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Purple halos on stars

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#1 Stille

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Posted 03 December 2019 - 05:16 AM

I'm getting serious purple fringing on images taken with a Samyang 16mm f2.0 on a Canon Rebel T7i - they look ehh straight out of camera, but once I apply PI's autostretch the whole image purples.

 

Does anyone have any idea what I did wrong here?



#2 Ishtim

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Posted 03 December 2019 - 07:35 AM

You may not be doing anything wrong... and If you haven't had it longer than the seller's return period, you might want to send it back for replacement.

I had a similar experience with a f/1.4 24mm.  

In my situation, I was past the seller's 30 day return period, but < 1 yr. warranty.  I had to deal with the Samyang/Rokinon.  After four months, and two trips to the technician, their response was "we spoke to an authority on astrophotography and he claimed that a purple coloration to stars is typically the norm and actually more seen as being more “natural”.

 

IMO, unless you are doing some HDR or other on-camera processing that would cause purple fringing, I'd swap it out for another copy...


Edited by Ishtim, 03 December 2019 - 07:36 AM.

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#3 Stille

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Posted 03 December 2019 - 08:00 AM

I got this second hand, so no returns possible. I've also used it before on a Canon 350D - didn't manage to hit focus right without live view, but saw no purple fringing either. 

 

LOL@the authority on astrophotography, would be fun meeting that "expert" face to face.


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#4 sg6

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Posted 03 December 2019 - 09:04 AM

Purple finging sounds like CA where the Green is focussed and so the Red+Blu is a little out and the 2 combine to a make Purple fringe.

 

Remembering that a DSLR is sort of built for Green.

 

PI autostretch may not be right I suspect for AP images. Auto would imply more everyday images like people on beach, mountains, landscapes not small, dim dots on a blackish background.

 

What is the result if you drop from f/2 to f/4 ?

Nice, wide and fast sounds good, just rarely works.


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#5 Stille

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Posted 03 December 2019 - 09:43 AM

PI = PixInsight, dim dots on a blackish background is p. much everything that program does :) 

 

One thing I realize now might be relevant - I did the focus on a star right at the edge of the FOV because it was bright and I was lazy, so I'm thinking that if the lens has a slightly curved focal plane (which it likely does, Samyang not being Zeiss) it's quite likely that what's in perfect 100% focus on the edge is on the edge of focus in the lens, just enough to throw the red + blue out of focus. Don't have access to my raws on a computer screen right now, but from what I can see on my phone, it looks like the edge stars don't show that same purpling, so hopefully the problem here was in front of the liveview screen, trying to focus in the corner of the lens :)

 

I'll likely end up dropping from f2 for tracked astro anyway, to get rid of the coma. Have to test this further next time we get a clear night...



#6 Stille

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Posted 05 December 2019 - 06:27 PM

Had another look at the raws and they seem less bad-looking than the stacked result. But then again I don't know that much about how much CA is acceptable. If anyone would like to take a look at them and tell me how bad it is, I'd appreciate it!

 

https://we.tl/t-hVrmU4aCK2



#7 Ranger Tim

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Posted 05 December 2019 - 09:24 PM

Utilize filter step-down rings to control aperture and you won't have diffraction spikes. Almost all camera lenses display some amount of fringing (CA), and stopping them down can help a lot. AP is demanding stuff for optics.



#8 Nightfly

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Posted 06 December 2019 - 06:39 PM

Purple fringing (PF) may be completely normal for a particular lens, especially fast lenses made with conventional glass.  Lens design at the edges of the short end of the color spectrum (violet) is a critical test.  Stopping down, even modestly, usually helps.  Alternatively, the use of a wratten 2A, or better yet, a 2E can eliminate PF.

 

Better lenses address the issue.  My Samyang 16/2 is well tamed. The 35/1.4 Sammy however is poor at 1.4, but virtually gone at f/2.  At f/2.8, it is a perfect lens.  



#9 KLWalsh

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Posted Yesterday, 08:11 PM

The physics of diffraction and wavelengths means that blue (or bluish) fringing will more apparent around stars than other colors.
A weak amber filter could reduce some of the blue you’re getting.

Diffraction spikes due to the iris diaphragm in a camera lens can be minimized by rotating the camera slightly, every so often, when imaging a given object.


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