Samyang/Rokinon 135 - to EAF or not to EAF. Generic equipment discussions · Steven · 2/2/2022 · 25 · 2026 · 4

Hello, just a quick one to get some info of someone with experience on the Samyang / Rokinon 135mm.
Just ordered this lens and hoping to turn it into a third little wide field rig for imaging.
(Playing around at F2 should be fun compared to the F5's of my other rigs)

I'm now working on figuring out the best mounting solutions, which comes with the question: 
how well does it maintains focus during the night? And how often do you adjust it?

as I'm seeing conflicting stories depending on where you're looking, which is understandable at F2. And I'm trying to figure out if I should be buying mounting solutions that are able to hold a ZWO EAF... or;  if I can save a bit of money and just go for a standard mounting solution + bahtinov mask.

And I'd rather not waste money on the more expensive mount + another ZWO EAF it if it isn't needed. 
Or find out later I should have spent the money because it was needed and need to buy an entire different mounting solution.


Thanks!

Dan Kearl:
Mine holds focus... they are so wide you hardly have to polar align.
Stars are so small that Bahtinov mask is not really necessary either to get them in focus.
I use mine in the most simple star tracker mode with just ball head and camera and it works fine.

I don't own the Samyang 135mm (yet) - too much Canon glass lying around - but based on my own experience in this focal range I can only agree with Dan's statement. However, you don't mention what camera you intend to use. In case you're going to use the ASI 183 - as in your published images - it will turn into an effective focal length of almost 400mm and things may look quite differently.

In your case I would try out Dan's recommendation to come up with an own conclusion. If you then need more precision you may still go for the EAF.

CS
Wolfgang

I don't have the samyang (I wish I have) sadly but to give a bit of prospective; with my nikkor 300mm AFS @ f/2.8 I have to refocus about every 2 hours if the temperature drops more 3 degrees.

I have Samyang 135 F2.0
It's a great optics but you write "could be exciting to play around f2 but in my experience it is almost impossible to work in as there is so much and not least short FL that exposure time will be so short but you are welcome to try .
I do not want to take the excitement from this but as it is it is impossible and I can max work in F2.8 or it will be overexposed.

Hi Steven, 

What a great question, and very timely from my perspective as I contemplating the same question for my Nikon 200mm f2 lens.  

I am moving towards an autofocus solution for my camera lens, largely influenced by @Michael Feigenbaum (whose work I very much admire) who has recently set up his Samyang 135mm with an Astrodynium ring + ASIAir automated focus system.  Indeed the more I looked, the more purpose-built automated focus solutions I found for the Samyang 135mm lens (DeepSkyDad).  

This existence of this market - specifically for the Samyang 135mm lens - suggests there is a need. Or at least a perceived one. 

My own experience with first my Canon 200mm f2.8 (operated at f4) and now my Nikon 200mm (operated around f2.5, courtesy of a milk carton wedge) is that the need is very much driven by lens quality, f-ratio and temperature change.  No surprises there.  My Canon lens rarely needed refocussing during the night, unless the temperature changed by over 5C.  In contrast, I notice significant de-focussing with the Nikon after 2degrees temp change.  Currently we are going from 30C days to single digit dawns, so temp change is a big issue here.    

Getting the focus point accuruate to begin with is also important (no s**t, Sherlock) and previously a Bahtinov mask was "good enough" to give me sub-2pix on the Canon.  But again the Nikon is much more challenging to focus accurately this way, and is really at the limit of my dexterity.

For these reasons, I have also chosen to go with an automated solution, and have ordered another ZWO EAF, a bracket from TS and a 3D printer cable and pulley to lash the focusser to the focus ring on the Nikon.  We will see how well this works.   All up this cost around USD300, which is a small extra on top of a system which I hope will give me many hours of (unattended) pleasure.

CS Brian

Thank you @Brian Boyle !  I am a big admirer of your work too…. I would highly recommend some kind of mounting apparatus like the Astrodymium simply because you can use an EAF with it.  My experience in Arizona where there can be 10°C or more temperature change through a night is that the Samy did not hold focus all that well. The EAF works wonderfully with the lens and the hardware for using it which comes with these Astrodymium type devices.  

When I bought the lens I had exactly the same thing in mind, a nice very wide field setup for traveling or home use.  But I really did not want to be refocusing every hour so I put it on the shelf.  Then I happened upon this solution and I could not be happier.  It is a fantastic lens and with that nice 67mm aperture at f/2 quite a little light bucket.

I use mine with the small pixel ASI183mm with an electronic filter wheel and it is very nice.  I do intend to use the ASI294mm in unlocked bin mode (2.315um pixels) for my next images because of the bigger 4/3 sensor. 

Hope this helps!

The Samyang @ f/2.8 is just 48mm of aperture. It would be no better than a Redcat 51 and in fact I expect it to be worse by a margin in terms of light collecting abilities.

No way to get decent focus without EAF, some examples with different sensor and f/stop and fiters:

F2.4/APS-C https://www.astrobin.com/g9jgyd/D/
F2.0/ZWO 533 https://www.astrobin.com/g9jgyd/D/
https://www.astrobin.com/4y9la8/?nc=&nce=
https://www.astrobin.com/h9q0cw/B/?nc=&nce=

Hope it helps

CS

I love the Samyang 135mm. When stopped down to 2.8 its supersharp. I highly recommend the EAF with this lens. The focus shifts a lot during the night, at least in mine conditions, so the EAF is a must for me. This was my setup until I recently bought the Astrodymium system. 

to those using an EAF with the Samyang 135: how do you deal with the hardstop at infinity? while mine focuses a bit "short" of infinity for stars, there is not much room right of focus to get a V curve.

Matthias

Let's try to use the "reverse" setting in your favorite acquisition software for the AF routine

CS

I had the same focus issue with the lens using a filter wheel.  Ended up that I had to remove the stop block in the lens to get it to focus at infinity. It was a simple procedure and can easily be reversed.  @Nico Carver has an excellent video showing exactly how to do this on his YouTube channel.

For anyone using the autofocus, what are your step size settings and other details for ensuring the autofocus both can get enough to create a V curve, and then can adjust the focus not too much?

I've got a ZWO EAF hooked up to the Rokinon using ASIAir, but keep having difficulty in that the EAF doesn't nail the focus and goes too far. So usually I have to manually adjust it to just nearly infinity focus half the time which defeats the purpose of EAF!

Phil Creed:
Mono might be a different story, but I've never felt the need for an EAF using OSC.

Even at old-school-Megadeth levels of screaming fast, it's still 135mm focal length.  Roughly 6"/pixel for many cameras.  I just focus with a Bahtinov mask and call it good.   By the time the delta T might warrant changing focus, I've already collected enough light at such fast f-ratios to call it a night, anyway.

Clear Skies,
Phil

Same here. I’ve had the lens imaging all night a few times now and never had to refocus. The 3D-printed kits that let you attach the EAF are neat but I generally prefer to secure the lens with a sturdy tripod collar.