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UBASIC/Scripts: Lightning Photography

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Revision as of 19:41, October 27, 2007 by 67.8.108.65 (Talk)

Lightning Photography


IMPORTANT!!! -- CHDK Builds #144 and later now makes these scripts obsolete! There is now the option to turn off all dark-frame-subtraction on ALL shutter speeds, providing you with an open-shutter up to 95% of the time! See more about this in the RAW Parameters Firmware Usage page. Instead, you are much better off using the OMNI Intervalometer script (if using an S-series camera) and using the high-speed continuous Burst option (#3) with shutter-speeds longer than 1-second. (Or write a script for your A-series camera that can make use of high-speed continuous mode.) Turn off "Noise Reduction" in the RAW menu (this works whether you are saving in RAW or not). And then using this freeware "BlackFrame Noise Reduction" utility on your own. Take one frame with the lens-cap on at the end or beginning of your session so you have a black-frame to use with this Black-Frame utility. (After is better because your sensor will heat up during use.) Then you can apply that black-frame to every long-shutter-speed exposure from your session. Manually doing what the camera used to do, but now this noise-reduction routine is not interfering in your shutter-open time. It would also be good to take a new black-frame at various intervals during a lengthy photography session since the noise level on your sensor may change over a period of time due to over-heating or ambient weather conditions. Keoeeit 22:09, 14 July 2007 (UTC)

NOTE 2: The new Motion Detection versions of CHDK now make this script even MORE obsolete!! YAY!! With that you can let the motion-detection feature let the lightning pre-strike flash trip your shutter for you in time. No need to even keep the shutter open hoping that one will strike during the exposure! See the samples page about Lightning Photography.

Keo, for the dark frame utility you mentioned, I believe a separate 'baseline' image from the capped lens is required for each 'fineness' setting. If all the photos from one session are shot using LARGE/FINE, only one baseline darkframe is needed, but if you change settings during the shoot (let's say the card was getting full, so you dropped back to shooting M1/FINE) this requires an additional baseline darkframe sample. Also, FWIW, for A620 cam 15sec exposures at ISO50, the amount of 'extra' visible noise (with the in-camera NR disabled) is minimal. Minimal, as in, having the in-camera NR enabled just slows down your shooting while delivering only minimal NR benefit.

Yes, and I found that even after 20 minutes of 15 second exposures only required one darkframe to correct them all.



Written for/on: Powershot S3 IS, CHDK Build 106 Changeset 159, or later, required.
Also works on: Should work on all of them.
Version 1.1 Updated 2007-06-03 c.e.(Fixed auto-set 1-sec exp. time.)

Trying to capture lightning from thunderstorms is a unique situation in that the lightning is bright enough to capture during a very very brief exposure. Yet due to the nearly random nature of lightning-strikes you have to keep the shutter open for many minutes sometimes to capture a good bolt or two. Since these cameras do not have a BULB setting on them, where you would usually keep the shutter open until you saw a good lightning-bolt, then close it before the next try at capturing one, we have to find a way to keep the shutter open as much as possible with as little shutter-closed time as possible. Using a shutter-speed of 1 second in high-speed burst mode will be the best possible scenario to accomplish this because it doesn't sit there working on a dark-frame subtraction (as happens on shutter speeds slower than 1 second). When using a fast SD card and a High-Speed Continuous mode, you'll get up to 42 seconds of open shutter time for every minute (42 1-second shots per minute). This means your shutter is open 70% of the time, greatly improving your chances of a lightning-flash occurring during the open-shutter time. Every time you see a lightning flash while this script is running means you'll have a 70% chance of it showing up on one of the frames. Best-case scenario using shutter speeds with dark-frame subtraction (more than 1") and that lowers to a 45% chance of capturing a lightning-flash during each exposure. (A 15" exposure on an S3 IS timed-out to 33" long with dark-frame cycle included.)

I also included a short half_press pause to allow the camera to set auto-focus if ambient lighting is bright enough, otherwise use MF at infinity, and if in Tv mode then also auto-aperture for any ambient lighting, in case you don't do these manually beforehand.

