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Installing a Canon EF-M split-circle viewfinder screen into an EOS camera.

PhotoNotes.org DonationsCopyright © 2003-2014 NK Guy

http://photonotes.org/articles/split-circle-screen/

Introduction to split circle viewfinders.

Back in the 70s and 80s it was very common for SLRs to have split circle (split prism) viewfinders. Such viewfinder screens feature a circle, bisected by a usually horizontal line, in the centre of the screen. To focus the camera you find a high-contrast vertical line (eg: edge of a wall, a tree trunk) and look at it through the viewfinder. You then adjust focus until the line appears unbroken in the split circle.

Split circle viewfinders use a pair of tiny prisms set in precise positions in the viewfinder to achieve this effect. They’re arranged such that the image you see lines up (becomes coincident) when the line is correctly focussed on the plane of the screen. Some cameras use four prisms so you can look for both horizontal and vertical lines and some use prisms mounted at 45 degree angles. Sometimes microprisms are used - even tinier prisms which give a textured look when the image is not in correct focus. All these types of manual focus assist aids make it much easier to focus manually, especially when light levels are low.

The primary disadvantage of all prism-based focus assist aids is that they can stop working and black out (become so dark as to be useless) when used with certain slow lenses, long telephotos, teleconverters or extension tubes, because of the angles at which light is entering the camera from the lens. They may also require some exposure compensation because they may affect the amount of light reaching the camera’s internal lightmeter.

Sadly, most camera makers ditched these manual focus aids with the advent of autofocus in the late 1980s, apparently on the assumption that autofocus is so fabulous that you’ll never need to focus manually again. And worse yet, most EOS cameras do not have interchangeable focus screens.

The Canon EF-M viewfinder screen.

In 1991 Canon produced the Canon EF-M for markets outside Japan. It wasn’t sold as an EOS camera, but was a stripped-down EOS Rebel/1000 without the ability to autofocus. Since it was a manual focus camera it shipped with a split circle viewfinder.

In fact, it has a split circle in the centre of the screen and a ring of microprisms (microprism collar) around it. So if it’s too dark or you can’t find a decent high-contrast line you can try the microprisms instead. I find them useful at night if there are bright points of light - such points appear broken up into little points by the microprism when not in focus.

Compatible cameras.

By happy coincidence a number of related EOS bodies have viewfinder screens of precisely the same size and dimensions as the EF-M (ie: they were all built around the same basic mirror box chassis design). As a result it’s fairly easy to remove the stock laser matte screen and replace it with an EF-M screen. These cameras were not designed with user-replaceable screens and no alternative screens were generally sold, but they’re nonetheless reasonably easy to work with.

Cameras to which this viewfinder screen can be installed are the following:

EOS Rebel/1000 series:

EOS 100/Elan.

It’s possible there are other EOS cameras of the same vintage into which this screen can be installed, but I haven’t examined many others. I do know that the EOS 10/10s screen is slightly larger and thus incompatible with this hack. Any EOS camera with different viewfinder specs - all the cameras listed above have 90% vertical and horizontal viewfinder coverage with 0.75x magnification - will be incompatible, in fact.

Note that the EOS 100/Elan is by far the most capable of the cameras listed here. It’s a midrange EOS camera with a decent cross-configuration autofocus sensor. So if you’re going to try this modification I’d personally do it to a 100/Elan.

Obtaining an EF-M screen.

Unfortunately Canon have not carried the EF-M viewfinder screen as a replacement part for many years, so they’re basically impossible to obtain from official sources. They still appear on some price lists, so some camera shops may claim they can order the part, but you’ll probably discover after months of waiting that they’re wrong. Your best bet is to find an old broken EF-M - or sacrifice a working EF-M - and remove its focus screen.

I’ve found a broken EF-M for $12 US and a working EF-M for $85 US on an auction site, so they’re not impossible to find. It just takes time searching now and then. Used camera retailers KEH and B&H sometimes carry them, but usually they ask well over $100 US for a working model.

Installing an EF-M screen.

Exchanging the screens of EOS cameras which do not have officially interchangeable screens isn’t difficult, but is very fiddly. The screens are made of very delicate and easily scratched plastic, so you may be best off taking your cameras to a repair shop and having them exchange the screens. If you want to try it yourself examine the rectangular metal frame which is used to clip the viewfinder in place. This frame has four tabs, two on each long edge, which fit into small holes in the black plastic mirror box and hold it in place.

Use a pair of very fine tweezers and slide the tip of the tweezers between the metal frame and the front edge of the camera’s plastic shell, where there’s a black piece of foam (the edge nearest to the lens mount). It will help to watch what you’re doing by looking into the camera’s mirror. Push the frame into the camera (towards the shutter) and down (towards the camera baseplate) and the frame should simply fall away. Be careful not to touch the screen or the mirror - both surfaces are extremely fragile and easily scratched.

Note that there may be a rectangular plastic shim located between the camera body and the viewfinder screen - you’ll need to keep that there. Note also that because of alignment issues it’s possible that the camera may not be truly in focus when it appears in focus using the split-circle screen, and it may need to be shimmed.

You may also want to test the camera’s lightmeter to see how metering is affected by the presence of the new viewfinder. It may be wise to do some controlled tests with the old laser matte screen before replacing it with the EF-M screen.

So that’s basically all there is to it. Hopefully you’ll find it as useful as I do. Adding the split circle screen to my EOS Elan has really changed it for me. I do a lot of low-light and night photography, and being able to focus manually with ease has helped me a great deal.

Finally, note that other components in the cameras are not so easily interchangeable. The EF-M’s plastic lens mount, for example, can’t be swapped out for the Elan/100’s steel lens mount. The two cameras have seemingly identical viewfinder lenses, but it’s hard to remove from the Elan/100 because the light metering board is in the way. In fact, Elan/100s are a real pain to work on as everything’s crammed in there. The EF-M is much simpler and has more open space. So it’s pretty lucky that the viewfinders just happen to be interchangeable!

Thanks to EOS user Julian Loke for discovering this possibility and posting about it to the EOS mailing list.


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- NK Guy, PhotoNotes.org.

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