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Author Topic: time-lapse exposure?  (Read 12152 times)
lightstand
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« on: June 08, 2008, 11:12:38 AM »
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How do you guys handle adjusting your exposure to give good detail from day through the evening hours into night? Any insights are appreciated, thanks Jeff
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bill t.
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« Reply #1 on: June 08, 2008, 01:46:01 PM »
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The dirtball solution is to leave the camera on autoexposure.  Won't work well for scenes with jittery exposure changes such as the sun moving behind clouds.

I have worked with HV30 shots done this way.  The worst problem was that the camera quantizes exposure changes in sudden jumps of something like 1/10 stop which is just enough to be visible on lock off shots.  These jumps blended out nicely with 2 second cross dissolves straddling the jumps.

If your subject can tolerate larger timing breaks, set up the camera on manual exposure with maybe 90% zebra bars floating on the brightest objects in your scene.  As the ambient light goes down, tweak the exposure to keep the zebras looking about the same.  Then do the x-dissolve thing in post to cover the fiddling around.

A wonderfully devious solution to this problem is to place two polarizing filters over the lens.  Rotating one polarizer in relation to the other gives you a variable ND effect.  Tighten the filter in the camera threads really tight.  But barely wind-in the outer filter just a little more than one turn.  Wind some string around the outer filter, attach the other end of the string to the rotating knob of a 60 minute, wind-up kitchen timer, let the time do the "exposure pull."  These timers don't put out a lot of torque, so you need to lighten up the filter rotation as much as possible. Good for about 15 minutes of light variation, about the length of the fast moving part of a sunset light transition.  You can stall the timer knob with a piece of tape to control when it starts.  Gotta love it.
« Last Edit: June 08, 2008, 01:47:44 PM by bill t. » Logged
lightstand
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« Reply #2 on: June 09, 2008, 02:52:13 PM »
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Thanks Bill for your informative response, I will definitely play with the two polarizers that's a great idea.

One little question on post production, you mentioned  "Then do the x-dissolve thing in post to cover the fiddling around." I'm not familiar with this idea, what do you mean - I'm just starting out shooting jpeg stills and editing via actions in photoshop.

Thanks again, Jeff
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bill t.
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« Reply #3 on: June 09, 2008, 05:00:13 PM »
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Quote
One little question on post production, you mentioned  "Then do the x-dissolve thing in post to cover the fiddling around." I'm not familiar with this idea, what do you mean - I'm just starting out shooting jpeg stills and editing via actions in photoshop. [a href=\"index.php?act=findpost&pid=200617\"][{POST_SNAPBACK}][/a]

In a running movie made of stills you will sometimes get gross exposure changes.  If you are very careful about preparing your original stills in PS, this will not be much of an issue.  But in those occasions where you wind up with a sudden dramatic exposure or qualitative change in the running movie shot, you can "cut" the movie into two pieces at that exact point, then connect the two pieces via a cross dissolve or perhaps 2 seconds length.  A cross dissolve requires you to slide the next piece of film a few seconds backwards and "overlay" it over the preceding piece of film, so you lose a short duration of time as well.  During the cross dissolve we slowly blend between the two pieces of film which takes away the sudden impact of the exposure change.  The bump gets stretched out over 2 seconds instead of 0 seconds so its much less noticable.

All the editor programs let you do a cross dissolve.  With Windows Movie Maker there is a menu somewhere that lets you change the default 1 second cross dissolve to 2 seconds or whatever you want.  1 second is probably a little too short for this kind of use.

In regards to the polarizer, the best time to start the rotation is just a few minutes before the sun touches the horizon.  It takes about 4 or 5 minutes for the sun to disappear after that first touch, then there is a much slower rate of exposure change.  You want to bias the starting position of the polarizer so you sort of eas-into the range of increasing polarizer effect which you should reach during the time the sun is grazing the horizon.  The biggest exposure issue is that the sky darkens much slower than the ground, you quickly get into a situation where you can't find an exposure that works for both sky and ground.  If you can shoot raw you will be able to tease a slightly better overall compromise exposure than if you shoot jpegs, but of course your post processing effort will escalate dramatically.

I like the way the Adobe Capture Raw program lines up the input files in a row.  By watching how the levels graph animates as you flip from picture to picture you can get a really good idea of how the images themselves with animate (or jump, or flicker, or whatever) on your final film.  That levels graph is the best way I know to equalize frames for movie work.

Do not expect success on your first attempt, these kinds of shots take a lot of experimentation.  If I were you, I'd probably see what could be done with autoexposure alone on the first attempt, and count making a few cross dissolves and lots of post processing twiddling.
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timescapes
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« Reply #4 on: July 14, 2008, 03:27:39 PM »
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The best solution to this would be some type of external laptop control or a hack of the camera itself that would make exposures ratchet up or down, similar to a roller coaster.  This way, as the sun goes, for example, your shutter priority can only change in ONE DIRECTION - ie, longer exposures.

The flickering results from the shutter changing speeds in both directions, back and forth.
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AndyF2
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« Reply #5 on: July 14, 2008, 08:10:55 PM »
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What quality images do you need?  If images from a Canon digicam would be adequate then you could use one of the scripts from the CHDK development.  

The folks there have already posted intervalometer scripts to either take shots every x seconds or minutes, or take N shots within X minutes.  There are other scripts that implement bracketing of exposure or focus distance.  The appropriate parts of each script could be combined to take an exposure bracketed set of shots every N minutes.  You could select the best from each once the series completes.

Scripts: http://chdk.setepontos.com/index.php/board,7.0.html
General info on CHDK: http://chdk.wikia.com/wiki/FAQ

Andy
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