Focus

[ravnostic]

It’s a good start! One of my tricks is shooting on nights when the moon is low in the western horizon early on, which I use to get the focus. Then, after it sets, you’re already focused for stars, as well (just remember to turn off auto focus before taking that next picture!) I believe the moon’s at 1st quarter right now; it would set about midnight and IIRC, you’re something of a night owl? For pics of the moon itself, I start with ISO 200 and 1/200th seconds exposure (you won’t get stars), and adjust from there (tonight, for example, the exposure would be a tad longer as the moon isn’t full.)

And I don’t know how you feel about marking up your lens, but I demarkated one of mine in increments near infinity, took several shots at each, noting which mark I used, and once I saw which images were clearest, lengthened that mark out so I could find it again in the future. Helped a good deal, as my Tamron 70-300mm’s exact-as-I-could-get-it infinity focus is a bit off from where it’s marked on the lens.

[Plamadude30k]

That’s not a bad idea, Rav, having some experience with lenses without any focus demarcation at all, making little marks could help. I can’t begin to tell you how many out of focus astro images I’ve taken. Sometimes you can focus on a star if you can find a sufficiently bright one, but that’s not always an option. I’ve used distant city lights sometimes, too-anything further away than ~100 meters should get you close enough, depending on the lens-further is always better, of course. I also like to focus on point sources rather than extended sources-it just seems to be easier. Of course, there’s plenty of other problems to worry about once you’ve got the focus down. I was out at Kitt Peak not too long ago, and out of ~50 images, only a few were usable. Turns out I was out on the catwalk with a ton of astro camp students who kept nudging my tripod-not on purpose of course, but it was enough to subtly ruin almost every image I took. Even a few with nice looking meteors. Grrr.

[chupathingie]

2 words: Bahtinov Mask.

There is no better way (that I know of, anyway) to get precise focus on stars.

Turning focus all the way out will, in fact, focus beyond infinity and setting via the infinity mark is imprecise at best. Live view works well, as does auto-focus on the moon or lights on the horizon, but using a mask is as precise as you’re going to get without breaking out the scientific gear.

/spent over an hour one night with a 300D trying to get sharp focus, don’t be me.

[orionid]

I might have to give that a try, as well. I made a Bahtinov mask for my 70-300, but it didn’t exactly deliver as hoped. Something about stopping an already slow (5.6) lens down to 11 or 13 to focus on a 3 or 4 magnitude star without a tracking mount is a little, ummm, difficult.

[chupathingie]

Focus on Jupiter, Venus, Vega or any other -mag object.

And yeah, not having a tracking mount will make things more difficult, but you can cheat and use anything distant enough to approximate a point source. A streetlight on the horizon will work pretty well.

I got really tired of *nudge*, 30-sec exposure, wait for download over USB1.1, “damn. still blurry”, *nudge*, etc, ad infinitum…

[chupathingie]

Oh, a side tip to save anyone some grief if you’re looking for multiple images to stack: double over a rubber band so that it rides 1/2 on the focus ring and half on the lens body. It will make focus a little more difficult (since it’s a bit springy and draggy), but will prevent your focus from drifting if you’re shooting overhead. I lost a whole night’s data a few years back because of this. First couple/three subs looked great, but as the night wore on my focus got softer and softer…

[ravnostic]

The Bahtinov Mask: I made one, but haven’t yet really got to employ it sufficiently. The one opportunity I had was with Plama, and dang if the sky didn’t cloud up ruining any chance to make use of it with the scope. The moon demarkation ensures I do well enough with my Tamron at 300mm, but then, I can’t get any exposure of decent length w/o a polar mount. I do better with the 50mm f/1.8, using any ol’ thing far enough out to focus on.

I’ll be down Tombstone way beginning Sunday; I’ll probably mark the 50mm like I did the 300 during daylight hours (or using the moon, which should be overhead at sundown. Unfortunately, on a 3-day photo excursion, I’m not up to bringing the scope, as I won’t have a place to keep it while I go wandering about (it fills the entire passenger seat, and the tripod has to go in the truck bed–that’s not a good feeling when I’m more than 20 fet away from the vehicle.) I’d never thought I wish I bought a smaller telescope but sometimes, I do.

Size does matter, and sometimes, big is too big. 🙁

Oh, and Orionid; I found that there is indeed a *real* graveyard with *real* old graves and nice gravestones, etc., down there, so I’ll be making use of that opportunity, as well.

[U-Man]

Clouddancer, I had a conversation with Mopsy a few years back about focusing her 18-55 kit lens. It was over the phone so neither of us could actually see what the other was talking about. I kept referring to the focus numbers on the lens 9 including infinity – and she kept saying she couldn’t find them. We got frustrated with each other until I looked like CISS and discovered that the lens has no such numbers. 🙁

I use cheapo shutter release cables too. They don’t click in as well and sometimes fall out, but they’re waaaaay less $.

When shooting star trails, I have never had any trouble just pointing my camera at a distant light source (star, planet, streetlight) and using auto-focus. I have is limited to the center sensor. If I’m shooting something on the ground at night, I use a flashlight for auto-focus. But I really like the laser pointer idea.

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