How to Convert

Your Minolta MC Rokkor 58mm f1.2 Lens

to Canon EOS Mount: A Step-by-step Guide

Photos and text Copyright 2008 by Paul Yates

Rokkor on 5D
Perfect Strangers: Minolta MC Rokkor-X PG 58mm f/1.2 on Canon EOS 5D

Introduction

If you've found this page, you likely already have a good idea why someone would attempt this procedure. The Minolta MC Rokkor 58mm f1.2 lens is simply legendary. Most well-known for its creamy-smooth bokeh, it also does not fail to deliver superb resolution, even wide open:

Rokkor Wide Open on 5D
Minolta MC Rokkor 58/f1.2 @ f1.2 on Canon EOS 5D

100%
100% crop of first photo

There are very few lenses that offer this magical combination of ultra-fast aperture, ultra-sharp resolution, and ultra-smooth bokeh, most of which cost thousands of dollars. The Nikon Noct-Nikkor 58/1.2 comes to mind; selling for as much as $3000. The Rokkor, however, is a bargain these days because its native mount is basically obsolete. Demand is rising though, now that a few people have figured out how easy it is to convert these lenses to other mounts. Nevertheless, this lens remains one of the best lenses I have ever had the pleasure to shoot with and an incredible value, even at today's slightly elevated prices (between $250 and $350). An incredible bargain at even twice that price when you consider that I have it on good authority from a prominent German optical designer and researcher that the Rokkor and Noct-Nikkor are identical in optical formula and calculation. No wonder they resemble each other so much.

What you'll need

The Lens - Any version of the Minolta MC Rokkor 58mm f1.2 lens, including the MC Rokkor-PG (both metal and rubber focus grip), MC Rokkor, and MC Rokkor-X. As long as it has "MC", "Rokkor", "f=58mm" and "1:1.2" engraved in the front, you are good to go.

M42-to-EOS adapter - This becomes the new mount on your lens. I prefer the flangeless, chromed-brass adapters. If you get the black, aluminum, flanged adapter you will need to file down the flange to make room for the rear element of the lens (as I have done with the one used in this example). You can optionally buy a focus-confirm adapter and use that for this conversion.

Micro-drivers (Jewelers Screwdrivers)- A good set of these are always handy. You'll need at least a size #0 or #00 +shape and a small flathead.

Small file - I purchased a "Needle File Set" from Canadian Tire. It includes 6 small files of various shapes. I used the plain flat, double-cut file.

Drill - Preferably a variable speed, cordless drill. A drill press would be even better, but isn't necessary.

Drill bits - 5/64ths and 7/64ths. You'll likely break 1 or 2 of the 5/64ths bits before you get the hang of it, so get a few. I find the brittle ones (high carbon, cobalt, etc.) sharper but easier to break.

80-grit sandpaper - I use "Norton 3X". A few sheets aught to do it.

Flat black metal paint - I use Tremclad Rust Paint.

Fine-tip permanent marker - A black or blue "Sharpie" does the trick.

Cotton swabs - For applying paint, etc.

Vacuum cleaner - For cleanup.

Goggles & face mask - For safety, of course.


The Procedure

Before beginning, ensure you have rounded up the necessary materials and tools (as above) and you have a clutter-free work area.

It is important to note that the following procedure is irreversible.

It is also important to note that I will take no responsibility for any loss or damage that may be suffered by attempting this procedure. If you are not good with your hands, just don't do it. There are a few people, including me, who may have a converted Rokkor for sale or may do the conversion for you for a reasonable fee.


1: Mark the Old Mount

Use your permanent marker to draw a line on the face of the mount that is parallel to the "white diamond" centre mark on the lens (unlike me, do this before removing any screws):

mark

2: Remove the MC Mount

Remove the 8 screws from the mount:

remove screws

You'll notice that 4 of them are only half as long as the other 4. You'll be using the 4 longer ones later, so don't lose them.

screws

Once the screws are removed, simply lift off the mount:

remove mount

3: Under the Mount

Under the mount, you'll see the the mount shim (silver ring) and a black ring that holds the aperture mechanism. You'll also notice some ball bearings and some curved tubular pieces between the mount shim and the black ring. Simply lift the whole works off:

lift off

...and then pour the loose parts out into a container:

pour
Note: One of these ball bearings may come in handy later, so keep this stuff around.

