Checking the focusing system of an Olympus OM-4. Definition of a workflow for checking and adjusting sharpness

[Andreas Thaler]

In several articles, we have discussed the flange focal distance. the infinity focus of lenses, and the autocollimator.

• We concluded that the flange focal distance of an SLR must first be verified through measurement.
• If this is correct, an objective lens on this SLR can be adjusted to infinity using an autocollimator.
• What is still missing is a test procedure that allows us to assess the accuracy of the focusing system (here we are limiting ourselves to lenses that are manually focused via a focusing screen).

It makes sense to use the autocollimator for this purpose, which projects a luminous target at an infinite distance relative to the SLR.

If we set the lens to infinity and use it to focus on the target, the image must also appear in focus on the focusing screen.

If this is not the case, and the flange focal distance and the infinity setting on the lens are correct, the problem can only lie with the SLR’s rangefinder system. Errors here may be due to the mirror or the focusing screen (misaligned).

So let’s continue working on our Olympus OM-4

whose flange focal distance we have already verified for and on which we adjusted an OM Zoom to infinity at its longest focal length (f/200 mm).

After checking the flange focal distance and the infinity adjustment on our Olympus set and confirming that they are OK, we can check the focusing system.

In the viewfinder of the OM-4, a subject at infinity must be sharply focused using the infinity setting on the lens.

To do this, we set up the autocollimator so that we can look through the viewfinder of the OM-4 into its lens.

As we know, the autocollimator projects a luminous target through its lens that appears to be infinitely far away for an SLR. So, if we set the lens to infinity, the target must be sharply focused in the viewfinder.

Test #1 with an Olympus OM Zuiko 65-200/4 auto zoom

The first image shows the target in the viewfinder at a focal length of 200 mm. Perfectly sharp.

That was to be expected, since we had adjusted the zoom's longest focal length using the autocollimator.

The second image at a focal length of 100 mm. Also sharp in the viewfinder.

The third image at 65 mm. Here it was difficult to capture the viewfinder image with the iPhone.

The sharpness isn't perfect. That's because we only performed infinity adjustment for the longest focal length (f/200 mm) but not for the shortest (f/65 mm).

Adjusting this focal length using the autocollimator should resolve the issue.

Conclusion

• We know that the flange focal distance of the SLR and infinity setting on the lens are OK for f/200 mm.
• Thus the image is sharply focused on the film. The image also appears sharp in the viewfinder.
• This means that the focusing system too works perfectly.
• If we use the autocollimator to adjust the zoom to f/65 mm as well, all focal lengths should produce a sharp image on the focusing screen.

Test #2 with an Olympus OM Auto-Macro 50/3.5

The infinity setting of the lens has not been checked with the autocollimator.

The first image shows the target in the viewfinder of the OM-4 with the focusing ring not set to infinity.

The split-image indicator splits the target as expected.

In the second image, the lens is set to infinity. The target is sharp and not split.

Conclusion

• The flange focal distance of the OM-4 had been verified before.
• The focusing system had also been verified.
• The macro lens produces an infinitely sharp image on the focusing screen.
• Therefore, it should be correctly adjusted to infinity.
• A check using the autocollimator should confirm this.

Definition of a workflow for checking and adjusting sharpness.

We will now combine all of our findings to check the sharpness of an SLR lens combination and, as far as possible, adjust it.

Step 1: Measure and adjust the flange focal distance on the SLR.

Step 2: Check and adjust the infinity setting on the lens.

Step 3: Check and adjust the focusing system of the SLR.

This allows for a systematic approach and helps to avoid experimentation and haphazard measures.

How can these three parameters be corrected?

1. To adjust the flange focal distance of an SLR, thin shims can be inserted at the junction between the front plate witmirror box, and body to increase the flange focal distance. To reduce it, material can be removed. According to SPT Journal, there were also modified lens mounts for certain SLRs
2. To adjust a lens’s infinity focus, we use the autocollimator and adjust its infinity stop. For zoom lenses, the shortest and longest focal lengths are adjusted. Here, it may be necessary to adjust the zoom mechanism as well (not yet researched).
3. To adjust the focusing system, we can check the position of the mirror and the focusing screen and attempt to adjust them. Anyway I cannot say whether this is possible without additional measuring instruments.

