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FORUMS Photography Talk by Genre Astronomy & Celestial Talk
Thread started 05 Sep 2017 (Tuesday) 15:16
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Is this normal?

 
TomCruise06
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Joined Jun 2017
Post has been edited 1 month ago by TomCruise06.
Sep 05, 2017 15:16 |  #1

Hi all,

I bought the Rokinon 14mm 2.8 recently for Astro photography.

Please see this image attached (cropped)and let me know if the lens is good? The stars are like distorted. I am an amateur so not good with the technical terms or to find if the lens is flawless. I am still u see the 1 year warranty if that would help.

Thank you so much for your help.

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katodog
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Sep 05, 2017 20:13 |  #2

First problem is focusing, that's always a trick getting perfect. The other thing is that you have to remember that the Earth rotates, which makes the sky move. Unless you're using a tracking mount you're not going to get pinpoint stars.


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TomCruise06
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Sep 05, 2017 23:22 as a reply to katodog's post |  #3

I totally agree that I am going to get star trails and I am fine with that. But if you see the pic above, those are not trails. Some sort of distortion like - each star has a beard. Is this something called the mustache distortion???

Is this something wrong with the lens or all the lens has this??

Should I return it for a replacement?




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Inspeqtor
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Sep 05, 2017 23:45 |  #4

I agree it looks like a focus problem. What was your shutter speed, f/stop, ISO?


Charles
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Roy ­ A. ­ Rust
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Post has been edited 1 month ago by Roy A. Rust with reason 'clarify details - add image'.
Sep 06, 2017 01:49 |  #5

TomCruise06 wrote in post #18445240 (external link)
Hi all,

I bought the Rokinon 14mm 2.8 recently for Astro photography.

Please see this image attached (cropped)and let me know if the lens is good? The stars are like distorted. I am an amateur so not good with the technical terms or to find if the lens is flawless. I am still u see the 1 year warranty if that would help.

Thank you so much for your help.
thumbnailHosted photo: posted by TomCruise06 in
./showthread.php?p=184​45240&i=i68009171
forum: Astronomy & Celestial Talk

That's what bokeh looks like in photos of stars - little round doughnuts where they are supposed to be specks. Bokeh is an indication that the stars are out of focus. You didn't indicate what camera you are using, but hopefully, it has a decent monitor screen with live view.

The best way I've found to focus properly, is to use live view, find a nice bright star for a rough focus, center it, then zoom in as much as possible with the live view, focus again, then switch to a dimmer star and repeat the fine touches to focus until the faint star is as sharp and bright as you can get it. I also use a magnifier to view the star in live view as I make the final adjustments. The reason for switching to a fainter star, is that it won't even show up unless it's really close to being perfectly focused, and it occupies only a few pixels on the screen when it's focused properly. Once it's focused, leave it alone when changing objects, too. Everything in the sky is at infinity, so when one star is in focus, everything else will be, too.

I had to go to a folder a couple of years old to find one of mine that looked something like yours. More recently, I just delete them, but I still take some out-of-focus pictures - I just don't keep them.

The little bokeh effects around your stars aren't symmetrical, though, which probably also indicates a slight bit of chromatic aberration around the edges of the image. A better focus will minimize that, too. That doesn't account for the flare on the left, hopefully, you know what caused that.

This is my out-of-focus shot...

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And a more recent one. I cropped this one to show at 100% with the stars in focus...

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MalVeauX
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Sep 06, 2017 04:43 |  #6

Heya,

Your lens is fine, you're simply a little out of focus.

Very best,


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TomCruise06
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Post has been edited 1 month ago by TomCruise06.
Sep 06, 2017 07:12 |  #7

Thank you very much to all.

@Inspeqtor - It was 13 secs at f/2.8 ISO was around 1000.

@Roy - Highly appreciate your detailed explanation. I used a 80D.

But, I did not focus using the method you mentioned and never knew about it. The flare to the left was due to the lighthouse Cape Hatteras.

The Rokinon 14mm f/2.8 I have is a AE chip version so when I focused on a bright star(I think it was Jupiter) it beeped confirming auto focus. My mistake, I relied on it instead of my eyes. ߙ

Does that chromatic aberration mean I cannot use this lens for night street photography where the picture will have bokeh - in this case the bokehs will be asymmetrical?

Pardon me for my ignorance.