Then use a flathead micro-driver to remove the screw that holds the spring from the aperture mechanism ring:

unsrew

Below are the pieces you can eventually discard. The mount will be used later as a template (and as a "handle" for sanding). Also, one of the ball bearings might come in handy:

don't need

Below are the pieces you will use to re-assemble the lens:

keep

4: Remove the Aperture Ring and Reduce the Aperture Ring Tab

There is a tab on the aperture ring that will interfere with things later, so lets get rid of that next. 

tab

First you need to remove the aperture ring from the lens. When you do, a spring-loaded ball bearing (for the aperture clicks) will almost certainly shoot out onto your shag carpeting. That's OK, because the ones that you recovered from under the mount are the same size.

remove

Once you have removed the aperture ring, simpy lay some sandpaper on your work bench and rub the tab right off:

tab off

Be certain to slow down the sanding near the end and finish it off nicely. I prefer to re-finish this this with flat black paint if the aperture ring is black (on the older models, the aperture ring is silver coloured). Also, make sure to clean the metal shards off the ring:

clean
 
Set the aperture ring aside for now. You'll re-install it later.

5: Reduce the Rear Element Retaining Ring

Note: This step can likely be skipped if you intend to use this lens on APS-C (1.6x crop) dSLRs only.

If left in its existing state, the rear element retaining ring will certainly cause issues with FF dSLR mirrors (and possibly 1.3x crop as well). Anyone planning to use this lens on a FF dSLR (the 5D in particular) will need to complete this step.

The method I use is quite simple, but does require some skill and concentration. Simply use a file to reduce the retaining ring as much as possible without filing the rear element.

file

A few extra strength and dexterity hit points will come in handy here, but anyone who calms down and takes their time will have no problem. Make sure to always hold the file at an angle so that it does not come in contact with the rear element:

file more

Despite the fact that the brass is quite soft, I work away at it slowly, making about 3 passes in total over the circumference of the ring, reducing it down within a few hair-widths from the surface of the glass.

6: Paint the Rear Element Retaining Ring

Once the retaining ring has been reduced appropriately, simply apply some flat black metal paint to the bare edge with a cotton swab, ensuring that you do not also paint the rear element, of course. (Note: ignore the fact that the aperture ring and mount shim have already been installed here):

paint

7: Prepare the New Mount

Place the old mount on the work bench and, using the permanent marker, make a second mark on the opposite side of the hole closest to the first mark:

mark
Make this second mark longer, in order to distinguish it from the first mark.
The two marks should be the same distance away from the closest hole.

Next, install the M42-EOS adapter (the new mount) into an EOS body and then use your permanent marker to mark the 12 O'clock position on the face of the adapter (please ignore the fact that I've already drilled the holes in this one):

mark adapter

Then, remove the M42-EOS adapter from the camera and set it flange-side down onto the work bench and place the old mount on top, so they are back-to-back:

holes

As shown above, align the second (longer) mark on the old mount with the 12 O'clock mark on the adapter. Then, making sure that the mount is perfectly aligned with the adapter, use a permanent marker to mark the closest hole (between the two marks, see above) and the 3 remaining holes (skip every other hole, you need 4 in total).

Once you are certain that you have all 4 holes marked accurately, mount a 5/64ths drill bit into your drill and drill the 4 holes:

drill
It is best to take your time with the drilling. When the bit breaks through the mount, it will have a tendency to bind against the flange on the other side and the bit may snap if there is too much torque. Try to make these holes a straight and clean as possible and do not "grind the bit around" which will result in the hole being too large.

