Samyang 135mm F/2 focus issues with ASI585mm pro585MM Pro camera

Forgot to add something; I know the sensor seem to protrude from the camera housing, so I initially thought perhaps it was too close, hence why I was adding 1-5mm of spacers. But as mentioned it only threw me off more.

Jan Erik Vallestad:
Forgot to add something; I know the sensor seem to protrude from the camera housing, so I initially thought perhaps it was too close, hence why I was adding 1-5mm of spacers. But as mentioned it only threw me off more.

TiffsAndAstro · Dec 23, 2025, 11:41 AM

Jan Erik Vallestad:
Forgot to add something; I know the sensor seem to protrude from the camera housing, so I initially thought perhaps it was too close, hence why I was adding 1-5mm of spacers. But as mentioned it only threw me off more.

There should be a mark on the camera body to show sensor position like a o with a line though it.
Might be useful to double check your back focus

Also, what is that spacer between camera and lens?

I’ll double check that! That is the ZWO Nikon F adapter. The one made for 1.25” efw and small format cameras.

Carastro · Dec 23, 2025, 12:16 PM

You may need to slightly modify your lens; https://www.youtube.com/watch?v=Yk2myVjAFlU

Don’t be afraid, I have two left hands with ten thumbs and still was able to perform the modification without issues. a magnetic mat is of great help to avoid losing the tiny screws. After all, Murphy is always watching us …

Good luck and clear skies

Wolfgang

I had my lens modified like this recently as I was having difficulty getting focus with some of the filters, and it worked a treat.  Great video and thanks so much Nico for posting it.  
I got some-one to do the Mod for me (who had done it befor) as I didn't have the tools to do it.  

Carole

Yes, the mod is helpful to many. I actually didn’t need to do it on any of my lenses, but did so anyways because I like the extra wiggle room for the EAF. I’ve used these lenses for years, but never had issues like this with any of my other cameras. Everything from 2600mc/mm, 533mm to 678mm/mc works a treat.

kuechlew · Dec 23, 2025, 06:18 PM

I’m extremely puzzled by your issue. In the ZWO ASI 585 the sensor is recessed by 6.5mm just like in the ZWO ASI 533. So if you reach focus with the 533 - as you claimed - it’s totally incomprehensible why you don’t with the 585. Finding focus with the 533 and then replacing the 533 with the 585 should do the job. With the additional wiggle room of the lens modification it’s even more puzzling.

I have not the slightest idea if it will work but you may try the ZWO Nikon Adapter for 2 inch Filter wheels: https://www.teleskop-express.de/en/adaptors-10/adapter-camera-bayonet-132/zwo-nikon-lens-adapter-to-m54x0-75-thread-e-g-for-2-inch-filter-wheels-14737

It has only 14 mm optical length and then you need an adapter to reduce to 48mm like this one: https://www.teleskop-express.de/en/adaptors-10/adapter-m48x0-75-206/baader-adapter-m54i-to-m48a-16779

It claims to have an optical length of only 1 mm, so you would end up with 15mm in total. Since you’re 2-3 mm off you would need another 2-3 mm of spacers

As am I. I used these lenses for a project only a few months ago, and have been using it with my ASI678 cameras since. I may have to do a teardown and test infinity focus manually during daytime. The fact that it “should” be correct, but just isn’t is what drove to the forums just in case there was something obviously wrong that I was missing.

I have all of ZWO’s adapters (the only difference is extra spacers that are detachable) so I may try that. I also have a better and 1mm shorter one from Aliexpress that I might try to fit in.

kuechlew · Dec 24, 2025, 12:06 AM

Jan Erik Vallestad · Dec 23, 2025, 10:10 PM

I may have to do a teardown and test infinity focus manually during daytime.

Does this mean you so far only tested to focus with autofocus? In this case your focus range may be limited by the auto focus unit. You may not reach focus due to this limitation. I assume you already thought about that but I point it out just in case.

With this specific camera, yes. But there aren’t any limitations except for the hard stop of course. The ZWO EAF has enough steps to play around with so the belt hits the hard stop and that’s that.

Hopefully it’s just a “d’oh!” moment once I decide to bring it in to delve into the problem a bit more but we’ll see over Christmas. I’ll update the thread once I figure it out.

Merry Christmas!