[JThorn]

Hey guys! I should probably start by saying that I'm relatively new to photography after accidentally discovering that its something I'm good at. I started last year with a Nikon D3100, which I broke a few months ago (I dropped it on the way to catch a sunrise one morning) and decided to upgrade my gear rather than repairing that camera and lens. I've learned quite a bit, what I need and don't need in a camera with the D3100, and decided the K-3 fit my needs best as I like to shoot in outdoor extremes and night photography. That said, I'm loving the K-3...huge Pentax fan now. It has really helped take my photography to the next level and has not failed me once, except last night...

So I'm learning how to do time lapses. I've done a few already and I've been shooting star trails for awhile - my methods seem to be working for me. Last night I setup to take a 4 hour time lapse of the milky way descending below the horizon at 24 second intervals with a 20 second shutter speed, ISO 6400, f/1.8 with a Sigma 18-35mm f/1.8 dc hsm lens focused to infinity. Only 30 minutes of frames from the entire 4 hours was in focus because the focus ring had moved from infinity to the opposite end of the focus range, and it happened between intervals - it wasn't a gradual move. I've been using back button AF in AF.C mode for awhile now after recently discovering the method, so I never switch between Manual and Auto focus anymore, lens and camera both stay on AF. Nobody touched the camera after it was set up initially. I was completely alone that night and I'm thinking the focus should not have changed...even if the camera tried to use Auto Focus, I would think it would have focused past infinity, because that's usually what it does in the dark when it can't lock on to a target. Also, when I shoot at night for long periods, in order to prevent dew or frost from forming on the glass, I put chemical activated hand warmers (like Hot Hands) inside of a sock, wrap the sock around the lens, and put a rubber band over that to keep it on the. I had a thought that maybe the heat caused the focus ring to move somehow, but as I've said the change wasn't gradual and I've shot several times, all night this way without the focus ever changing.

To be safe in the future, I'm going to just switch the lens to MF so the camera doesn't try to communicate with it. However, I am wondering what could have caused this...is there an aspect to Auto Focus that I'm missing? Or was this maybe some kind of weird glitch with either the camera or lens?

[semaca]

I am no expert in timelapse, but I know some guys with more experience and they never use autofocus. They focus on some bright star or the Moon and even tape the focus ring.

[Steve.Ledger]

Not really of any help to the OP, but by way of letting you know that you're not alone with this anomaly, I recently had the Q with the 01 Standard Prime lens lose focus gradually during an all day time lapse after about 14 shots. I used MF, but as the focus ring on the Q lenses is electronic, not mechanical, taping it would likely not be effective.
Very annoying.

[enoeske]

Any timelapse work should be done with everything locked down. You should have your rig all set up before you need to be shooting. Get the focus locked on and switch it to manual so it doesn't move.

[Ole]

Manual focus is the way to go as said above.

The Sigma 18-35 is particularly problematic due to its inconsistent autofocus performance:

Sigma 18-35mm Autofocus: A Second Look - Previews and Reviews | PentaxForums.com

[jatrax]

I put chemical activated hand warmers (like Hot Hands) inside of a sock, wrap the sock around the lens, and put a rubber band over that to keep it on the.

Just speculating but any chance the sock and hand warmers shifted during the night and moved the focus ring? I think those hand warmers use fine ground iron that oxidizes and that might change the weight distribution so it spun around.

Other than that, I'm lost. Just put the camera on MF.

[VoiceOfReason]

Any time I do something like that I focus, then put it in MF, and then stick a tiny bit of scotch tape to hold it there. It has never went wrong and doesn't make the camera want to refocus on something like a little condensation on the front element or a bird or bat.

[JThorn]

Manual focus is the way to go as said above.

The Sigma 18-35 is particularly problematic due to its inconsistent autofocus performance:

Sigma 18-35mm Autofocus: A Second Look - Previews and Reviews | PentaxForums.com

Yeah, I've certainly had problems with AF with this lens but it focuses perfectly at 35mm, then I can zoom back to 18mm once focused. It's annoying, but I'm willing to put up with it for the sharpness I get and the fast, constant aperture..plus I shoot only landscapes with it, so it's not like I have to worry about chasing a moving target.

Good ideas on taping the focus ring. I'll do that in the future and...I guess I'll stop being lazy and flip over to MF

Thanks for sharing your experience Steve.Ledger. Hopefully this doesn't become a similar problem because that sounds like a tougher fix.

[Stone G.]

Only 30 minutes of frames from the entire 4 hours was in focus because the focus ring had moved from infinity to the opposite end of the focus range, and it happened between intervals - it wasn't a gradual move.

To be safe in the future, I'm going to just switch the lens to MF so the camera doesn't try to communicate with it. However, I am wondering what could have caused this...is there an aspect to Auto Focus that I'm missing? Or was this maybe some kind of weird glitch with either the camera or lens?

You cannot really auto focus on the soft, structureless Milkyway glow. My guess is, that as luck would have it, you have been (auto) focusing on one or more brighter stars in your field of view. The K-3 is very capable on auto focusing on even not-too-bright stars. However, after some time the star has drifted away from your focus point(s) and then AF begins to hunt - ending up in the opposite end.

As others have said: Always use MF for astrophotography and if there is a risk of focus creep with a particular lens, use some tape.

[JThorn]

You cannot really auto focus on the soft, structureless Milkyway glow. My guess is, that as luck would have it, you have been (auto) focusing on one or more brighter stars in your field of view. The K-3 is very capable on auto focusing on even not-too-bright stars. However, after some time the star has drifted away from your focus point(s) and then AF begins to hunt - ending up in the opposite end.

As others have said: Always use MF for astrophotography and if there is a risk of focus creep with a particular lens, use some tape.

That makes sense. I guess my thought was the camera only auto focused when I pushed the AF button. When I shoot normally, the camera never refocuses unless I push the button, it acts as if the camera is in MF mode. That was one of the main reasons I got in the habit of using back button AF because I was switching between focus modes so often that I would forget which mode I was in.

[Aaron28]

I really enjoy time lapse photography and is mostly what I do with more to learn about it, for instance what mode to use to maintain a certain aperture during a sunset rather than using the 'sunset' scene mode but I have learned that I must pay attention to switch to manual focus...have gotten some weird stuff when I don't......

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