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The autocollimator, with its 3-watt LED, had its work cut out for it during all these tests. As a result, the heat sink gets quite warm and is mysteriously illuminated by the LED.

 

[Andreas Thaler]

Uncertainties in testing the rangefinding system with the autocollimator

Since I set up the autocollimator, I used it to test five manually focused Nikon prime lenses on a Nikon FG at infinity.

See

Nikon FG: A closer look at service and technology

I had previously verified the FG's flange focal distance by taking a measurement.

All were purchased secondhand; their condition ranges from like-new to visibly used, with no damage.

The results were sobering:

• two clearly cannot focus to infinity on the film plane,
• two focus well,
• and one focuses very well.

This means that at least two lenses need to be adjusted for infinity.

Normally, this isn’t a big deal. Under the autocollimator, the adjustment screws on the lens barrel need to be loosened and the infinity stop readjusted.

What surprised me, however, was that all of these lenses still produce a sharp image at infinity in the viewfinder.

I can’t say yet why that is.

Because, as I understand it, the distance to the film plane and to the focusing screen via the mirror should be the same. And if lenses produce different images at infinity on the film plane, that should also be the case on the focusing screen. I rule out any misoperation of the autocollimator.

New questions always arise when working on cameras and lenses—that’s how you learn

So I asked Matt Bechberger, the developer of the Reveni Labs Autocollimator and Reveni Labs Camera Tester, about this.

Here is his response which I am allowed to quote with his permission:

Hi Andreas,

Yes, this can be because the Autocollimator is much more sensitive than your eye on the ground glass. You are viewing the projected star on the film, which is very smooth, and the star is very small, only a few mm in diameter. You are viewing it through the Autocollimator with a fair amount of magnification. You can place some scotch tape at the film plane and see the star yourself.

When viewed on the viewfinder ground glass, you are limited by the „fineness“ of the texture of the glass, and you are not seeing the star as large, so it is harder to see the defect in focus at the very centre.

Thanks,

Matt Bechberger
Reveni Labs
www.reveni-labs.com
@revenilabs on Instagram

It follows, then, that I still have quite a bit of research work ahead of me.

Perhaps our experts @ic-racer and @Dan Daniel could share their experience and knowledge on this and offer their insights?

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A word of caution

Please keep in mind that you’ll be using highly flammable solvents for service work, and their fumes are harmful to your health. Soldering also produces fumes that should not be inhaled. Dangerous high voltages can be present when handling electronics, especially in conjunction with electronic flash units and mains. Therefore, familiarize yourself with the safety regulations beforehand and ensure your safety. Depending on the activity, protective gear may be advisable.

All information provided without guarantee and use at your own risk.

Acknowledgments

Without the work of Master Larry Lyells, in particular, we wouldn’t have the detailed technical information we need today for service/repair of SLRs.

The SPT Journal and The Camera Craftsman, which he helped shape significantly as an author, and which contain all the necessary information and instructions are available for a fee through Learn Camera Repair.

We owe our access to them to Eugene Pate, the founder of Learn Camera Repair.

 

[ic-racer]

Oh yes, the collimator magnifies the projected image on the film plane so it’ll be under more magnification than what you see with your eye through the viewfinder on the focus screen. Some cameras do have accessory magnifiers, for example, I have a right angle finder for my OM1 and that has a magnification function on it where it will increase the magnification of the viewfinder’s optics on the focus screen.

 

[ic-racer]

Also, you can use the collimator to project its little star onto the underside of the focusing screen and observe that with cameras mirror down.

 

[ic-racer]

Somewhat related thread here,

Setting SLR Mirror Angle With A Collimator

In most all SLRs the mirror pivots at the upper portion of the film frame. Mirror angle has very little effect in that area, as the pivot is not moved with the mirror angle adjustment. In the viewfinder, the top of the film frame is the bottom of the viewfinder image. So, adjusting the mirror...

www.photrio.com

 

[Andreas Thaler]

Thanks!