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Should I keep this lens?

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#1 nyx

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Posted 19 June 2021 - 07:09 AM

Sigma Art 135mm, 40s sub shot with an astromodified 2000D. Look at the color shift on the stars: https://i.imgur.com/w6hdl3F.jpeg

 

Keep or return?



#2 Boeglewatcher

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Posted 19 June 2021 - 07:59 AM

sorry, I can't see any stars in the pic.

cs Joachim



#3 Aleksandr Naumov

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Posted 19 June 2021 - 08:01 AM

Looks like the lens is miscollimated: red halos are too fat and appears to be thicker to the left side, on a whole shot.

I would return it.


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#4 Aleksandr Naumov

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Posted 19 June 2021 - 08:05 AM

sorry, I can't see any stars in the pic.

cs Joachim

There is no stars visible when looking at the Imgur preview;

you could press three dots pictogram there at top-right and choose "download" to examine it using your favorite image viewer.

Attached Thumbnails

  • sigma135.jpg


#5 Boeglewatcher

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Posted 19 June 2021 - 04:05 PM

lens doesn't look suitable for taking astro photos (for my taste).

cs Joachim


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#6 nyx

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Posted 19 June 2021 - 04:09 PM

lens doesn't look suitable for taking astro photos (for my taste).
cs Joachim


Mine neither :)

#7 calypsob

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Posted 20 June 2021 - 05:39 PM

Im not sure you got a bad copy or what but my samyang 135's do alot better than that 



#8 Kevin_A

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Posted 21 June 2021 - 06:58 PM

Return and get a Rokinon/Samyang 135 f2.



#9 mrflibbles

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Posted 21 June 2021 - 08:37 PM

While I have never used the 135 samyang. I have a 16mm f/2 rokinon (Rokinon and Samyang are essentially the same) & it is excellent. Their lenses are optically really good. Their drawbacks being they are manual focus and manual aperture, but for astro this is a non issue, as you don't need it anyways. 

 

To me that looks like chromatic aberration. 

 

Every lens maker has their drawbacks. Sigma lenses are good walking around lenses but less than great for astro, that tend to be cheaper than their on brand counterparts, but suffer from CA and sharpness (when not expensive or sometimes even when they are).  Rokinon and Samyang are IMO good astro lenses, especially for wide angle and milkyway. They tend to compromise on bells and whistles rather than image quality. 

 

IMHO for what it's worth



#10 nyx

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Posted 22 June 2021 - 12:25 AM

Thank you all for your responses and suggestions. The reason I made this post was to pretty much confirm my fears. Finding a really decent lens for astrophotography seems difficult if not impossible nowadays.

 

As far as Samyang/Rokinon is concerned, it's a firm NO from my side. I have ordered so many Samyang lenses trying to find a decent one and they were all bad. If you think the Sigma above is bad, you haven't seen the Samyang samples I had to return. In fact, I've ordered so many of them (especially the 135mm) that I've been banned from ordering more products on one specific online vendor. So thanks, but no thanks. I wouldn't touch a Samyang lens unless there is an explicit note from the manufacturer inside the box that says "Allowed to return indefinitely till good sample is found". I'm glad you guys are happy with your Samyangs, though.

 

Oh and I already have the 16mm f/2. Also terrible. It was my very first lens and back then I had no clue about how bad lenses can be. Of course when I found out, it was too late to send it back.

 

Anyway, I've already returned the Sigma. I'll stick to telescopes for now.

 

Clear skies!



#11 Andy Lucy

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Posted 23 June 2021 - 05:19 PM

You might be interested to try the Askar FMA135 telescope (135mm focal length, f/4.5).  I have one and I've been impressed by it - much better quality stars than my Samyang 135mm f/2.

 

Andy



#12 nyx

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Posted 24 June 2021 - 02:22 AM

You might be interested to try the Askar FMA135 telescope (135mm focal length, f/4.5).  I have one and I've been impressed by it - much better quality stars than my Samyang 135mm f/2.

