Adapting Lenses To Micro Four Thirds

From a technical standpoint, the Micro Four Thirds sensor format is very well suited for adapting both modern and vintage lenses, even if they were initially designed for other formats and recording media.

From an aesthetic standpoint, an adapted lens can provide a unique look both for photography and videography.

Note: Although Four Thirds lenses need an adapter to be mounted on Micro Four Thirds cameras, they behave like native lenses and will be covered in a separate article.

Optical Aspects – Sensor & Lens

Preliminary remark: Adapting a lens does not change its f-number or focal length.

Sensor & Crop Factor - Advantages
When a lens made for larger formats is adapted to a Micro Four Thirds camera, only the center portion of the image circle is utilized. The interplay between the Micro Four Thirds sensor design and the crop factor is highly beneficial.

1) The light rays in the center area of the projected image circle are predominantly perpendicular to the sensor. As the micro-lens arrangement of a Micro Four Thirds sensor is designed with telecentric lenses in mind, the resulting match is perfect.

2) Lens vignetting is largely cropped as well and therefore drastically reduced.

Usually, adapting does not introduce any additional vignetting and existing peripheral darkening is cropped out. Edge-to-edge sharpness and contrast are maintained very well.

Sensor & Crop Factor – Disadvantages

1) The lens has to meet higher requirements concerning resolving power, as a smaller portion of the projected image is enlarged more than originally intended by the lens designer.

2) Assuming same sized prints, lens defects are enlarged more and therefore carry more weight than originally envisioned by the lens designer.

Lens designed for 24x36mm adapted to Micro Four Thirds.

Only adapt high quality lenses to get high quality results or embrace the lower fidelity as part of the aesthetic of the adapted lens.

Adapters

At its most basic, an adapter provides a connection between the lens and the camera.

Novoflex Leica M to Micro Four Thirds adapter.

Some adapters include optics to act as telecompressors and others even transmit limited information to the camera body.

Regardless of adapter type, smallest manufacturing tolerances are key to ensure parallelism and infinity focus.

Therefore, only adapters of the highest quality should be utilized.

Metabones Nikon F to Micro Four Thirds x0.71 telecompressor.

Telecompressors

A Telecompressor or focal length reducer (aka “Speed Booster”) utilizes an optical system to compress the diameter of the image circle. The effective focal length is decreased and more light is concentrated on the sensor, reducing the effective f-number. This has several consequences in terms of image quality:

+ Due to the compression, optical defects are compressed as well and therefore less pronounced. The image will also appear to be sharper.

- The optical system of the telecompressor might not concentrate the light perpendicularly to the sensor plane. This reduces the sharpness, contrast and resolving power in the periphery.

- The vignetting will no longer be “cropped” as effectively.

- The additional optical elements can affect the rendering, especially if cheap adapters with low quality optics are used.

Effect of a telecompressor on a lens designed for a 24x36mm sensor.

Autofocus Adapters
Some adapters allow for autofocus operation of the mounted lens. The performance can vary drastically depending on the camera, lens and adapter used. Expect to have to focus manually.

Tilt-Shift Adapters
Lenses made for larger formats project a larger image circle. Therefore, they can be adapted to a Micro Four Thirds sensor with a tilt and/or shift adapter to allow adjustment of the plane of focus and/or perspective. Expect compromised sharpness and resolution.

Recommended Manufacturers
For mechanical adapters: Novoflex.
For telecompressors & AF-adapters: Metabones.

Working Aperture & Data Transfer

Most adapters do not transmit aperture information or any other data to the camera. This has three major consequences in practice:

1) Lenses can only be used at the working aperture. On the plus side, focus shift is of no concern and depth of field is always previewed accurately. However, the plane of critical focus is harder to discern at smaller apertures. Also, Live View frame rates drop if too little light reaches the sensor (e.g. in low light or when using a combination of slow shutter speed and large f-number).

2) The exposure metering can be impaired. Depending on the camera model, using center-weighted averaging or spot metering can mitigate the issue.

3) The camera cannot perform automatic corrections at the RAW processing stage. In order to get the same image quality as with native lenses, manual post processing is required (e.g. removing CAs, sharpening).

Calibrating Sensor Stabilization

If no focal length information is transmitted to the camera, the IBIS system has to be manually calibrated.

Micro Four Thirds cameras provide an interface to enter the focal length of the mounted lens.

However, the focal length values provided by manufacturers are almost always rounded and even if they would be exact, they are only applicable when the lens is focused at infinity.

A native Micro Four Thirds lens mitigates any issues as it relays precise information about the focusing distance and focal length to the camera body.

Theoretically, in order to ensure best IBIS performance, the real focal length of the lens at the given focus distance should be set.

If the set focal length is too short, the IBIS will under-compensate and deliver less stabilization.

If the set focal length is too long, the IBIS will over-compensate and the resulting excessive sensor movement will blur the photograph.

Calibration

In practice, entering the standard focal length usually leads to acceptable results as camera manufacturers have incorporated tolerances in their IBIS systems.

To ensure peak performance and satisfying results, follow these recommendations for advanced users:

1) Test & Adjust
Test the IBIS calibration with the adapted lens at the desired shooting distance. Fine-tune for best results. At closer focus distances, focus breathing usually allows for longer focal lengths to be set. This will in turn increase IBIS effectiveness.

2) Zoom Lenses
Set either the correct focal length the lens is used at or set the shortest focal length of the lens, accepting compromised IBIS performance.

3) Turn Off
Deactivate IBIS when in doubt and utilize a tripod.

Note: If a telecompressor is used, take the compression factor into account.

Entering Lens Information

Some cameras like the OM System OM-1 allow the photographer to enter lens information including a custom lens name and the focal length.

When a registered lens is selected, the focal length setting is automatically transferred to the IBIS system. Additionally, the lens designation is stored in the EXIF-data.

Note

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