Hey everyone, I can’t get any of my M12/CS/C mount lenses to focus on my Raspberry Pi HQ Camera. I’m running a Raspberry Pi 4B. I’ve been adjusting the back ring and the front ring on the lens in just about every combination I can think of, and still nothing sharp. The weird part - if I hold the lens in my hand and move it closer to the sensor (with the back ring taken off completely), an image starts to come into focus. It really feels like the lens just isn’t sitting close enough to the sensor when it’s fully adjusted down. Did I get a bad set, or are these simply not compatible with the HQ camera? The kit didn’t come with any instructions, so I’m sort of guessing my way through this. I’ve honestly never gotten it to work.
Thanks for the details, that’s a good clue already. A few quick questions so we can narrow it down:
- Which lens are you testing (focal length)?
- Are you using any C/CS-to-M12 adapter, and which one?
- When you say ‘back ring’, do you mean the adapter ring that screws onto the HQ camera, or the lens’s own back-focus ring?
The fact that it only focuses when you physically bring the lens closer to the sensor usually points to a back-focus / flange distance problem rather than a dead lens. Let’s confirm before blaming the hardware.
It’s the M12/CS/C mount lens set, and I’m testing on the Pi 4B with the HQ camera. I’ve tried both the back ring and the front ring adjustments on the lens itself, in various combinations. The only time I ever see anything sharp is when I hold the lens by hand a bit nearer to the sensor with the back ring removed entirely. When it’s screwed down all the way it still doesn’t get close enough.
Okay, that’s really useful. Here’s the thing with the HQ camera - the sensor needs a specific distance from the lens mount (back focus). C-mount is roughly 17.5mm flange distance, CS-mount is about 12.5mm. If you’re stacking the back ring/adapter and the lens can’t get any closer, you’ll never reach focus no matter how you turn the rings.
Two things to try:
- Remove the back ring/adapter completely and see if you can reach focus (even if only held in place for a test).
- Double-check whether your lens is actually C-mount vs CS-mount - a C-mount lens needs different spacing than CS.
Also make sure you’re turning the actual focus ring and not just a lock/thumbscrew ring.
So removing the back ring does let me get focus when I hold it, but obviously I can’t mount it that way. And I honestly can’t tell if these are C or CS - there are no markings and no paperwork in the box. Is it possible the back ring itself is too tall and stops the lens from sitting close enough? Are these defective, or just incompatible with the HQ camera? That’s the part I’m stuck on.
From what you’re describing it sounds more like a spacing/back-focus issue than a manufacturing defect, but let’s try to confirm. If you can, tell me the height of the back ring and which focal length you’re testing. Also, if you have any other mount or camera to test the lens on, that would tell us whether the lens itself is fine.
The behavior you’re seeing - focus only when the lens is closer than the mount physically allows - is the classic symptom of a back-focus limitation, not necessarily a bad lens.
I don’t have another mount to test with right now, and no calipers, but the back ring looks like it adds a good few millimeters. The lens I’ve been testing is a wide one (maybe ~6mm? not sure, there’s no labeling on it). Is there any way to make this work on the HQ camera, or should I be looking at a different approach?
Got it. Here’s where we’re at: the lens only focuses when you physically move it closer to the sensor and skip the back ring, which points squarely at a back-focus / flange distance mismatch - the back ring doesn’t let the lens sit close enough to the HQ camera sensor. That’s a compatibility/spacing issue rather than a confirmed defect.
I can’t give you a fix that guarantees focus with this exact combination as-is, since every mount/adapter stack changes the distance. If your setup allows, try a shorter or no adapter; otherwise a lens designed around the HQ camera’s spacing is the safer bet. It’s also worth posting this in the Arducam forums (https://www.arducam.com/forums/) so other HQ camera users can weigh in, and there are mount/back-focus diagrams in the docs at Arducam Wiki. If you can get the lens focal length and back ring height, reply back and we can dig further.