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Comment by ryandamm | Hacker News Reader

A note of possible interest for this crowd: all these motor types are open loop, so you can't actually command a specific state with any accuracy / repeatability. In other words, if you send a signal to the lens to focus at, say, 3m, it won't necessarily be in exactly the same state as the next time you focus at 3m.

This is why camera calibration can be tricky, and you often don't want to touch the cameras after you've done a calibration pass.

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The system stops the motors once it reaches focus. Therefore is a closed loop system.

It’s open loop in that the measurement of it being in focus is reliant on the subject matter, and a different measurement.

I’m not making this up, camera manufacturers have told me to my face that focus is open loop, period. They can’t guarantee repeatable focus.

Notably the measurement isn’t of the state of the motor/gearing. Furthermore, being “in focus” means the subject matter’s out of focus blur is below some threshold; there is a range of focus states that qualifies — but those seemingly small differences can affect camera calibration, with >pixel-level differences in effective focal lengths.

It’s open loop.

I can back you up on that. I work on a highly custom system that uses EF lenses through an EF-to-serial adapter. The lenses don't even know what position they're at when you power them on. If you want to move them to a specific focus step you have to go through a homing procedure that drives the focus motor to a stop (I don't recall if it's the near or far stop).

Lens-to-lens it's definitely not consistent. Step 142 on two different lenses will have a different focal depth. We calibrate each lens ourselves (thankfully you can read the serial number through the EF mount) and then still have to do closed-loop image-based sharpness estimation to guarantee that things are as good as we can get.

How could they then tell to what distance they're focused?

I used to have two Ultrasonic lenses, the 17-40/4L and the 17-55/2.8. They both had distance scales which would move around as the lens focused.

My current Olympus lenses have a focus-by-wire manual mode, with a distance scale on the barrel. The camera also reports the focus distance in the EXIF. Are these just vague ballparks?

Some - not all - of the EF lenses have a distance encoder as well. It's fairly approximate; I think it exists mostly for the benefit of the flash system, which needs a rough starting point for the distance in some of the open-loop modes.

For the current Olympus ones, I think there's a broadly similar encoder on the ones with proper manual focus scales, and the pure fly-by-wire ones reset focus at connection so the camera can work it out.

There's a list of the EF lenses and which forms of distance information they provide here: https://web.archive.org/web/20130425064359/http://www.lenspl...

Sorry for the late reply. What we’ve found is that they’re all close but not identical. Our system has a point LIDAR pointed at the ground and then uses that to drive the focal distance for the lens (as well as input for some other sensor fusion stuff). We generally run the lenses wide open (f/2.0 or f/2.8). Mapping step counts to distance would get us close, but unless we did calibration for each individual lens it wasn’t close enough to get the focus tack sharp. When you’re looking at the focal distance in your EXIF metadata you’re probably not validating whether that 2.4m distance to the subject was actually 2.3m and for the vast majority of cases the absolute accuracy of it doesn’t matter. Same with the distance markers on the focus ring… you’re not setting the focus within a few cm based on those marks, just getting it close enough that you can do closed-loop focusing with an eyeball in the loop.

Once we did the per-lens calibration curve we didn’t have to do it again. I’m assuming the small differences are just manufacturing tolerance and they’re relying on the focus/phase sensors to get it the rest of the way.

Correct. The AF sensor continually takes more samples even while the motor is moving. You can tell because sometimes the motor overshoots and then you can see it come back.

Yup… but it’s not necessarily in the same state if you do it twice. It’s close, but not identical. Even if the image metadata claims the focus point is identical, the lens in all likelihood is not in the same state and will have some deviation in its intrinsics.

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