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

[Andreas Thaler]

In The correct flange focal distance: a prerequisite for the correct adjustment of a lens to infinity, we saw that

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.

We will put this into practice

and measure the flange focal distance of an Olympus OM-4 with the depth gauge.

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



I use a lockable cable release, the depth gauge, two gauge blocks and a rubber cylinder.

Calibrating the depth gauge



This depth gauge has a digital display. The measuring rod can be turned in and out of the bridge using a knob and handle. The device measures the movement of the rod with an accuracy of 1 micrometer (1 micrometer = 0.001 mm = 1×10⁻³ mm).



Before use, the depth gauge must be calibrated using two high-precision gauge blocks. For this purpose, I use a 30 mm high gauge block, which I place on a larger gauge block (50 mm) as a base.

Both blocks are made of steel and lightly oiled to prevent corrosion. Before use, they must be cleaned and then coated with a film of oil again. I use Ballistol, which is also known as gun oil.

The depth gauge is firmly placed with its bridge on the smaller gauge block and the measuring rod is turned until it touches the lower gauge block and stops. This position is entered into the device as 30 mm. The depth gauge is now calibrated.



Measurement on the camera

1. The larger gauge block (50 mm) is placed on the two inner film rails and must lie completely flat. (There are also the outer film rails, on which the film pressure plate rests. The specific rails to which the flange focal distance refers is often listed in the manufacturer’s service manual and in the SPT Journal for the respective SLR).
2. The rubber cylinder is put on the gauge block, the camera is rotated 180 degrees and set down.
3. I use the cable release to open the shutter in B(ulb) mode. The mirror flips up and the shutter opens. Both remain fixed in this position. The surface of the gauge block is now visible on the film plane.
4. The depth gauge is placed on the lens mount with its bridge (I measure the centre of the film plan here).
5. The measuring rod is extended downwards until it touches the gauge block and stops.
6. Now the measured value, the flange focal distance, can be read: 46,000 mm. That corresponds exactly to the specification.

Several comparative measurements recommended

Since we are working in the micrometer range here, even the smallest irregularities affect the measurement result.

The depth gauge must be carefully calibrated, the gauge blocks must be meticulously clean, all levels must be precisely aligned with each other.

The depth gauge must be pressed lightly against the gauge blocks and the lens mount, and the handle must be turned slowly in the final phase until the final value is displayed.

Minor deviations are unavoidable, so it is advisable to take several measurements in order to obtain a valid result.

Whether the film plane is parallel to the lens mount can be determined by taking measurements at various points on the gauge block.

 

[Andreas Thaler]

Flat granite surface plate as a reference

After writing this post, I invested in a heavy granite surface plate that is flat and serves as a reference for my measurements.






When calibrating the depth gauge, two contact points provide greater stability than one. The gauge blocks must be placed flat on a level surface to prevent them from tilting and causing measurement errors. The flat surface plate ensures that.

For this purpose two measuring blocks or several composite gauge blocks can be used. The surfaces of the blocks are flat and can be slid together (wringing).

The SLR camera also rests stably on the measuring block with its film rails. In the photo, we see a Nikon FG being measured on a surface plate.

The rubber cylinder could lead to measurement errors due to its elasticity; as said we are dealing with micrometers here.

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In the final part of this series, we'll explore whether we can define a workflow for the entire focusing system of an SLR.

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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.