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Rokinon 135mm f/2.0 - keep or exchange?

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#1 DubbelDerp

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Posted 03 December 2019 - 11:12 AM

Hello all,

 

So after struggling with vintage prime lenses and the CA associated with them, I decided to spend the cash on a Rokinon 135mm f/2.0 prime lens. I got a chance to try it out last night, but after looking at some of the light frames today, I have some concerns about the quality of the particular lens I got, and am hoping for some more experienced feedback as to whether this one is a keeper for AP or if I should exchange it for a different one. Specifically, I think I'm seeing a tilt in the lens that when focusing at a star in the center, the left and right sides of the frame are out of focus in different directions on an APS-C sensor.

 

Here's a crop from the center of the frame with a Bahtinov mask...

center_mask.jpg

 

Left and right sides of the frame:

left_mask.jpg  right_mask.jpg

 

Center of the frame on M45:

center_m45.jpg

 

Left and right sides of the frame:

left_m45.jpg  right_m45.jpg

 

I'm not seeing much difference between the lens wide open and stopped down three stops in terms of what I think is OOF. I had the lens supported by the camera on my skyguider pro, so I suppose there could be flex from the weight of the lens causing a tilt. Would supporting only the lens with a collar be preferable, or should they both be supported on a rail? I did take a single frame at M31 right about at the zenith, and I did not see any noticeable difference in how the sides of the frame were out of focus, so I don't think I can entirely blame flexure of the camera body. How would you recommend mounting this lens on a camera tracker?

 

Any feedback would be greatly appreciated on what I should expect with this lens!



#2 Jim Waters

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Posted 03 December 2019 - 01:06 PM

The quality control on Rokinon lens is hit-or-miss.  Especially on lens 35mm and below.  I would send the lens back.



#3 tkottary

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Posted 03 December 2019 - 01:29 PM

This does not look right .

#4 Poochpa

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Posted 03 December 2019 - 03:06 PM

I think you have a bad one and would send it back. I had to do that with a Rokinon 16mm lens. I just acquired a used Rokinon 135mm. Here's the PI Aberration Inspector image of one of my initial test shots with the 135, using a Canon T4i, 60s, ISO 800. Not perfect, but not bad. Focus was a little off.

Mike

IMG_0871_AbInspect3xmosaic.jpg


Edited by Poochpa, 03 December 2019 - 03:21 PM.

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#5 DubbelDerp

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Posted 03 December 2019 - 04:02 PM

Thanks for the feedback.. It confirms what I was thinking, that this would be best sent back for another. I do like how this lens gulps down the photons, but the reason for getting it was to get away from poor performance at the edges of the frames, not to replace one set of issues with another. But for less than 90 minutes of integration time, this lens is pretty incredible! This is a center crop, but you can see some blobby stars on the left side of the frame. Hopefully the next one is better.

m45_135mm.jpg



#6 DubbelDerp

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Posted 03 December 2019 - 04:03 PM

I think you have a bad one and would send it back. I had to do that with a Rokinon 16mm lens. I just acquired a used Rokinon 135mm. Here's the PI Aberration Inspector image of one of my initial test shots with the 135, using a Canon T4i, 60s, ISO 800. Not perfect, but not bad. Focus was a little off.

Mike

attachicon.gif IMG_0871_AbInspect3xmosaic.jpg

Thanks for the visual reference! If mine had performed that well, I would have been happy indeed. Back it goes...



#7 Poochpa

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Posted 03 December 2019 - 05:00 PM

Thanks for the feedback.. It confirms what I was thinking, that this would be best sent back for another. I do like how this lens gulps down the photons, but the reason for getting it was to get away from poor performance at the edges of the frames, not to replace one set of issues with another. But for less than 90 minutes of integration time, this lens is pretty incredible! This is a center crop, but you can see some blobby stars on the left side of the frame. Hopefully the next one is better.

attachicon.gif m45_135mm.jpg

Nicely processed M45, despite the lens aberrations!

Mike



#8 Kevin_A

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Posted 03 December 2019 - 06:09 PM

the 135mm f2 lens is a very sharp lens and unfortunately it is a lens that you will love only when you get a non de-centerred sample. Fairly common to have one or 2 bad corners.... so send it back and try again as most of us who own them did. Use it at f2.8 as it is even better stepped down ever so slightly.


Edited by Kevin_A, 03 December 2019 - 06:11 PM.

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#9 DubbelDerp

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Posted 03 December 2019 - 06:10 PM

Nicely processed M45, despite the lens aberrations!

Mike

Thank you! I keep thinking I’ve pushed the background too high, but I just love seeing the billowy dust clouds start to appear. The fact that there’s even a hint of them after 90 minutes has me excited about getting a good copy of this lens. 



#10 DubbelDerp

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Posted 03 December 2019 - 06:15 PM

the 135mm f2 lens is a very sharp lens and unfortunately it is a lens that you will love only when you get a non de-centerred sample. Fairly common to have one or 2 bad corners.... so send it back and try again as most of us who own them did. Use it at f2.8 as it is even better stepped down ever so slightly.

Thanks, will do. I just wanted to double check that this was unacceptable before doing the exchange. Appreciate the feedback!



#11 Coconuts

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Posted 03 December 2019 - 10:05 PM

My third Samyang 135 mm f/2.0 lens was a keeper.  B&H is great in that regard.  But I'm not sure if the tilt issues I ran into on units 1 and 2 was due to improper build, or simply a delicate design, where differences in shipping trauma resulted in different performance.  If the latter is true, then transportation while in my hands could eventually degrade the performance of my good unit.

 

One other comment: at f/2, image degrading focal plane tilt can easily arise for objects off the zenith from the torque of this lens on your camera bayonet mount.  I prefer to rigidly mount the lens, and allow the lighter and shorter camera to hang off the rear of the lens.  Details on how to do this are in my CN post, here:

 

https://www.cloudyni...-camera-lenses/

 

Before you send the lens back, try imaging at the zenith...

 

All the best,

 

Kevin



#12 ChristopherBeere

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Posted 04 December 2019 - 07:45 AM

Definitely send that one back those stars are unnaceptably defocused at the edges


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#13 DubbelDerp

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Posted 04 December 2019 - 10:37 AM

My third Samyang 135 mm f/2.0 lens was a keeper.  B&H is great in that regard.  But I'm not sure if the tilt issues I ran into on units 1 and 2 was due to improper build, or simply a delicate design, where differences in shipping trauma resulted in different performance.  If the latter is true, then transportation while in my hands could eventually degrade the performance of my good unit.

 

One other comment: at f/2, image degrading focal plane tilt can easily arise for objects off the zenith from the torque of this lens on your camera bayonet mount.  I prefer to rigidly mount the lens, and allow the lighter and shorter camera to hang off the rear of the lens.  Details on how to do this are in my CN post, here:

 

https://www.cloudyni...-camera-lenses/

 

Before you send the lens back, try imaging at the zenith...

 

All the best,

 

Kevin

Thanks for that... I saw your thread previously but didn't find it in my 30-second search.

 

Here's a single shot of M31 near the zenith. I'm definitely seeing the same focus pattern. There's almost an in-focus band from about the 11 o'clock position to about 5 o'clock, with the stars progressively worse farther outwards. I think if I overlaid the single frame and the m45 stack, the pattern would be pretty much the same.

 

M31-singleframe.jpg




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