Samyang (Opteka) 500mm F6.3 - Another low cost lens for AP

[bojan]

Being a fan of catadioprics, I was intrigued by this lens for some time..
When I saw the add on ebay, I decided to bite in.
The preliminary results are attached below.
Lens is decently sharp in the centre, a bit of coma in corners.
Not easy to focus, but this has to be done only once in a while anyway... I plan to add the ring which will fix the focus barrel..

Not bad for $150

[mikerr (Michael)]

The results are impressive for $150.00 lens.
What is the object in the third photo?

Michael

[bojan]

Hi Michael,
Yep, it is a good deal..
The object on third photo is C/2014 Q2 Lovejoy

[Regulus (Trevor)]

Thx Bojan, I have been considering these for a while now but just wasn't sure. There is also a 600, and an 800 I think, that it would be good to see some tests from. Astro test, rather than land shots that is.
So it looks like it is quite capable for the price they ask.
Trev

[bojan]

Here you can find more star tests for this lens (and some others):
http://www.astrobin.com/search/?q=sa...se=5&license=6

It is interesting to see Bob Atkins website:
http://www.bobatkins.com/photography..._500_f6-3.html
The focus he managed to obtain with this lens is unnbelievably bad.. I think he made a mistake (or he didn't have enough patience to focus properly, and this is not easy task due to backlash in the focussing mechanism. The focussing when lens is pointing to the horizon may be disturbed by changing the direction... to the zenith for example.. because the corrector lens can potentially slide back.
This problem can be sorted out like on those images:
http://www.iceinspace.com.au/forum/a...e.php?a=142353
http://www.iceinspace.com.au/forum/a...e.php?a=143393
If the lens is used only for AP, this solution is not a problem, as the focussing must be done only once.. and then checked from time to time.
I focussed my Rubinar last year.. and the focus is still spot on.

[ReaPerMan (Paul)]

I have just received this lens today. I'm hoping to get out and try it with some moonage tonight but currently the clouds are starting to roll in! Here is a shot of the school opposite my house with a range of about 400 meters. Not bad and I agree that the focus can be very precious. The focus was helped by the A7S having focus peaking where you can magnify the focus area up to 8X.

I'll try to get some astro shots up as soon as I can.

All the best

Paul

[ReaPerMan (Paul)]

Quick Moon Pic Centre crop from A7S

EDIT:

Now second pic after cleanup and processing

[gregbradley]

Nice. Magnified view is better for critical focusing rather than focus peaking. If you use focus peaking its better to have it set to low so the area highlighted as in focus is minimal. I find there is still a small adjustment to be made from magnified view over focus peaking.
The image is good but it looks like the focus is off a touch.

Greg.

[ReaPerMan (Paul)]

Thanks for the advice Greg. The difference in focus between in and out is a travel of 1-1.5mm. The action takes place completely within the borders of the infinity mark! The ring is fairly stiff and I could of taken a longer time fiddling to get it right but i was getting ready to go to work. I'm also thinking of making a Bhatinov mask and using this with the 8X zoom to get as close to focus as possible.

all the best

Paul

[bojan]

This spring arrangement (there is another one on the other side of the lens) removes focus backlash.
Because the focussing tread is quite steep, springs are intentionally not parallel with lens axis, to compensate for the tendency of the focus ring to rotate in the opposite direction due to spring tension.