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05-04-2021, 11:36 AM   #1
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Manually Focusing on Stars with Standard K-3 18-135mm lens

Is there a trick to manually focusing the standard Pentax-DA 1:3.5-5.6 18-135mm ED AL [1F] DC -WR lens that comes with the K-3 on the stars or moon? For some reason I wasn't able to use the focus ring to get it to focus on the stars. The focus ring works in daylight and the autofocus works. But I'm trying to take star shots and have not been able to get it to work.

05-04-2021, 11:48 AM - 1 Like   #2
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Find a bright star in live view. Hit the OK button to magnify and minimize the starís diameter.
05-04-2021, 12:35 PM   #3
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Also make sure you have the body on M/F so the shutter will release and it can't attempt to autofocus. A Bahtinov mask can be useful too.
05-04-2021, 12:52 PM   #4
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In addition to the suggestions above: When you want to focus on the moon in LV it sometimes is shown too bright to see any details on the surface, in that case try spot metering and position the moon in the centre of the frame for focusing.


Last edited by othar; 05-04-2021 at 02:46 PM.
05-04-2021, 12:57 PM - 3 Likes   #5
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With a shorter lens like that you likely have to just use magnified live view. Here is the process I suggest that works best for me when I am doing very wide astro shots (yes you are doing wide field astro with that and most any actually camera lens you will use):
1. Disable focus peaking, this is key as it will bloat a bright star and it becomes hard to tell if you actually have gotten it as small as possible.
2. put the camera into manual focus
3. center a really bright star in the view finder and get it as focused as you can through the view finder
4. start live view and zoom in all the way on the bright star
5. Make the bright star as small as you can. while doing this you may notice that there are some spots that consistently filcker make a mental note of these
6. Slowly and carefully adjust the focus until you can get those consistently filckering spots to turn on all or most of the time.
At this point you have achieved a proper infinity focus and can reframe on the object or scene you actually want to shoot.

Those spots that consistently flicker are actually dimmer stars and when you get a perfect focus you are focusing enough photons from them to continually illuminate a pixel.

If you go a bit narrower you can likely use a focusing aid like a bahtinov mask. For my longer lenses (200mm and above) I have ones I made but you can buy one that will work for wider lenses that I use with my shorter stuff.

One thing to be aware of is that you may have bloated stars with purple fringing. I haven't used that lens for astro so I don't know how it performs. One other thing to keep in mind is that you will have to stop it down some to get the best out of it. You are not stopping down to increase depth of field, only to correct for various design and manufacturing defects. Depending on the lens you will likely get acceptable results by stopping down 1 to 2 stops but some lenses require more and some a lot more. A few lenses are near perfect wide open and become perfect very quickly.
05-04-2021, 05:49 PM   #6
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@MossyRocks - pretty well says it all.
05-04-2021, 09:49 PM   #7
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In live view, if you switch to manual focus, you can magnify to 10x. For some reason, in AF you can only do 6x.
05-04-2021, 10:40 PM   #8
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I do all these things, and then I always find it best to do a quick couple of test shots, and zoom all the way in on the preview and see that they are looking sharpest.

05-04-2021, 10:42 PM   #9
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QuoteOriginally posted by MossyRocks Quote
you can likely use a focusing aid like a bahtinov mask.
I'll second that, it makes things soooo much easier
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I would say, set it to infinity focus and start shooting. Nothing is farther away then the stars
6 Days Ago   #11
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QuoteOriginally posted by Sakura Quote
I would say, set it to infinity focus and start shooting. Nothing is farther away then the stars
However the infinity mark on the lens is just a somewhat reasonable guess. I don't have a lens that is actually focused at infinity when infinity is centered on the focus mark. Most lenses will focus a bit past infinity and there are good reasons for this. It allows for easier calibration and manufacture from the factory. Also it allows a lens to contract when getting cold and still be able to focus to infinity.
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this hasn't been mentioned and not sure if you're having trouble with it but get a remote or a cable, or at the very least shoot with the timer so the pressing of the shutter release doesn't shake the camera too much
5 Days Ago   #13
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QuoteOriginally posted by MossyRocks Quote
However the infinity mark on the lens is just a somewhat reasonable guess. I don't have a lens that is actually focused at infinity when infinity is centered on the focus mark. Most lenses will focus a bit past infinity and there are good reasons for this. It allows for easier calibration and manufacture from the factory. Also it allows a lens to contract when getting cold and still be able to focus to infinity.
The 18-135 has no marks on its focus ring and the infinity stop is "soft" where you can turn the focus ring with only slightly increased turn resistance for as long as you want.

I tried to use my copy of this lens to shoot fireworks once and it was a mess due to these aspects of the focus ring. I should have brought one of my 28mm primes instead or my M 20, etc. The focus ring is the one aspect of the 18-135 that I do not like.
5 Days Ago   #14
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Use a fine marker and mark infinity on your scale in daytime with a distant object. At night to test for focus adjustment due to the cold do two bracket shots an mm or two each side of this mark as well of course one on the mark.
5 Days Ago   #15
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QuoteOriginally posted by pres589 Quote
The 18-135 has no marks on its focus ring and the infinity stop is "soft" where you can turn the focus ring with only slightly increased turn resistance for as long as you want.
I would not like that at all. A hard stop somewhere past infinity seems like a good option with distance marks to help you gauge things. I do miss the DoF gauge on most modern lenses though.
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