Next, you'll need to use a 7/64ths drill bit on the other side of the mount to "countersink" the holes:

countersink

It is almost better to use a dull bit for this. You do not want to let the bit take off and chew right through to the other side. The point here is to simply open up the mouth of the hole so the screw heads will sit in there. Take it really slow here. Very little pressure is required (almost none). Drill a little and then use a screw to see if the fit is right. If you get too carried away here, the screws will simply fall right through. You do not want that to happen.

8: Reduce the Mount Shim

This is where the "work" starts. In order to attain infinity focus, you'll need to reduce the thickness of the mount shim (silver ring). In order to do so, I place 80-grit sandpaper on the workbench, then I place the mount shim face down on the sandpaper:

shim
shim

...and place the old mount on top:

place

...then I simply use the old mount as a "handle" to rub the shim against the sandpaper in "a vigorous circular motion hitherto unknown..."

 rubit

The loose fit of the old mount on the shim will allow the shim to rotate at random while you are sanding, resulting in a very even plane on the sanded edge of the shim. Initially the sanding will go quickly, as the little rim is reduced:

initialsand

Eventually, it will begin to look like this (below) and the sanding will start to slow down. Many might opt for a power tool here, but I find this the best way to get a perfectly plane surface:

ringagain

I highly recommend wearing a dust mask while doing this. Breathing in aluminum dust is not a good idea. I also recommend vacuuming the area periodically. The sandpaper will work better if you vacuum it, as well. Once the ring looks like the one above, it is a good idea to assemble everything to check for a good fit. It will need to be disassembled, more sanding done, and reassembled repeatedly in order to tune infinity focus, so it makes sense to make sure all the pieces fit first.

9: Assemble and Test

The paint aught to be dry, by now:

dry

The first thing to do is re-install the aperture ring. This can be challenging, but if you are careful you'll only have to do it once (not each time you reassemble during infinity testing). First you'll need to place one of the ball bearings back in its spring-loaded hole on the lens. If you are lucky, a bit of grease will be there and it'll help hold the ball in place:

ball

Line up the slot in the aperture adjustment arm (silver) on the aperture ring with the aperture mechanism in the lens:

align

The ball likely fell out by now, but keep the aperture mechanisms lined up and partially engaged:

engage

...then get the ball back in there and use a small flathead screwdriver to compress it into its hole while you slide the aperture ring over:

ball

Once it is in place, check to make sure the aperture clicks work properly and the blades stop-down appropriately, then simply take care not to remove the aperture ring again throughout the rest of the procedure (you won't need to, so make it easier on yourself by being careful).

Next, simply place the silver shim on the base of the lens so the holes line up and then position the new mount properly (red dot on lens should line up with red mark on the new mount) on top of the shim and install the 4 (longer) screws that you saved from before:

install

Do not tighten these screws too much. They need to be nice and snug, but cranking them is unnecessary and will likely only result in either stripped screwheads or the screw may break through the soft metal, rendering that hole useless.

below

You'll want to double-check that the screwheads sit below the surface, otherwise they will scrape against the camera's mount. Also, check for any barbs of metal and/or rough edges around the holes. If necessary, use your file  (or some sandpaper) to lightly knock off any of the barbs and smooth those areas out. Beyond that, I choose to throw the lens on my half-broken 10D and turn the lens back and forth inside the camera to ensure that any issues are discovered on a "cheap" body. I highly recommend this.

Try focusing the lens to see how far you can focus. You are likely not even close to infinity yet, but at least you know that the end is near.

10: Lather, Rinse, Repeat

Remove the screws, lift off the new mount, remove the shim and resume sanding it as described earlier. Use the photo below as a rough guide:

infinity
The lens on the left will focus to infinity. The one on the right will not.

The last thing you want is a lens that focuses past infinity. It is annoying and it ultimately will make camera mirror issues even more difficult to overcome. The closer you get to infinity, the more likely that you will have mirror issues on a 5D (for instance). One could simply leave the lens like the one pictured at the right above, and subsequently have no mirror issues and no infinity focus. The best way to approach it, if you do not want to shave your 5D mirror, would be to very gradually thin the shim until a good compromise is found.