The correct flange focal distance: A prerequisite for the correct adjustment of a lens to infinity

[Andreas Thaler]

In the article Reveni Labs Autocollimator: Lens adjustment in the DIY workshop. An Olympus Zuiko 65-200/4 auto zoom is set to infinity, we looked at the use of an autocollimator to adjust the infinity setting of a lens.

To do this, we mounted the lens on an SLR, checked it with the autocollimator and adjusted the infinite stop on the lens.

To simplify and illustrate the process, we limited ourselves to adjusting the zoom lens to one focal length and did not check the infinity setting for the other focal lengths.

We saw that the autocollimator enables two things:

1. the light target, which represents a test subject at infinite distance, and
2. direct observation of how sharply this target is imaged on the film surface.

Doing this we assumed that the SLR itself on which the zoom was mounted was perfectly adjusted.

In particular, we assumed that the

Flange focal distance on the SLR

corresponded to the specification.

The flange focal distance is the distance between the contact surface of the lens—the lens mount on the SLR—and the film plane.

This distance is precisely specified for each camera system from a manufacturer. This ensures that a correctly adjusted lens focuses accurately on the film plane at infinity. If the flange focal distance is incorrect, the image will be either in front of or behind the film plane and therefore out of focus.

For Olympus OM, the flange focal distance is 46 mm.

The correctly set flange focal distance is therefore a prerequisite for adjusting a lens on an SLR with the autocollimator.

There are two methods for checking the flange focal distance of an SLR:

1. Viewing a test subject at infinite distance through a correctly adjusted lens directly on the film plane. This could be a tower on the horizon or the moon. For this purpose, a focusing screen is used, which is placed on the film plane and on which the test subject is imaged. If the flange focal distance is correct, the image will be sharp. This is a complex process that requires a correctly adjusted lens. Richard Haw describes this process in detail.
2. The flange focal distance is measured with a depth gauge. This does not require a test setup with a lens, and test subjects that are infinitely far away (from the perspective of the lens). The device measures the distance directly on the SLR.

Conclusion

1. In order to be able to adjust the infinity focus of a lens using an autocollimator, the flange focal distance of the SLR used must be correct.
2. The flange focal distance is the distance from the lens mount to the film plane of the SLR. This distance is specified for each camera system.
3. The flange focal distance can be measured using a test setup or directly with a depth gauge.

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In

Olympus OM-4: Measuring the flange focal distance with a depth gauge

we will look at how the flange focal distance of an SLR can be measured using a depth gauge.

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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.