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repete7
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Sep 06, 2017 07:41 |  #8

I don't have one with an AE chip, but for astrophotography you really to need to focus manually with live view. You also can't rely on the distance markers on the lens. On my version of the lens, it is focused on the stars close to (but not at) infinity. On a friend's version, the stars are in focus close to the 2 meter mark on the lens.

Roy A. Rust wrote in post #18445672 (external link)
The best way I've found to focus properly, is to use live view, find a nice bright star for a rough focus, center it, then zoom in as much as possible with the live view, focus again, then switch to a dimmer star and repeat the fine touches to focus until the faint star is as sharp and bright as you can get it. I also use a magnifier to view the star in live view as I make the final adjustments. The reason for switching to a fainter star, is that it won't even show up unless it's really close to being perfectly focused, and it occupies only a few pixels on the screen when it's focused properly. Once it's focused, leave it alone when changing objects, too. Everything in the sky is at infinity, so when one star is in focus, everything else will be, too.

Is there some trick to finding a bright star to focus on? I really struggle with finding a star in live view to magnify.


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Roy ­ A. ­ Rust
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Sep 06, 2017 14:14 |  #9

repete7 wrote in post #18445777 (external link)
I don't have one with an AE chip, but for astrophotography you really to need to focus manually with live view. You also can't rely on the distance markers on the lens. On my version of the lens, it is focused on the stars close to (but not at) infinity. On a friend's version, the stars are in focus close to the 2 meter mark on the lens.

Is there some trick to finding a bright star to focus on? I really struggle with finding a star in live view to magnify.

If you aren't pretty close to infinity with your focus, even bright stars won't show up in live view. There are a couple of simple tricks I use to get me there really quickly.

During the daytime, I focused the camera on a really distant object with the lens in Auto-Focus mode. After it was focused, I turned off the camera. The focus ring locks in place while it's off, so stays at the infinity setting. Without changing anything, I stuck a couple of small pieces from a white label on the focusing ring and the body of the lens, adjacent to the ring. Then, I used a straight edge across the labels, and made a mark across them to line up the focusing ring at the infinity point. This gives me a starting point for a rough manual focus on a star when it's dark. A sharp focus will be somewhere really close to where the lines on the labels line up. On one of my lenses, the barrel of the lens rotates as I zoom, so I put a label at both ends of the zoom range. There's enough space between the elements on the lens barrel that the label doesn't interfere at all with any movement of the lens.

On some lenses, the distance markers are in a window, and the focusing ring will rotate freely without changing the focus in Auto mode. This suggestion won't work with those, but I can use the infinity mark in the window to help get in the ballpark when I'm using one of those lenses. All of the auto-focus lenses I have now have white labels on the barrels to help with manual focusing at night.

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Then, in order to help me find a star to focus on, and later, to point it at the part of the sky I want to photograph, I made a mount for a red-dot rifle sight to point it at a star, and adjusted it so when an object is sighted in with the red-dot sight, it's right in the middle of the frame. When I get a star lined up with the red-dot sight, I can depend upon it being in the frame when I start to focus, and with the lines on the label lined up, I know it's pretty close to being in focus. Then I can make fine adjustments to get it as nearly perfect as I can.

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I found out a long time ago, that it's hard to tell where the camera is pointing, so I made the mount for the red-dot sight. Now, it's a simple matter to point it exactly at the object I want in the picture.

Hope this helps....



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repete7
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Sep 06, 2017 14:41 |  #10

Roy A. Rust wrote in post #18446062 (external link)
If you aren't pretty close to infinity with your focus, even bright stars won't show up in live view. There are a couple of simple tricks I use to get me there really quickly.

During the daytime, I focused the camera on a really distant object with the lens in Auto-Focus mode. After it was focused, I turned off the camera. The focus ring locks in place while it's off, so stays at the infinity setting. Without changing anything, I stuck a couple of small pieces from a white label on the focusing ring and the body of the lens, adjacent to the ring. Then, I used a straight edge across the labels, and made a mark across them to line up the focusing ring at the infinity point. This gives me a starting point for a rough manual focus on a star when it's dark. A sharp focus will be somewhere really close to where the lines on the labels line up. On one of my lenses, the barrel of the lens rotates as I zoom, so I put a label at both ends of the zoom range. There's enough space between the elements on the lens barrel that the label doesn't interfere at all with any movement of the lens....

Hope this helps....

