I am trying to improve coma affecting one of my catadioptric telephoto lenses (Opteka 500mm DG), by adjusting the distance between two glass elements (FF group.. I have a crazy feeling the problem could be there because I know of good specimens out there and maybe Opteka decided to cut cost by omitting spacers..).
The idea is to glue the right lens to threaded retaining ring, so the distance can de varied.
I was thinking of using shellac or similar, because it is easily removed by isopropil alcohol.. but not sure where to get it from
Is there a better stuff for the purpose?
Perhaps PVP-based glue stick is OK?
Gluing the lens to retaining ring was successful, now I can vary the position of the assembly and see if coma can be improved by increasing the distance between elements.. If it gets worse, I will try to reduce their distance by machining the barrel by ~1mm or so..
In the meantime I tried the lens w/o field assembly on the Moon last night..
The focal length of the system is reduced from 500m to 278mm (from f/6.3 to f/3.5).
This could be useful for guiding.. maybe.
This weekend I combined the FF from Rokinon and the rest of Opteka..
Initial results strongly suggests the coma at Opteka is indeed caused by FF group, because this combination doesn't have coma any more.
It seems they (Samyang and Co.) are not very good in terms of assembly and quality control..
Samyang and Rokinon are DEFINITELY no good at quality control. I've not only experienced it with a new lens, I've read so much about the inability of many of their lenses to obtain focus at infinity.
Unfortunately I have an 85mm f1.4 I bought used from eBay and when I couldn't achieve focus at infinity the seller told me I had to use live view and p*ssed in my ear about how to obtain focus via some setting I knew nothing about because I didn't have live view and he knew that.
I should have returned it for an immediate full refund. It may focus stopped down to f4 or f5.6 but that's not why I bought an f1.4 lens.....
Yep, but new, return if focus can not be achieved at infinity.
I got my new 14mm f2.8 ED replaced immediately, the new one will achieve good focus but it's nowhere near the infinity mark on the lens and it's considerable fidgeting to obtain focus. I now have a red nail polish mark on the focus ring where infinity is sharp for astro. I spent hours out adjusting, running inside to look at the image on a screen I can actually see and back out to adjust focus again, and again, and again.. Four hours in one night running in and out after every 2 or 3 shots to see where it was closest.
Don't ask me about my new PK lens to Nikon F mount adapter from K&F with the glass element, it's a nightmare, infinity is at around 3 feet on the scale and far from clear. It works with a lens locked on macro at 80mm but nothing else and I have several old Pentax lenses, PK and M42.
... and when I couldn't achieve focus at infinity....
Leo,
Inability to reach infinity focus probably could be corrected by moving the mechanical stops.. I did this on my FD Canon lenses when converting them to EOS..
