgoswami and I also have the back focus problem, although the prisms are pointing to the pentamirror. I think that it's due to a little "step in" on the edges of the original D5000 screen. Do I have to try to cut the same step? It's barely measurable and it would be very difficult to cut it accurately. But maybe that's not the problem. I want to ask pjgoswami how he got his right.

Thanks for any help.

Alex said...

I'm not familiar with the "step" you're describing since I haven't actually looked at any D5000 focus screens.

As long as the ground side of the new screen is exactly where ground side of the old screen was, you should have the same focusing.

That said, it's possible that your camera's original focus screen was not adjusted right, and the new focus screen is making it more obvious.

Depending on whether the screen is too far or too close to the mirror, you could try adding tape around the edge of the mirror to add some distance, or you could check to see if there's a shim inside. If there's a shim, try removing it.

pjgoswami said...
This comment has been removed by the author.
pjgoswami said...

I want to clear one thing to be mention to this as a addition for Nikon D5000 that the actual size of the original focusing screen is

Length = 25.33mm
Breath = 17.6mm

not as I have mentioned earlier. I have measured it by very precession digital caliper. And the center point at the center of it.

I followed as Mr. Alexander S. Yip. And I kept the shim in its position. I used the focus screen look like Pantax MX camera. I can’t say it definitely because I got it from an old camera repairing shop. The thickness of the re engineered focusing screen is 0.11mm & the original Nikon is 0.16mm and it is nearly plain in the middle portion. Now I use one net type focusing screen. During cutting operation I missed some precautionary steps so I damaged two of them. Out of those, one from Pantax (which can't be re use) and one from Zenith. The Zenith one I glued with Fevi Kwik (one drop instant adhesive) and inserted its oblique side to the panta mirror base side and found it work fine.

Mr.Bertino Araújo, please follow as Mr. Alexander S. Yip, mentioned. I got result after following his advise.

I am very much sorry to all that I have not written these earlier.


Bertino Araújo said...

Thanks for your quick answers.

The thing about the "step" was just a distraction of mine: the face with the step was the one facing the main mirror so it wouldn't affect its location.
So I put some tape on the edges of the screen facing the pentamirror and now the focus is closer to be correct.
Conclusion: I can only have lost my shim! Dam it!
I'll look for it where I took the screen of the camera. If I do not find it what can I do? Can I buy one somewhere?

Thanks a lot for your help!

Alex said...

Can you add enough tape to put screen into focus? If so, I'd just use the tape. If not, try cutting some thick card-paper, or ask the people at Kateyez if they have any shims.

Bertino Araújo said...

Hello again.
I went looking in the place I took the original screen of the first time for the supposedly lost shim. I didn't found it and than I decided to try the original screen again. Just to make sure there was actually a shim in the first place. Well, the screen focuses correctly, like it did before. The strange thing here is that pjgoswami's D5000 had a shim.
But, back to the split focusing screen, why didn't it worked? Is it because of its thickness? I didn't mentioned but it is not a K3, it is a Nikon Type E2 for the FA/FE2.
Any ideas what went wrong?


pjgoswami said...

Mr. Bertino Araújo, I found "shim" every set where from I have taken out the focusing screen. Why it is not there in your set it could not understand. I have open Zenith, Pantax, Yasika, Patrika, I found every set it is there. One thing may be there, that you’re focusing screen thickness may be that much where it is not requires any shim.

You can check thickness of both focusing screen and take necessary action as per the thickness. If thickness of your "E2" focus screen is more than do not go for filing. The middle portion is fixed by molding method, so please do not any modification to reduce the thickness it may change the material property if we not get the right instrument.

The finale solution you will get from Mr. Alexander S. Yip. This is my first time I am doing this type of job, so my knowledge is very less.

Alex said...

I can't think of a good reason why the E2 screen would be out of focus. Optically, the image appears on the rough side of the screen, and if that rough side of the screen is flush to the camera in the same exact place, then the focus should be exactly the same.

On possible reason is that the original screen has a larger depth-of-field, and it makes things look like they're in focus even when they are not truly in focus.

Or maybe the screen is not completely flat. Is there a ridge around the outside of the screen perhaps?

Jani said...

Hi! Interesting mod. Did anyone find some seller of K3 screen from Europe.

Jecki said...
This comment has been removed by a blog administrator.