Modify a Canon EOS-300 Into a Manual M42 Mount Camera!




Introduction: Modify a Canon EOS-300 Into a Manual M42 Mount Camera!

About: I love fixing things...

You love your M42 manual lenses, but fed up with your heavy, stinking old SLR? You want a decent motor drive and flash on your camera?.. Hmm. Today we'll convert an EOS-300 to take your old M42 lens stock and install a split-image screen to ease manual focusing.

M42 lenses are so 1970s. They are built to last. And among them you'll find spectacular ones that rival modern lenses in so many ways. Check out these:
- Carl Zeiss Jena - Flektogon 35mm f2.4,
- Pentax Super-Takumar 50mm f1.4,
- Carl Zeiss Jena - Sonnar 135mm f3.5, and many others.

A few years ago, you could get them for half the shipping price on ebay. Nobody wanted them. People loved their 'wonderful' autofocus/autoexposure Canons, Nikons, Minoltas... Now people went digital, and experimenting is virtually free. So they started looking into alternatives. That's why we have a boom in old M42 lens market! People love them on their Canon digital camera bodies. I said Canon, because the lens register space in Canon DSLRs allows using an adapter to convert M42 to EF lens mount. You focus manually, set your aperture manually, and use 'aperture priority' mode for auto-exposure.

Since everybody is into DSLRs, mass production film cameras go dirt cheap on ebay. I mean some lower end modern Canon EOS series and some Nikons. You can get a mint EOS-300 body for $40 including shipping. EOS-300 makes a great light body to use for your street photography. Put a wide-angle M42 lens on it, and you'll have a very compact, reliable, quick camera. It has 1.5 fps burst shooting. Built-in flash. Reliable auto exposure. Low power consumption. For its price, these are great features.

However, the mirror mechanism slaps some M42 lenses, we have to modify the mirror. The original focusing screen doesn't help you for focusing manually, so we'll change it into a split-image screen. The M42-EF adapter needs to be taken off each time you change a lens. I prefer it to stay on the camera, so we'll solve that problem, too.

Tools needed: Utility knife, pliers, watchmaker screwdrivers, fine-grit sandpaper, adhesive tape...
Parts needed: Canon EOS-300, M42 lens, M42-EF mount converter, an old SLR focusing screen (F-1)...

Below are the photos of the camera before and after the modification!.. In the middle you have the M42-EF mount adapter.

Step 1: Start With the Focusing Screen!

In this step we'll take out the existing focusing screen from EOS-300 and modify an old focusing screen to fit into our camera. For the split-image focusing screen you have to dig into an old SLR camera to salvage a focusing screen. I had an old Canon F-1 modular focusing screen laying around, so I decided to make use of it. This was an expensive decision since I could sell it on ebay for at least $15. Well, I want this camera now! :-)

The first photo is from the original focusing screen on Canon EOS-300. The second photo is the split-image focusing screen that we'll fit into our EOS-300.

Step 2: Take Out the Original Focusing Screen!

It is pretty easy if you have the right tools. All you need is a long screw driver with a sharp-pointed tip. If you look closely, behind the mirror padding you'll see a metal notch that you can insert the pointed tip in. Carefully pry the metal notch towards inside the camera, and pull it up. The thin metal frame will come out easily. Then carefully take the original focusing screen and brass spacer out. If you want to use the focusing screen in future, pay attention to not scratching/greasing it. It is made of plastic and virtually impossible to clean.

Look at the photos below for the step-by-step disassembly process.

Step 3: Prepare the Split-image Focusing Screen!

Now you have to disassemble the old focusing screen that you'll fit into EOS-300. It depends on where you getting it from. Only make sure that it is something that you can take out without damaging. Also, you have to make sure that the size of the new screen is similar or larger than the old one so that you can fit it in the camera after some alterations.

Below are the photo steps for a Canon F-1focusing screen I found laying around in my drawer.

Step 4: Modify the Split-image Focusing Screen to Fit in EOS-300!

Now is the hardest task. Your split-image focusing screen may be of different size than the original EOS-300 focusing screen. Differences in thickness can be solved by adding spacers, or by reversing the focusing screen. For the length/width differences, you have to cut the screen to the size of original one.

In my case, the length was OK, I had to trim two sides to fit the width. I used an X-Acto knife to mark it deeply on two sides, then I used pliers to snap the edge off. Then I sanded the sides in water to smooth the edges. Pay attention to not damaging the split-image prisms at the center. And don't scratch the focusing screen! When you are finished, wash them with some lens cleaner solution so that you won't have any grease/finger prints on them. This is not easy actually... :-(

Step 5: Mount the Split-image Focusing Screen!

