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04-25-2019, 04:14 AM   #1
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Problems Focusing on Infinity with K-70

Is at times a problem, especially when I'm out in the field using the K-70 for Astrophotography. It's maddening, actually.

Am I doing something wrong? I Mainly use a Sigma DG 70-300mm lens, but also have a Rokinon 1:2.8 14mm and Pentax 18-135.

Advice both needed and appreciated.

04-25-2019, 04:36 AM   #2
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is it with all three lenses or just one of them?
04-25-2019, 07:52 AM   #3
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What is the actual problem: lenses won't focus to infinity, or just trouble achieving/locking focus?

Note that many lenses will focus "past" infinity - you shouldn't just turn the focus ring as far as it will go - that may well put you past the point of infinity focus. This is frustrating to astro photographers! It may be hard to find something to focus on at night. Try manual focus and magnified live view on the moon or the brightest star/planet that is up. Autofocus is fairly useless at night on the sky, unless you have the edge of the moon to work with.
04-25-2019, 09:09 AM   #4
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QuoteOriginally posted by AstroDave Quote
Note that many lenses will focus "past" infinity - you shouldn't just turn the focus ring as far as it will go - that may well put you past the point of infinity focus.
This is true with many AF zooms. Note also that the infinity stop on many manual focus lenses is well short of what one may consider to be infinity.


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04-25-2019, 10:57 AM - 1 Like   #5
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Personally I would ditch using the autofocus for astrophotography and go over to manual focus, use a focusing aid, and check the focus using magnified live view on a bright start. Sometimes I've gotten the autofocus to nail the focus when pointing at the moon but it misses slightly far more often that it gets it right. For a focusing aid use a bahtinov mask and there are online generators for them available. I generated one for my Sigma 300mm f/4 and converted the SVG into a 3d printer file and had a friend print a nice plastic one for me. When getting a template you don't need the big filled in center circle as most people who use them are using them with a Newtonian (reflector) telescope that has the mirror there. Make as many slots as you can and have them cover the most area so that when focusing you can get the brightest image in live view possible. The settings I used (find by trial and error) for my 300mm f/4 were:
Focal length (mm) = 300
Aperture (mm) = 77
Edge thickness (mm) = 2
Structural bar thickness (mm) = 2
Horizontal slot count = 18
Angled slot count = 20
Angled slot offset = -5
This allowed the mask to fit inside the hood but rest on the filter ring

So I would suggest creating a bahtinov mast for the 70-300mm and 18-135 finding a bright star, be sure it isn't a planet, and get it more or less centered in the view finder, put the mask on the front of the lens, go into magnified live view and adjust the focus until you get the diffraction pattern correct and then point it at the object you want to shoot. Another thing to keep in mind is that you will really want to stop those lenses down at least 1 stop as astrophotography is really punishing on lenses and will show most of the problems it has. For the 14mm I don't think a bahtinov mask would work given how short of a focal length it is so I would suggest just using magnified live view for focusing to make the star as small as possible and stop it down to f/4 or more. In all cases here you will want focus peaking turned off as it isn't perfect and the added bloat around a star makes it difficult to tell when you have truly gotten it as small as possible.

Since you are doing astrophotography I would consider some form of tracking if you haven't already. The best is a good equatorial mount, but if you don't want to take that rather expensive plunge there is always the O-GPS1 add on for you K-70. It works well once you get the hang of it and understand it's limitations. I use one on my K-3 and chase deep sky objects with fairly good results.
04-26-2019, 04:28 AM   #6
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The problem is that the lenses wonít get to infinity; just a tad short. I prefer
If they get past infinity so I could focus back to infinity. I never use AF in astrophotography and I do use iOptronís Sky Guider Pro mount. The focusing device (Bahtinov mast) is intriguing. Will explore.,
04-28-2019, 08:38 AM   #7
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QuoteOriginally posted by Roger Dier Quote
The problem is that the lenses wonít get to infinity; just a tad short. I prefer
If they get past infinity so I could focus back to infinity. I never use AF in astrophotography and I do use iOptronís Sky Guider Pro mount. The focusing device (Bahtinov mast) is intriguing. Will explore.,
Focus at long distances with low light and smallish apertures can be tricky.
I tend to use live view AF for long distance shots of still subjects, for better focus accuracy.
04-28-2019, 10:29 AM   #8
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QuoteOriginally posted by Tigs Quote
Focus at long distances with low light and smallish apertures can be tricky.
Smallish aperture because that is all the widest the lens goes??

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