How Does a Leica S Camera Autofocus a Hasselblad Lens?

I investigate a question that has been bothering me. The answer can help unlock the value in Leica S autofocus lenses.

Introduction

I love photography and I am a Leica enthusiast. Because Leica photography equipment is expensive, I don't have much equipment and the most I can do is spend a significant portion of my free time reading posts in the Leica Forum and following the Red Dot Forum.

Recently, I have spent a lot of time reading on the Leica S medium format cameras: the Leica S2, Leica S 006, Leica S 007, and the Leica S3. I have been reading about the Leica 120mm S macro lens because it is one of Leica's only macro lenses that have autofocus. While reading about the 120mm lens, I started thinking more about how the Leica S autofocus system works. When Leica announced the S2 on September 23, 2008, Leica not only released native Leica S autofocus lenses for the S2, but Leica also announced a number of adapters that allowed other brands' medium format lenses to be used on the S system.

I was especially surprised when I read that the Leica S can autofocus a Hasselblad lens. As a camera enthusiast, this amazes me because the Leica S system and Hasselblad H system are entirely different systems; I am very curious as to how these 2 systems communicate. Hence, I decided to focus this article on answering this question. As a software engineer, I tend to look at things through a system design lens so I may scatter some comparisons to software engineering concepts like API's.

A Hasselblad H lens on the Leica S2. Source: Leica Rumors

Leica S-Adapter H

It turns out that Leica offers an adapter that "allows all the Hasselblad H-system lenses to be mounted on a Leica S body." Amazingly, "all lens functions, such as autofocus, central shutter control, and automatic diaphragm are retained." It seems that this adapter has electronic contacts on each side: the one on the front touching the lens and the one on the rear touching the camera body. There doesn't appear to be a chip inside the adapter that translates signals, but I could be wrong on this.

Questions & Assumptions

This leads me to a number of questions about how communication between the Leica S and Hasselblad H autofocus lens works. Below I have listed my questions and my assumptions which may or may not be accurate. By answering these questions, I can then prepare a system design workflow for how the Leica S autofocuses a Hasselblad H lens.

What types of electronic functionalities does a Hasselblad H lens have? More generally, what features does any autofocus lens have?

  • The lens has an autofocus motor.
  • It has a chip with a firmware that translates the mechanical state of the lens into electronic representations.
  • The chip writes signals to the camera: current autofocus distance, current aperture setting, current zoom distance.
  • The chip also reads signals from the camera: what distance to autofocus to right now, do I use the built-in leaf/central shutter, what aperture should I set this shot to, how fast do I trigger the leaf/central shutter (if one exists)

Does the Hasselblad H lens operate under some industry standard communication protocol for the features mentioned in the question above?

  • I am not sure about this, but chances are that some parts of its communications are industry standard communication protocols, and other parts are proprietary.
  • If Hasselblad collaborated with Leica on developing this mount and the software to support autofocusing its lenses on the Leica S, it can be assumed that Hasselblad provided Leica with the exact hex codes / communication messages to communicate with its lens firmware.

How does the Leica S determine autofocus distance?

  • I believe that autofocus distance is lens-agnostic: that if you were to mount any lens on the Leica S, it would be able to determine the distance to autofocus to.
  • The Leica S is a DSLR (digital single lens reflex) system. I won't go into the details in explaining that in this post, but the main thing to know is that there is some type of sensor close to the mirror that looks through the lens to determine whether a point is in focus, too close in focus, or too far in focus.
  • The Leica S uses a Phase-Detect AF system (Source: Neo Camera). This means that there is some beam splitter that splits the optical view and looks at the waves in order to determine if the point is in focus, too close in focus, or too far in focus.
  • The beam splitter probably uses some type of PID control to say, focus farther if the current image is focused too close, or vice versa, while making sure not to overshoot the correct focus distance by too much. The PID control system is not too dissimilar to how an autonomous car controls its steering wheel (don't turn too far left, don't turn too far right).
  • In conclusion here, the Leica S - regardless of what lens is mounted on it - should be able to determine whether the image is currently focused too close, just right, or too far. As a result, the camera should be able to communicate to the lens which direction to focus.

Can the Leica S do continuous autofocus or face detection autofocus?

  • In reality, I don't think the Leica S has a face detection autofocus feature, but for this exercise let us assume that it can. In other words, when the camera is pointed at a scene, if there is a face in that scene the Leica S should command the Hasselblad H lens to autofocus on that face.
  • I think that this is very doable, given the assumptions to the previous question.

System Design

After thinking through some questions before, here is my estimate of how a Leica S can autofocus a Hasselblad H camera with the use of the S-Adapter H:

  1. A photographer looks through the Leica S optical viewfinder and holds down the shutter button halfway to trigger autofocus.
  2. The phase detector within the camera sends a pulse of light out through the lens and reads it back. The phase detector determines if the lens is focused too close, too far, or just right.
  3. Without loss of generality, let's consider the case where the Leica S determines that the lens is focused too close relative to the subject.
  4. The Leica S sends an electronic signal from its pins to the S-Adapter H, which then sends that electronic signal to the Hasselblad H lens. We assume that Leica knows the "API", or communication protocol in order to send a command to the lens.
  5. In this case, the Leica S sends a signal to focus farther, since the lens is currently focused too close.
  6. As the lens actuates its motor to focus farther for a split second (e.g. 50 times a second), the camera's phase detector looks at the new image and determines whether the lens is focused close, too far, or just right. Repeat steps 2 to 6 in very very fast loops until the object is in focus, using some type of PID control to overshoot to be too far in focus, then overshoot to be too close in focus, then get the focus just right.

Conclusion

I have realized that the original question proposed in this article can actually be extrapolated to "How does a camera autofocus a lens?" It turns out that the only layer that the Leica-Hasselblad model adds is the communication protocol between the Leica S pins and the Hasselblad H pins. Unfortunately, I did not have time to investigate the electronic communication protocols, but I might follow-up with the Leica product specialist in San Francisco with some questions.

I think it is very fascinating that Hasselblad's lenses can be used on the Leica S system, and I wonder when the Leica S lenses can be used on other systems. Currently they cannot, but the ability to would surely increase the value of Leica S autofocus lenses!

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