Samyang 14mm possible lens problems need some advice please

[Danjc]

I'm trying to work out if I have a decent Lens, I have done a little research but I'm still non the wiser.

My focus may not be the best but the attached image does not seem very consistent focus wise and there is a slight lightness to the image in the bottom left corner that was present in all the images I took so not cloud.

This is a guided 300s sub iso 800 stopped at F4 Canon 1100D with a Skytech CLS

Cheers, Dan.

 

[michael.h.f.wilkinson]

Without more accurate focus it is difficult to see if there is an issue with the lens. A lighter area in a corner might be amp glow more than a lens problem (did you use darks to correct for this?)

[Danjc]

No unfortunately not on this occasion as time was extremely limited 

4 minutes ago, michael.h.f.wilkinson said:

did you use darks to correct for this?

 

[jgs001]

Try mounting the camera setup on a tripod, and take a picture of a wall (boring I know ;)). It may help to highlight any issues... it does seem, from that shot that the focus on the right of the image may be a little better than the left.

[Danjc]

1 minute ago, jgs001 said:

Try mounting the camera setup on a tripod, and take a picture of a wall (boring I know ;)). It may help to highlight any issues... it does seem, from that shot that the focus on the right of the image may be a little better than the left.

I will give that a go now and post up the image. 

[tooth_dr]

Dan - I've just started lens imaging myself and I've found focus can be really critical.  If it's 99% the defects are exagerated and also where in the frame you choose to focus has an impact.  I've found I have to focus in one corner in particular with my Nikon lens.  It does look off though what you have posted above.  Can you try focusing on the left side of the image next time?

[Danjc]

7 minutes ago, tooth_dr said:

Can you try focusing on the left side of the image next time?

I can but must admit I was struggling to find focus even when I had the iso @ 6400 and zoomed @ x 10 in live view I couldn’t see anything even on a bright star.  I was also using APT. 

[AbsolutelyN]

I'd also suggest taking a pic of a brick wall - keep it parallel to avoid any distortion. I bought one of these a few years ago and the test revealed one side of the frame completely out of focus, even when stopped well down. They can be great lenses but you need to be sure to get a good copy as I don't think they are the most consistent regarding quality control of the optical alignment.   

[Danjc]

[kirkster501]

To the OP.  I have experienced this too and your picture is almost as if you grabbed one off of my laptop.  I think this is down to focus and it being minutely critical at such a sort FL.  I also agree, even at high ISO with ten sec exposures and a Bhat mask, it is very tricky to focus.  I think this is the problem we both have, not a bad lens.  And I am all ears as to how to get the focus right.

[kirkster501]

EDIT: When I am home later I will share one of my stretched subs.

[jgs001]

I'm gonna guess that the camera sensor was not totally parallel to the wall, as you have a little more barrel distortion on the right hand corners than the left. The focus, to my eye, does look a little softer to the right of the image, that might be due to the distortion effects, but...

 

For focussing, if you have an android device (tablets work really well) with a USB OTG port... connect your camera and use "DSLR controller". This allows you to use the liveview and really zoom in on a much larger screen. You can't use all it's features with the Samyang, as it's fully manual focus, but the app will allow you to remotely control the focus of the lens directly at varying speeds, and works really well. You may need to look into back button focusing to decouple the focus activation from the shutter button if you want to give this a go.

[SteveNickolls]

Not sure on your particular model but there are 4 lens reviews here for Samyang 14mm lenses which might shed light on the design-https://www.lenstip.com/lenses_reviews.html

Cheers,
Steve

[Alien 13]

1 hour ago, Danjc said:

I can but must admit I was struggling to find focus even when I had the iso @ 6400 and zoomed @ x 10 in live view I couldn’t see anything even on a bright star.  I was also using APT. 

I am a bit surprised by this, I can normally get hundreds of stars on the live view screen but only if the focus is good.....I expect the live view image on a PC screen using APT or similar is no where near as good at spotting stars as the camera one.

Alan

[Danjc]

4 minutes ago, Alien 13 said:

I am a bit surprised by this, I can normally get hundreds of stars on the live view screen but only if the focus is good.....I expect the live view image on a PC screen using APT or similar is no where near as good at spotting stars as the camera one.

Alan

I also tried with the camera screen but I suspect my focus was out/user error as I was surprised I couldn’t see anything at all !

More clear nights needed ?

[Danjc]

47 minutes ago, jgs001 said:

I'm gonna guess that the camera sensor was not totally parallel to the wall

This should be better, and only apple devices.

 

[iapa]

6 hours ago, Danjc said:

I can but must admit I was struggling to find focus even when I had the iso @ 6400 and zoomed @ x 10 in live view I couldn’t see anything even on a bright star.  I was also using APT. 

To check star focus I invested in some bahtinov masks (from Gerd Neumann https://www.gerdneumann.net/deutsch/astrofotografie-parts-astrophotography/bahtinov-masks-bahtinov-masken.html); basically replacement filter but slotted to give a cross pattern you can use to get very close focus.

If you use one of those you could set for focus on any particular star, then take an image. and inspect it to see if there is a significant focus issue across the field.

[kirkster501]

A Bahtinov mask for a small refractor will work just as well.  Or make one.

[Danjc]

I do have a Bahtinov mask that will fit/work spot on. I guess I’m used to focusing live with my ED80 so was in that train of thought opposed to taking a short sub with the mask on and finding focus in that manner.