Canon Vs Rokinon 14mm 2.8 lens for Astrophotography

Discussion in 'Photography Beginners' Forum' started by WhoFlungPoo, Feb 7, 2018.

  1. WhoFlungPoo

    WhoFlungPoo TPF Noob!

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    Hung up a newspaper started taking a few pictures to test the corners. I had the camera on a tripod and stood about a foot away which allowed me to fill the camera frame with the newspaper. Took the images at ISO100 and f/2.8. The pictures are posted below. Zooming in and inspecting them , I feel the upper right and lower right corner is not as sharp as the upper left and lower left. Will try and get some test shots at infinity focus in the next few days. What do you guys think about the focus at the corners?

    Thanks!

    [​IMG]

    [​IMG]
    on tripod with camera sensor parallel to newspaper


     
  2. TCampbell

    TCampbell Been spending a lot of time on here!

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    I agree. The right edge isn’t equal to the left edge with respect to focus symmetry. I also notice the barrel distortion (very wide angle lenses often have at least a tiny bit) looks a little off-center. As if the “optical” center of the image is a little left of the center of the frame.

    I see that your camera is on a tripod and appears to be parallel to the wall. If the camera were aimed just slightly right then the right side would be farther than the left and that could explain the non-symmetric focus... but it “looks” parallel to me.
     
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  3. WhoFlungPoo

    WhoFlungPoo TPF Noob!

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    Thanks, TCampbell. I appreciate your input.
     
  4. WhoFlungPoo

    WhoFlungPoo TPF Noob!

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    Its been awhile but finally got around to returning my original lens and testing the new one they sent me. This one is better than the last but feels like the image is darker on the right compared to the left and the upper right corner is blurrier than the other corners (it got better in the second picture at 1/13sec, i think i did a better job at focus here than I did in the first picture at 1/10sec). What do you all think and are these issues something easily corrected in post? I think this lens is better than the first one I got but not sure if I want to send it back as the next lens could be worse than this current one. Any input is appreciated. Thanks!

    [​IMG]
    iso100 f/2.8 1/10sec

    [​IMG]
    iso100 f/2.8 1/13sec
     
  5. TCampbell

    TCampbell Been spending a lot of time on here!

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    While it does appear darker on the right, the question is... is that the lens' fault? How is the paper lit? Is the light source symmetrically placed or is the light source coming from one side?
     
  6. kalgra

    kalgra TPF Supporters Supporting Member

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    @TCampbell which tracker did you use for this? Any thoughts on the iOptron SkyGuider Pro EQ 3550?
     
  7. TCampbell

    TCampbell Been spending a lot of time on here!

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    I use a Losmandy StarLapse: Losmandy StarLapse System

    Losmandy is a machine shop... everything they make is precision machined. They are well known for their fairly high-end telescope mounts... the Titan, the G11, etc. are very popular among astro-imagers. The StarLapse is actually just the right-ascension drive part of a Losmandy GM8 mount (there’s no declination drive). But they’ve modified the base... on a real GM8 there would be a latitude adjustment wedge, but the StarLapse is modified to fit on a photo tripod.

    It’s expensive as trackers go... I have the STLA-SYS version ($695) and the optional polar scope ($200) ... so I’m into it for about $900.

    It is a very good mount. The only “plastic” in the whole thing is the hand box. Every part of the tracking head is metal (they are a machine shop).

    I bought this back when the mount choices were the AstroTrac, the original iOptron SkyTracker and the Vixen Polarie.
    The AstroTrac was very expensive but has limited travel and not as versatile (but quality). The iOptron was the most versatile but the build quality and load capacity wasn’t great. Losmandy had a very high-end system and very versatile... no complaints about quality... just not cheap. So I went for it and I’ve been very happy.

    But SINCE THEN... Sky Watcher (who had no dog in the hunt at that time) has entered the market with their Star Adventurer. It really stole the market from everyone because it was higher quality than the iOptron and at a competitive price. This forced iOptron to redesign their product and they now have two products (both are competitive with what Sky Watcher has).

    Today... iOptron has two choices and Sky Watcher has two choices... I think they both have a high end version that claims to handle 11 lbs... and a lower end choice that handles roughly half the capacity.

    BTW, there does not seem to be an industry standard for that load capacity. Marketing people can probably say anything they want. In normal astrophotography (telescope) mounts, we typically tell people to only load the mount to about half (or maybe maybe 2/3rds) of what the manufacturer claims it can handle... that safety buffer to make sure it really handles the load.

    I’ve seen images from the “low end” mounts that were very good... so I don’t doubt the higher load versions are great.

    I do like my Losmandy... but the mounts on the market “today” were on the market when I bought my mount, I probably would have picked up one of them.

    Make sure you have a solid tripod (this is not the time to cheap out on tripod strength). You don’t want that thing to vibrate or flex while the shot is happening.
     
  8. beagle100

    beagle100 No longer a newbie, moving up!

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    for testing a tripod and a still subject ...... e.g. a wall is most important
    www.flickr.com/photos/mmirrorless
     

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