 

Andy

I did consider the FMA135, I just couldn't find any reports/reviews of it. There's also a video on youtube about a flexing issue:

 

https://www.youtube....h?v=QmbKmVpJx6o

 

Smells like QA problems.



#13 Andy Lucy

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Posted 24 June 2021 - 05:45 AM

QA problems are a nightmare. However, I can say that Askar appear to do some quality control as my FMA135 (which does not have a flexing problem) - came with a multipoint inspection check list (including optical collimation), signed and dated.

 

There are a few images in my gallery taken with the FMA135.  I reckon that centre image quality is very good, APS-C edges OK and APS-C corners less good, showing some aberration when viewed at 100%.

 

Andy



#14 calypsob

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Posted 24 June 2021 - 03:28 PM

You might be interested to try the Askar FMA135 telescope (135mm focal length, f/4.5). I have one and I've been impressed by it - much better quality stars than my Samyang 135mm f/2.

Andy

how about if the samyang is stopped down to f4.5?

#15 Andy Lucy

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Posted 24 June 2021 - 07:11 PM

I find that the stars are sharp with a Samyang 135mm if the lens is stopped down, but as the sharpness improves unsightly red fringes become more obvious (although not as bad as the example from the Sigma 135mm Art in the original post of this thread).

 

Here's an example (M92) from the Samyang which was taken with a 37mm stepdown ring (effectively f/3.6), on a ZWO ASI462MC camera (which has a very small sensor).  The subs had an average FWHM of 8.5 arcseconds (1.9 pixels), so sharpness was fine.  I've shown the aberration inspector view so that the red fringes can be seen clearly.  

 

integration_ABE_solve_PCC_HT_mosaic.jpg

 

Here's an equivalent image (different target - the Eastern Veil Nebula) from the Askar FMA135 using the same camera.

 

integration_ABE_solve_HT_aberrationinspector.jpg

 

Andy


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#16 chanrobi

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Posted 24 June 2021 - 11:52 PM

I find that the stars are sharp with a Samyang 135mm if the lens is stopped down, but as the sharpness improves unsightly red fringes become more obvious (although not as bad as the example from the Sigma 135mm Art in the original post of this thread).

 

Here's an example (M92) from the Samyang which was taken with a 37mm stepdown ring (effectively f/3.6), on a ZWO ASI462MC camera (which has a very small sensor).  The subs had an average FWHM of 8.5 arcseconds (1.9 pixels), so sharpness was fine.  I've shown the aberration inspector view so that the red fringes can be seen clearly.  

 

attachicon.gifintegration_ABE_solve_PCC_HT_mosaic.jpg

 

Here's an equivalent image (different target - the Eastern Veil Nebula) from the Askar FMA135 using the same camera.

 

attachicon.gifintegration_ABE_solve_HT_aberrationinspector.jpg

 

Andy

interesting info, I have the same lens I always thought it was my post processing that was fucken it up with the red around all the stars... now I know.

 

Any way of mitigating this? Either in lens or post?
 



#17 Andy Lucy

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Posted 25 June 2021 - 04:02 AM

I can't see how you could mitigate the behaviour of the Samyang lens, since stopping down the lens doesn't seem to help.

 

There are ways of removing red fringes in processing.  I believe that in Pixinsight it is possible to separate the image into the R, G, B layers and use morphological erosion or deconvolution to reduce the size of the stars in the red layer, then recombine the layers.  However, this is beyond my level of competence in Pixinsight.  Lightroom has a fringe removing tool:  I just tried this and it does help.

 

Andy



#18 whwang

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Posted 25 June 2021 - 09:11 AM

Sigma Art 135mm, 40s sub shot with an astromodified 2000D. Look at the color shift on the stars: https://i.imgur.com/w6hdl3F.jpeg

 

Keep or return?

 

Is this shot at maximum aperture?  The red fringe can be caused by the sphero-chromatic aberration.  It may go away if you shift the focus a little bit, or stop it down, or combine the two.  Focusing lenses like this can be tricky.



