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12-02-2019, 03:28 PM   #1
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Astrophotography with DFA 15-30 - any hints to focusing?

Hi all, this is going to be the third winter for me trying some astrophography with DFA 15-30. So far, the issue has been the focusing to infinity in pitch dark. It has been mostly trial and error and there have been quite many disappointments when viewing the images later and realizing the stars were not sharp.. So what is the best way of focusing to infinity when it is pitch dark and horribly cold, and keeping the focus there when moving the camera?

12-02-2019, 03:48 PM   #2
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Liveview, pick a star, zoom in and adjust focus until it’s as small as possible, lock the focus ring with duct tape or similar, shoot
12-02-2019, 03:59 PM - 1 Like   #3
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Focusing in the dark is always the challenge!
I have used various strategies in the past including:
1. Focus in daylight and then apply a strip of tape to hold focus position for the night session. Can work well with a manual prime but not so practical with with a modern zoom.
2. Focus on object on horizon at night with LV and 100% magnification. Can sometimes use AF sometimes but usually manual focus. No good for pitch black (in middle of no where)
3. Most commonly: use manual focus and Live view at 100% - usually against a planet which is clear to see wiithin the live view noise.

Definitely want to avoid the trial and error if you can though.
12-02-2019, 04:10 PM   #4
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Basically what DrawsACircle said, maybe take a test picture after taping the focus ring to check if you accidentally moved the ring.
I think the DFA 15-30 does support some kind of ring to warm the lens during the night so you don't get condensed water on your front element (if that has ever been an issue for you).
I don't know about the 15-30mm lens but some lenses have problems with thermal expansion if the temperature difference between locking the focus ring and taking the pictures is too high (I assume this warming tool will help with that a little too), so look at your results from time to time (except you are making a timelapse video -> in that case hope for the best, because sudden movements (beeing just so tiny) will be seen in the result (if not edited in post)).

12-02-2019, 04:30 PM   #5
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All previous replies are good advice. You might find that a red or green light with a head mount will help with set up. The color lights will not mess with your night vision and the head mount will leave your hands free. I like the red. Just turn off before starting your exposure.
12-02-2019, 04:34 PM   #6
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Good advice, thanks! Although it is basically just confirming that there is no magical trick that I have not been aware of. I just recently got a lens warmer, using USB power bank to keep the lens warm. In some of my earlier shooting sessions, ice on the lens has been an issue.
12-02-2019, 05:21 PM - 3 Likes   #7
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One addendum to what DrawsACircle said is turn off focus peaking so that it doesn't outline the star. The focus peaking will basically bloat the star on the screen so it becomes impossible to tell when it has been minimized.
12-02-2019, 07:25 PM - 1 Like   #8
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Perhaps a bit of heresy for astrophotography, but back off the f-stop a notch or two, to get better depth of field, after you've done your best at focus. Cranking up the ISO to compensate may still give you an overall better result. I more than occasionally go to f/4.0 or even 5.6 with a 2.8 lens.

12-02-2019, 07:50 PM - 3 Likes   #9
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Funny you ask this now. I wish I had known some of these things a week ago!
I went to the GC for TG and it was my very first attempt at trying astrophotography (not the purpose of the trip at all, but the sky looked beautiful). I used the 15-30. It was dark...very dark...too dark and focusing was veeeery difficult. I didn't even know what I had in front of me. I managed to follow a couple of tutorials and put together some of the less than ideal shots I took. This is the result:
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12-02-2019, 08:17 PM   #10
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QuoteOriginally posted by sergysergy Quote
Funny you ask this now. I wish I had known some of these things a week ago!
I went to the GC for TG and it was my very first attempt at trying astrophotography (not the purpose of the trip at all, but the sky looked beautiful). I used the 15-30. It was dark...very dark...too dark and focusing was veeeery difficult. I didn't even know what I had in front of me. I managed to follow a couple of tutorials and put together some of the less than ideal shots I took. This is the result:
For that type of shot I would be looking to stacking a set of images twice. Once for the sky and another for the foreground and then combine them. I assume that was a single shot and turned out pretty nice. I really need a faster better wide/ultrawide lens.
12-02-2019, 08:22 PM   #11
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The sky is 2 shots. The foreground one (with a 3 minute exposure approx.). You can even see people (lights) going down the canyon!
12-02-2019, 08:38 PM   #12
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I've read but never tried that a "bhatinov mask" helps focus stars that you can see in live view. Wondering if anyone has experience with using them. Seems like you can make your own too instead of buying one? Bahtinov mask - Wikipedia
12-03-2019, 01:25 AM - 1 Like   #13
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Bahtinov masks work brilliantly, but not that good on shorter focal lengths. The number of slits you need depends on the focal length (try one of the online mask generators) and for such short focal lengths you'll need a very fine mask, probably out of reach for a diy solution.
12-03-2019, 07:42 AM - 1 Like   #14
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QuoteOriginally posted by HoutHans Quote
Bahtinov masks work brilliantly, but not that good on shorter focal lengths. The number of slits you need depends on the focal length (try one of the online mask generators) and for such short focal lengths you'll need a very fine mask, probably out of reach for a diy solution.
I have created Bahtinov masks and the fineness of the slits that give ideal results really gets fine with short lenses so at the 16mm focal length I think you would need some very good machine tools to make one. A little under 1mm is about as fine as one could reasonably expect to create and that is the size you would want for a 300mm lens for good results. I have a 3D printed mask like this for the 300mm lens and I can use it on shorter ones but it is almost completely unusable below 100mm and at 50mm I end up with 7 very faint very tiny very close together dots that are almost on top of each other so I am at best making an educated guess on focus correctness. You may be able to do a laser print of one fine enough onto a transparency but I'm not sure that will work all that well but it would be better than nothing

I have used this generator in the past to create a SVG file and to convert it to a STL file for 3d printing I used this to extruded. In looking at the generator the slit sizes for various lenses I see:
500mm - 1.7mm
400mm - 1.3mm
300mm - 1mm
200mm - 0.7mm
100mm - 0.3mm
50mm - 0.2mm
35mm - 0.1mm
28mm - 0.1mm
16mm - 0.1mm

It looks like the generator has a resolution of .1mm and we hit that limit at 35mm but extendign things out you would probably want a slit size of something like 0.05mm for a 16mm lens.
12-03-2019, 07:57 AM   #15
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QuoteOriginally posted by aaacb Quote
I've read but never tried that a "bhatinov mask" helps focus stars that you can see in live view. Wondering if anyone has experience with using them. Seems like you can make your own too instead of buying one?
I use them all the time and they work great on long glass. The longer the glass the better and easier they are to make The best way to make one is by generating a SVG of one, converting to an stl (extrude about 3mm works), and then 3D printing. I've printed 2 of them down at my local library, one for my 400mm lens and one for my 300mm lens. The feature size starts getting really small for shorter lengths so I'm not sure that extruded plastic from a 3d printer is a good solution for ones for lenses below 200mm and using one for a longer lens on a shorter one produces progressively worse results.
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