Split-image focusing screens and DSLRs - how to get one without spending a fortune

There are tonnes of old manual lenses out there. But most modern DSLRs don't have any focusing aids at all, making it difficult to use old glass. This is a pity, since many of these old lenses have wonderful optics. For top-of-the-line DSLRs there are OEM split image focusing screens to buy, but for the cheaper ones there are very few alternatives. And for those few that do exist, the better ones cost a fortune, and the cheaper Chinese ones keep on getting very mixed reviews. So I decided to make my own!

My to-go camera is a Nikon D3100. Sure, it does lack some functions, but for a low price it gives you good IQ, and the small size makes it a great camera for carrying around. Another plus is that it can take almost any old Nikon lens you can find. I have some older manual lenses, but focusing with them has always been difficult. The camera has a small focus confirmation dot in the corner of the viewfinder, but it is quite difficult to use when trying to snap a picture on the go.

So, first of all I needed an old camera. I bought an old Canon T70 from the 80s for 3 euro on the web. It really doesn't matter what model you use. Just make sure it has a good focusing screen. I chose between this one and a Revue from the 70s, but according to what I read on-line the Canon had a brighter focusing screen. That it is newer also means the technology might be better.

This is not a model with exchangeable focusing screens, so I had to open it up. I should perhaps warn that this is not something you should do if you are not used to a bit of hobby work. Since the pentaprism had to be removed before the focusing screen could be taken out, it was practically impossible to do it without damaging the camera. But for 3 euro, that's ok with me! I removed the plastic cover with the help of a small screwdriver, cut a few wires so that I could lift the pentaprism, and then pulled out the screen. It didn't take more than ten minutes.

Once the screen was out I needed to cut it down to the right size. I started with putting transparent adhesive tape on the screen not to scratch it by mistake. Make sure to be very careful all the time. If you scratch the screen it might become useless. I then took a white paper and drew the size I needed it to have on it as a template. The more exact you can do this the better. The D3100 focusing screen is 28,6×19,5mm, which is quite a lot smaller than the old Canon one. Once I had the template drawn on the paper, I placed the screen over it and fixed it with tape. Make sure to be careful when doing this, since you have to measure so that is right on spot. Also be aware that you should cut about a millimeter more at the bottom than at the top. The prism should be closer to the bottom, but only by a little (I cut a bit too little, so the prism is just below the middle focusing spot in the viewfinder). 

I took a metal ruler and a sharp knife and cut along the lines a few times. Don't use a plastic ruler, as that might give you an uneven cut!

After that I cut the screen loose from the template and carefully broke the excess plastic off the screen with a pair of pliers.
I carefully filed down the edges so that they were smooth and the dimensions correct. I removed the adhesive tape, scratched away rests from the filing from the edges not to leave small cuttings in the camera, and carefully washed the screen in destilled water and a little washing up liquid (without rubbing it - only rinse it to avoid scratches).

The next step was to remove the old screen. I followed instructions I found on-line (just search for how to install a focusing screen). Be very careful so you don't scratch it! You might want to put the original back one day, and then you don't want it full of scratches. Also make sure to do this in a dust free environment. A dust blower is the best way to clean inside the camera. In worst case, the screen and the bottom of the pentaprism can be carefully wiped with those kits for cleaning camera sensors.

I then replaced the old screen with my new.  As you can see, the prism ended up a little but to the side, but that is possible to correct afterwards (if it is too far to the right or left, file a little on the side of the screen and put it back again. Move it around in extremely fine movements, a micro millimeter at a time, to get the placement correct. I used a round toothpick for this, which was a bad idea since it left dust. A needle could do it, or just shake the camera slightly to move it).

Out of focus:

In focus:

Make sure that the coarser side is upwards towards the pentamirror and the prism closer to the back of the camera than the front. I left the original shim in the camera, and snapped the screen into place. It had some front focus from the start, so I took it out again and put in two thin shims I cut from paper. How many shims you need might be different from screen to screen. I put the original shim on top of the focusing screen to make sure it stays firmly in place, since the Canon screen is slightly thinner than the original Nikon screen.

Paper shims:

Most old screens should work, since the important thing is that the coarser side, which is the one the image is projected on, should be towards the pentamirror/prism (that is upwards). Now I can use my old lenses and focus correcty without trying to use the focus dot in the corner.  So, a working focusing screen for the price of a glass of beer. Not bad!

A few thoughts about using a split-image focusing screen on modern cameras

In the good old days, before autofocus became standard, all cameras had some kind of focusing aid. The split-image had become standard in the 80s, but there were also other ways to focus, for example microprisms and ground screens. Not seldom were they combined (as is the case with the screen used above). However, when using a modern autofocus camera, they will never work as well as they did back then.

Why? Simply because modern cameras aren't built for them. In an old, manual focus camera the mirror reflected all the available light up towards the focusing screen and the pentaprism. Modern cameras have mirrors that let some of this light through to the autofocus sensor, which is usually placed at the bottom of the camera house. This means that the amount of light that hits the focusing screen is smaller than on an old camera. Add to this that cheaper DSLRs have a smaller viewfinder than what used to be standard. They can, of course, since with autofocus we don't need as big and bright viewfinders. But when we want to use older manual lenses, we will run into problems.

A traditional focusing screen with any kind of focusing aid will make it easier to use old lenses, even on a small, entry-level DSLR. But we have to be aware of that it will always be a compromise. The cameras weren't made for it, and this means that the viewfinder will be darker, no matter if you use an old screen or buy a new. We have to choose between a brighter viewfinder or focusing aids. Personally, I find the split-image so helpful that I am prepared to take the slightly darker viewfinder (and to be honest, it is not that much darker). But I do most of my photographing outdoors where there is more light available, and my manual lenses are relatively fast. For those who shoot mostly indoors or only have slow lenses, the darkness might very well be an issue.

Text and images (C) 2015