Now we'll put the split-image screen in its place. It is no big deal, just pop it in and lock the metal frame back in its place. But we are not done yet! We have to make sure that focusing screen is in correct distance from the mirror to tell us the correct focusing information.

For this purpose you'll need the original EOS focusing screen, or a translucent paper.

Open the film cover, place your camera on a firm place. Set the exposure to 30 seconds or bulb if you have a remote cable, and direct your camera to a vertical line (i.e. a post) at 4-5m distance. Use a steel meter to measure the exact distance between the vertical line and the film pane. Then focus your manual lens on the object by looking through the film chamber with the focusing screen (or translucent paper) on it at 30 second exposure. Once you feel that it is perfectly focused, read the meter reading, it should be close to what you measured. If so, you know that your mount adapter is a good one. Be careful no to touch the shutter curtain or put something on its way when it's open, you can damage it!

Now, without changing the focus, look at the same object through the viewfinder. Is the split-image screen showing a perfectly aligned image? If so, you are done... If not, check:

- If you need to turn the lens towards infinity to get it aligned, you need to put spacers between the focusing screen and the body.
- If you need to turn the lens towards closer focus distance, then you need to take spacers out (if you put any), or put the prism side upwards (it works both ways)...

When you are done, you are good to go! In the last photo below I have two strips of adhesive tape to adjust the spacing between the focusing screen and camera...

Step 6: Trim the Mirror Area for Letting Your Manual Lens Fit!

Now, you gotta modify the mirror chamber to accommodate your manual M42 lenses.

There are two problems, the first is when you are screwing the M42 in its place, the diaphram pin slightly hits a board on top. There is a risk of breaking the pin, which is not nice. So, we'll shave plastic piece on the route off. The pin in discussion is shown in the first photo, the steps are shown on the next three photos. If you don't want to modify (damage) your camera like this, you can always put the lens at Manual diaphram mode and screw the lens in with front facing down.

The second problem is the mirror slapping the rear group of the lens when focus is at infinity. This problem varies with the lens make/model. With Takumar 50mm, the problem is very serious, the mirror doesn't even move up completely (check out the fifth photo). Takumar has a gigantic rear element, and we cannot trim the mirror glass (so I am listing my Takumar on ebay soon). But with Zeiss Flektogons I have, the mirror only gets stuck on the way down, so trimming (shave material off with an X-Acto blade) the bottom edge of the mirror is enough to solve the problem. Check out the sixth and seventh photos. As you can see, it works great now (photos eight and nine).

Now finally we have an EOS-300 body that we can use M42 lenses on and focus manually easily...

Use P or Av modes for auto exposure with evaluative metering. If you use exposure lock, the exposure will be measured using partial metering. In M mode you can use center-weighed metering by exposing according to needle-scale.

Use the Manual/Auto switch to control the aperture on your lens. You can keep the aperture at desired setting and switch to Manual immediately before you are shooting. Or you can do it by changing the values slowly with a depth-of-field preview opportunity!

Good luck!

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    13 years ago on Step 6

    That's impressive  but carl zeiss lens are build for old practika camera *45.5 mm...better is to use Optical M42 Lens - Canon EOS digital camera adapter


    Reply 13 years ago on Step 6

    Well, this Carl Zeiss Jena is M42, like few other Carl Zeiss lenses (Flektogon 35, Tessar 50, Pancolar 50 and 80, and Sonnar 135). Are you confusing them with Carl Zeiss Jena lenses in Praktica B bayonet mount?



    16 years ago

    A tip: when collecting the bits for this Instructable, try to find an EF-to-M42 adapter that fits flush into the camera (doesn't have that flange), so you can focus all the way to infinity with all lenses. _


    Reply 16 years ago

    Well, the one I have is too thin, so it focuses well beyond infinity which I really don't like... At manual focus forum ( there are people who found better crafted EF-M42 adapters... Check'm out!.. Cheers. K.


    Reply 16 years ago

    I've always thought that the ability to focus beyond infinity is an indicator that "infinity" is an inaccurate term, lens-wise. You can't have something behind the furthest thing.


    16 years ago

    Very nice instructable, well documented. Is it safe to say that a split focusing screen is going to be hard to find?


    Reply 16 years ago

    Thanks! Finding split focusing screen is not a big deal I guess. People are selling lots of broken camera bits and pieces on ebay. It shouldn't be a big deal to take a focusing screen out of them. The only problem is finding one with correct size/thickness. Since EOS-300 has 90% coverage for viewfinder, most of the old focusing screens will be larger anyways. Good luck!