Adjusting Focus Distance on Adapted Lens

Hi, all--

I recently got a Voigtlander 15mm f/4.5 lens and an M39-m4/3 adapter for my G1. I'm planning on scale-focusing it in dim conditions where I need maximum depth of field (with a tripod).

According to the online DOF calculator I used, which accounts for sensor size, the lens should produce images that (for example) are in good focus from about 3.25 feet to infinity when the lens is set at f/8 and focused at 2m.

In trying out the lens, I could tell that this wasn't the case. At that setting, distant subjects looked clearly out of focus to me in pictures at full-screen monitor size, which seems roughly similar to a reasonable-size print.

So after several tests, I decided to check to see if the focus scale was accurate. In the attached picture (a 100% crop of a small printout 2m away) the left image was scale-focused at 2m (with the focus plane measured to be 2m away, of course), and it seems poorly focused. I then made the right image focusing carefully by eye, and it seems better. However, the focus scale showed that the lens was focused at a point about halfway between 2m and infinity. (There are no marks between those numbers.)

Now for the question. If the lens is focusing closer than the scale indicates, and given that infinity focus happens with the lens mechanism closest to the sensor, doesn't this mean that I need to move the lens *back* (probably by somehow shortening the adapter)? I've heard of people shimming lenses *out*, but if I understand this correctly, I have the opposite problem.

And if that's right, does anybody have any suggestions about the best way to do this? Any help would be greatly appreciated.

 

0.1mm! That's going to be hard to accurately adjust.

Mine is, iirc, from rainbowimaging. I've used their adapters before, but haven't needed to scale focus with those other lenses.

I've looked at the distance the lens moves front/back between focusing at 2m by scale and the point where it actually focuses at ~2m, and it's very small. There's no way I could accurately move the threaded mount that distance, even if it will move front/back (which I don't know). I suppose I could try to sand or grind the mount face bit-by-bit until 2m=2m, but that seems like a cheeseball solution.

 

The adapter has two pieces, and has a gap of about 2.5mm between the back of the M39-threaded piece and the m4/3 piece. I can't see any sort of spacer between those two pieces. The M39 ring is held in by three set-screws. I'm assuming these control the orientation of the lens to the top of the camera, but I can't see how they would control front/back distance.

 

Hi, Barrie, and thanks for those details--good stuff to know about.

But this is where I think I must be misunderstanding something. Shimming the lens or the face of the adapter outward would push the lens away from the sensor. But I thought that increasing the distance from sensor to lens *decreased* the distance to the in-focus subject. [EDITed for clarity] If that was true, then moving the lens farther out would make its maximum focus farther from infinity than it was before. But now I don't know if I've got this right or not. If somebody could put me straight I'd appreciate it.

 

OK, I was all ready to tear into the back of the lens looking for shims when, just in time, I remember my personal DIY rule-set:

1. Stop.
2. Put the expensive, delicate, and/or hard to replace thing away, well out of reach.
3. Breathe.
4. Think. Come up with something less risky.

During (4), I remembered reading that the slot holding the M39 threaded portion is asymmetrical. What if, I thought, somebody put it in backwards?

So step 5 was to remove and reverse the M39 ring. Now when I focus on a target at 2m, the focus scale says just over 1m! So the lens has gone from focusing short to focusing way beyond the infinite. But at least that means I could shim the adapter ring our rather than grinding it down or opening the lens.

Step 5(alt) was to also order a Fotodiox M39-m4/3 adapter. They claim to be able to guarantee infinity focus, so I'll give them a chance. (And return the adapter if the claim isn't true.)

In the meantime, the delicate expensive thing is going back in the case until the other adapter arrives. I hope to have this lens for years, so I can wait a few days in the interest of not destroying.

 

Yeah--that's part of why I think I must be missing something.

For now, though, I'm just going to keep the tools locked up and hope that the Fotodiox adapter will be better.  

 

Well, I'm still waiting for the Fotodiox adapter, but I've had the adapter apart a couple of times. I cleaned it up a bit and put it back together the original way, and then realized that focus would be correct if I used the f/4.5 DOF mark (for 35mm) as the focus mark, since the 2m mark now fell there when I tried to focus on a test sheet. So I put a piece of yellow striping tape on that mark so I wouldn't forget. Time permitting, I'll try to get some test shots to see if I'm getting the DOF I need.

 

Thanks, Barrie--I've been trying to find some good materials to do that with. I tried some aluminum-foil shims, but didn't quite find the right thickness. Plus they not very durable. I may go by the hobby shop and see if they have something a a bit thicker.

Do you make your shims circles (well, doughnuts...)?

 

Here's a boring-but-useful test picture with the offset focus mark. This was at f/8 with the focus at 2m, and it looks like everything's in focus:

 

Thanks, Barrie--I'll see what I can find that's a bit thicker.