Fitting a K3 Focus Screen to a D70/D70s

Overview

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!

Materials:
1 x Nikon K3 focus screen from bhphoto.com
1 x Roll of Scotch tape



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

Dimensions

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.

Experiences

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!

SLR Focusing Screens and Split Prisms

A couple of years ago, I upgraded from a 35mm film SLR (Nikon FM) to a DSLR with an APS-C sized sensor. It has been a good experience; I've been very happy with with output from the camera, and recently, I sold my film camera equipment.

One thing that does bug me about my new DSLR (a Nikon D70s) is the viewfinder. You can read all about it on the Internet. Everyone complains that the D70 viewfinder is small, dim, and "the worst viewfinder Nikon ever made". I'm ok with that part of it. I have decent vision, and I can see into the viewfinder with my glasses on... The problem I do have is that the focusing screen shows everything in focus all the time, even if it's actually out of focus!

This is most noticeable with a fast lens like an f1.4, but did notice something funny even with an f2.8. I thought I was going crazy, but when I looked through the D70s viewfinder, I had none of the defocus I saw in my FM's viewfinder. Then I learned that some focus screens make a compromise between brightness and the ability to manually focus (which means you can see when it's in focus and when it's out of focus). It looks like the D70s focus screen chooses brightness rather than viewing accuracy. You can confirm this by opening up lens aperture, looking in the viewfinder, taking a shot, and comparing the viewfinder image with the photo. You'll see a big difference in the defocus. I don't believe this is specific to the D70s, I think many new cameras make this trade-off, but I think it's wrong.

I think that half the point of using an SLR is to actually SEE what's going to hit the film. If the focus screen is just going to show me some distorted, version of it, then why wouldn't I just use a rangefinder? Smaller, lighter, quieter, less shake... Plus, I think it does a disservice to beginners who don't know how to use defocus. They won't know what they're missing, because they'll never see it in the viewfinder.

Anyway, now that I know what's going on, I'm going to change the focus screen to one that comes from a manual focusing film body. It has the added bonus of manual focusing aids which I also like.

In my investigation of this problem, I stumbled upon this article
Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras, written by Douglas Kerr about split prism focusing aids and AF techniques. It's pretty interesting and explains how these focusing techniques work.