HI all! I hope you are well.
I am trying to get to the bottom of an issue I am having with some new equipment I have (Modded Canon 6D, Askar ACL 200 lens).
I've been doing some tests and from what I can tell, there appears to be some misalignment - perhaps with the adaptor (m48), the lens itself or the newly modded 6d itself. Difficult to isolate so to try and mitigate/reduce any tracking errors I used only short exposures (8 seconds). I shoot the lens wide open at f/4. I am using an iOptron SkyGuider Pro, no guiding.
Anyway, the problem itself - it appears to me that I cannot get the top right corner and bottom left corner in focus at the same focal plane. If I favour the top right for sharpness vs out of focus/coma effects - the lower left suffers - and vice versa. I can try to 'balance' the focus by sacrificing each corner a bit for better stars across the frame but minor deviations in focus throw the whole frame out of symmetry.
I should add, that I am VERY new to this hobby so I am not 100% sure what my expectations should be of the equipment I have. I am a very demanding landscape photographer, shoot zeiss lenses and focus stack - so pixel peeping is a sin that I am guilty of - I confess! When I buy new gear for my landscape rig, I usually perform quite rigorous lens tests (particularly for skew/tilt/asymmetry). What I usually do is focus on a planar subject (a distant cliff face for eg) where everything should be on the same focal plane. A star field is good for this too. This is a very demanding subject for any lens so it reveals flaws in the lens (as well as the user - focus issues, for eg). After I think I have critical focus, I shoot, then I refocus the focus ring as small as possible clockwise, reshoot. I do this many times, I then do the same anticlockwise (twice as far, usually). The results can be very revealing of lens behaviour - skew/field curvature etc. Lens MTF charts are great eye candy but real lenses are built, and there are tolerances (Sigma, rokinon in my experience are not great at building lenses to the stated specifications. But I've also had Zeiss lenses with skew so no brand is immune). IMHO, real world tests are more relevant than design specifications. Plus, tiny adjustments in focus can yield very different results with the same lens - this is why I don't think it's realistic to compare lenses. You have to compare a lens to itself first! Anyway.... Here are some images I took (performing the same test of rotating the focus ring as little as possible per frame).
They don't look too bad but the asymmetry is evident.
Now, look at this one:
The centre and lower left are clearly out of focus, but other parts of the frame are better (top right is ok, not perfect, I confess).
IMG_5827 shows the opposite, the lower left is better, at the detriment of the top right, and top left - to a lesser degree.
This one might perhaps be the best in the sequence, there are many in this sequence - it was quite a though test. I can show more images if needed.
The key to examining these is to view at 100% and toggle between them, locking the view position. Lightroom is good for this. I am aware that stacking might help some of these issues but I believe it's best to get the best baseline data before working on it.
Please excuse the poor white balance, I live in bortle 8 and due to the covid situation there are fines for leaving my geographic area (which I am not happy about, I am an avid hiker and I have lovely bortle 2/3 skies only 1 hour from me!).
Thanks, and let me know if my expectations are unrealistic, or if there is something silly I am doing (I am new to this) or if there is a clear problem here.
P.S. I should add that I already tried the 180° camera rotation method (thanks Mark) but when I tightened the M48 adaptor, the focus was thrown out totally (I did not touch the lens or focus ring). Curious!
Edited by LooseFur1, 14 October 2020 - 05:04 AM.