Olympus Pen FT lens focus error

alanrockwood said:

Hi.

I just bought a 42mm f/1.2 lens for my Olympus Pen FT. It has a focus error. When I set the lens to the most extreme focus position (nominally at infinity) it actually focuses to about 13 meters. That's pretty close to correct focus, but not quite.

I wonder if there is a way to repair the lens so it focuses properly at infinity.

I calculate that at infinity setting the optics of the lens need to be about 0.13 closer to the film plane.

Any thoughts?

For example, in theory a negative lens of about minus 13 meter focal length (anywhere between about 10 and 15 meters), mounted in a screw in filter setting would work, but I doubt if one could find such a lens.

Alternatively, if there are some shims inside the lens maybe thinner shims could replace the ones that are in the lens.

Thanks.

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4season said:

Assuming that the camera body is known to be good, I'd suspect that the lens focus helicoid was screwed together incorrectly. Often, the things can be fit together in a number of different ways, but only one will be correct.

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alanrockwood said:

When I set the lens to the most extreme focus position (nominally at infinity) it actually focuses to about 13 meters.

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Ian C said:

Usually, helicoid-focus lenses have 6 thread starts. If the helicoids were off by one thread, that would spoil the focus in a major way. As you’ve described it, the error is smaller than I’d expect with the helicoids off-index by one thread start.

I’m not familiar with Pen F lenses. I found a file showing an exploded view of the 38 mm f/1.8 lens. It appears to be of conventional design. I think a more likely cause of the problem is the focus ring out of position relative to the outer helicoid body. The two are connected via several setscrews.

You can see this in the file penview3.jpg accessible in the following link:


Usually, you can index the infinity focus on an SLR lens by focusing on a sufficiently distant target, setting the focus ring to the infinity index, and snugging the setscrews (carefully, since they are threaded into the aluminum alloy focusing ring).

If you don’t want to deal with it, a competent camera repair shop can do this quickly and easily. It would likely need the camera and lens to make the adjustment.

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koraks said:

Does the viewfinder image agree with the image as projected on the film, or are these out of alignment?
With SLR's, problems sometimes occur with the mirror stop, resulting in focus inconsistencies between the ground glass/viewfinder image and the film plane image. You can verify viewfinder and film plane image consistency by placing the camera on a tripod and aim it at a distant spot (looking out of a window at the horizon etc.), then open up the back and position a piece of ground glass (or any suitable substitute, as long as it's rigid and flat) on the film gate. Then compare the image projected onto the film plane with the viewfinder image (use bulb mode or a very long shutter speed). This IMO should be the start of any further diagnosis, since the routes towards a solution will be fundamentally different depending on the outcome of this simple test.

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alanrockwood said:

I should add that I also have a 38mm f/1.8 and a 40mm f/1/.4 lens, and for those lenses the infinity focus agrees with the microprism focusing aid in the viewfinder.

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koraks said:

OK, I wasn't aware of this, but that isolates the issue to the one lens. Ignore my post and focus (heh) on the lens in question. Sorry!

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Ian C said:

To see the drawing and be able to resize it as needed, in post #4, click onto “Repair manual olympus pen f.”

Then scroll down to the blue link “service manual for the Pen FT” and click on it.

Scroll down to near the bottom of the page where you see the blue link

>> Download document <<

and click on this. It will take several seconds to download. You’ll see a list of 4 files. Save them. The drawing you need is the jpeg file “penview3.” It’s easy to resize to zoom in to see the details.

The setscrews that lock the focusing ring to the outer helicoid body are located on the periphery of the focusing ring. They thread into the focusing ring and their ends bear against the outer helicoid body. When the focusing ring is turned, that rotates the outer helicoid body to attain focus.

In the exploded view of the 38 mm f/1.8 lens, the relevant parts are:

Setscrews NU-L7 UB 20 and NU-L7 23.

Focusing Ring 5680

Helicoid Assembly 5673

There is also screw 5025 that attaches to the outer helicoid body that might have something to do with setting the helicoids as well. I’m uncertain of its function.

Here's the drawing as an attachment.

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