#19 nyx

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Posted 25 June 2021 - 09:42 AM

Is this shot at maximum aperture?  The red fringe can be caused by the sphero-chromatic aberration.  It may go away if you shift the focus a little bit, or stop it down, or combine the two.  Focusing lenses like this can be tricky.

Focused with a bahtinov mask and shot at f/2.8



#20 whwang

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Posted 25 June 2021 - 10:04 AM

I suggest you to focus without the mask with maximally enlarged live view on mag 1 to 3 stars that are not extremely red nor extremely blue.  Avoid stars brighter than mag 1, as its color can be completely washed out.  Focus at the aperture that you plan to use (or at most one stop wider).  Watch the change of star color when the focus transits from one side to the other.  You may see the star being red/purple in one side, and changing to green in the other side.  The mid point where the star has the least color is the focus you want to aim for.  And this focal position may not be identical to what a Bahtinov mask tells you.

 

I gave up using Bahtinov mask on lenses shorter than 200mm.  I can't consistently get satisfying focus from that, and I suspect that sphero-chromatism is part of the reason.  On telescopes and longer lenses, however, I find Bahtinov masks very reliable.


Edited by whwang, 25 June 2021 - 10:05 AM.


#21 Andy Lucy

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Posted 25 June 2021 - 05:58 PM

I suggest you to focus without the mask with maximally enlarged live view on mag 1 to 3 stars that are not extremely red nor extremely blue.  Avoid stars brighter than mag 1, as its color can be completely washed out.  Focus at the aperture that you plan to use (or at most one stop wider).  Watch the change of star color when the focus transits from one side to the other.  You may see the star being red/purple in one side, and changing to green in the other side.  The mid point where the star has the least color is the focus you want to aim for.  And this focal position may not be identical to what a Bahtinov mask tells you.

 

Having read Wei-Hao's comment above, I realise that posting an image showing red fringes (as I did in post 15) does not show that the lens will always produce red fringes - as it depends on the focusing position and the amount of sphero-chromatism.  Looking back through my library of Samyang 135mm images, almost all have severe red fringes but I found one that only had fringes in part of the image.

 

My Samyang mounted on a Nikon APS-C camera (I have used 3 different Nikon cameras) always showed clear differences in focus between the right and left sides.  The Pixinsight aberration inspector image below reveals this - and shows that the right side is in focus and has clear red fringes while the left side is out of focus and has no fringes.  

 

I now suspect that the methods I have used for focusing with the Samyang (mostly Bahtinov mask, sometimes FWHM measurement) result in a focus position that gives red fringes.  In the image below I appear to have mis-focused and serendipitously set a position which gave less overall fringing. 

 

It hardly seems ideal that a lens can give either sharp well-focused stars, or minimal fringing, but not both at the same time.

 

Andy

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  • Samyang 135mm f2 Nikon D7100 aberration inspector.jpg


#22 whwang

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Posted 25 June 2021 - 09:06 PM

My Samyang mounted on a Nikon APS-C camera (I have used 3 different Nikon cameras) always showed clear differences in focus between the right and left sides.  The Pixinsight aberration inspector image below reveals this - and shows that the right side is in focus and has clear red fringes while the left side is out of focus and has no fringes.  

This must be caused by a tilt in the optical axis.  If all three cameras show the same symptom, then the problem is on the lens.

 

I have a Nikon 80/1.8G.  It's sharp, but produces red/purple stars in one side and green stars in the other side.  The stars are almost equally sharp, but have different colors.  I shimmed the lens mount with adhesive aluminum tape and the problem is greatly reduced.  This is one way to save the lens if you don't want to return it.  I did this because I missed the return period.

 

There can be all kinds of manufacturing errors on camera lenses.  Most of these will not show up on normal photography but will become very annoying on astrophotography. I often tell people to avoid Samyang because of its QC.  But the truth is, this can happen on all brands, Nikon, Sigma, and even Zeiss.  We have to live with it.




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