The trick with the tape is definitely helpful. The distance marks on the Samyang/Rokinon lenses aren't reliable. Thanks for sharing your rifle sight rig. Cool set up!


Karen Flickr (external link)
Canon 6D2|Canon Eos-m|Canon SL1|Canon ef-m 11-22|Canon ef-m 22|Samyang 14mm f/2.8|Canon 24 stm|Canon 40 stm|Canon 50 f/1.8 stm|Canon FD 50mm macro|Canon Macro 100L|Canon 10-18 stm|Canon 55-250 stm|Canon 24-105L IS USM II|Canon 70-300 IS II USM|Canon 100-400L|

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Roy ­ A. ­ Rust
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Joined Dec 2015
Dallas, Texas
Sep 07, 2017 09:32 |  #11

TomCruise06 wrote in post #18445762 (external link)
Thank you very much to all.

@Inspeqtor - It was 13 secs at f/2.8 ISO was around 1000.

@Roy - Highly appreciate your detailed explanation. I used a 80D.

But, I did not focus using the method you mentioned and never knew about it. The flare to the left was due to the lighthouse Cape Hatteras.

The Rokinon 14mm f/2.8 I have is a AE chip version so when I focused on a bright star(I think it was Jupiter) it beeped confirming auto focus. My mistake, I relied on it instead of my eyes. ߙ

Does that chromatic aberration mean I cannot use this lens for night street photography where the picture will have bokeh - in this case the bokehs will be asymmetrical?

Pardon me for my ignorance.

Your lens is fine. Virtually all lenses have a bit of chromatic aberration around the edges, especially zoom lenses. Viewed at normal size on-screen or in usual print sizes, CA won't be apparent. If you print posters, it might show up around the edges. Bokeh will only be apparent on bright objects that are out of focus in the image. Unless you are taking fairly close up street photos of objects at night, most of the images will appear to be focused, and bokeh won't be a problem. I doubt that your lens has any more CA than most I use, and I take quite a few night street photos... love the effects of stars above the neighborhood with lights on!




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TCampbell
Senior Member
Joined Apr 2012
Sep 10, 2017 10:20 |  #12

That looks like 'coma'. A lens element isn't inserted correctly.

See: https://en.m.wikipedia​.org/wiki/Coma_(optics (external link))

If this is a new lens, then you should probably swap it for another copy.




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Inspeqtor
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Sep 10, 2017 11:06 |  #13

TCampbell wrote in post #18448536 (external link)
That looks like 'coma'. A lens element isn't inserted correctly.

See: https://en.m.wikipedia​.org/wiki/Coma_(optics (external link))

If this is a new lens, then you should probably swap it for another copy.

You have a bad link here sorry to say...

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Charles
Canon EOS 60D Gripped * Canon EOS XSi * Flickr Account (external link)
Tokina AT-X Pro DX 11-20 f/2.8 * Sigma 17-70 f2.8-4 DC Macro OS * Sigma 150-500 f5-6.3 APO DG OS HSM
Canon 18-55 IS Kit Lens * Canon 70-300 IS USM * Canon 50mm f1.8 * Canon 580EX II *** iOptron 3302W Star Tracker

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TCampbell
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Joined Apr 2012
Post has been edited 1 month ago by TCampbell.
Sep 11, 2017 08:42 |  #14

Inspeqtor wrote in post #18448575 (external link)
You have a bad link here sorry to say...

thumbnailHosted photo: posted by Inspeqtor in
./showthread.php?p=184​48575&i=i260365758
forum: Astronomy & Celestial Talk

Hmm... it seems the website has embedded a character in the link that breaks it. That tiny blue link icon is supposed to be after the closing parenthesis but for some reason it's located just before the parenthesis and it breaks the link. I'm guessing the website software is used to some links being in parenthesis and assumes the closing parenthesis isn't really part of the URL (but in this case, it is part of the URL.)

I'll try using the website's feature to add a named link...

Wikipedia: Coma (Optics) (external link)




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Davenn
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Oct 01, 2017 21:57 |  #15

TomCruise06 wrote in post #18445762 (external link)
T...............
The Rokinon 14mm f/2.8 I have is a AE chip version so when I focused on a bright star(I think it was Jupiter) it beeped confirming auto focus. My mistake, I relied on it instead of my eyes. ߙ
...............

Pardon me for my ignorance.


never ever use auto focus for astrophotography
ALWAYS use manual focus :-)

Dave


A picture is worth 1000 words ;)
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