When shooting in daylight conditions (not quite as spectacular for lightning shots), you'll have to use ND-Filters (neutral density filters) or two crossed polarizers to cut down the amount of available ambient light -- if an aperture of f/8.0 and lowest possible ISO isn't enough, the same way that film photographers used to accomplish this. If that is not possible, then you can delete the set_tv 0 command so you may adjust your shutter-speed higher. Keeping in mind that this will VASTLY increase the number of shots saved to your SD card and to sort through in trying to find ones with lightning on them. Doing so will also decrease the shutter-open time to camera-busy time, lessening your chances at getting a strike.

Documentation/Help (save as a small "lightning.txt" file to your /CHDK/SCRIPTS/ folder)


Set your camera to Tv or M(anual) mode and to Continuous mode, use "High-Speed Continuous" if you have it. If ambient light is bright enough it will auto-focus, if conditions are dark use Manual Focus mode set at infinity. When run the script will automatically set your shutter-speed to 1 second. If using Tv mode it will help you to properly expose for any ambient light during each exposure, otherwise adjust your aperture accordingly in M mode. Choose how many minutes and seconds that you'd like the camera to take exposures, keeping in mind that there will be up to 42 individual frames per minute.


Script Code (save as "lightning.bas" to your /CHDK/SCRIPTS/ folder)

rem Author ~Keoeeit~
rem Written for S-series cameras
rem Should work okay on others

@title Lightning Photos
@param a Duration Minutes 
@default a 1
@param b Duration Seconds
@default b 0

t=60000*a+1000*b

if t<1000 then let t=1000

print "Use Tv or M mode."
print "Use Continuous (burst) &"
print "High-Speed if available."
print "Use MF mode at infinity."
sleep 2000

set_tv 0
press "shoot_half"
sleep 1500
press "shoot_full"
sleep t
release "shoot_full"
release "shoot_half"

end


Lightning Photography (Script 2 for S-Series Cameras)
Written for/on: Powershot S3 IS, CHDK Build 106 Changeset 159, or later, required.
Also works on: S-Series Cameras Only
Version 1.1 - Changes: click "timer" commands were replaced with press/sleep/release "timer" commands, as it has more stability, using click statements it would sometimes skip a command if running too fast during initialization and exiting phases.

After working out why I couldn't get the camera to set the shutter-speed from the script command (seems it's touchy in that the set_tv (value) command can't be inside/after a press command), I thought I should take advantage of what I learned from making the OMNI Intervalometer subroutine too and make a script just for S-Series cameras. The only advantage between this and the previous "lightning.bas" script is that you don't have to manually put your camera into Continuous (burst) mode, the script will do that for you, and when ending it will return your camera back to Single-Shot mode (provided you started out in Single-Shot before running this, or it won't work).

Documentation/Help (save as a small "S_lightning.txt" file to your /CHDK/SCRIPTS/ folder)


Set your camera to Tv or M(anual) mode and leave it in Singe-Shot mode. When run the script will put your camera into Continuous mode and set your shutter-speed to 1 second. If using Tv mode it will help you to properly expose for any ambient light during each exposure, otherwise adjust your aperture accordingly in M mode for a 1 second exposure. If ambient light is bright enough it will auto-focus, if conditions are dark use Manual Focus mode set at infinity. Choose how many minutes and seconds that you'd like the camera to take exposures, keeping in mind that there will be up to 42 individual frames per minute.


Script Code (save as "S_lightning.bas" to your /CHDK/SCRIPTS/ folder)

rem Author ~Keoeeit~
rem For S-series cameras ONLY!

@title S-Lightning Photos
@param a Duration Minutes
@default a 1
@param b Duration Seconds
@default b 0
t=60000*a+1000*b
if t<1000 then let t=1000
print "Use Tv or M mode."
print "Use MF mode at infintiy."
sleep 2000
set_tv 0
press "timer"
sleep 200
release "timer"
sleep 500
press "shoot_half"
sleep 1500
press "shoot_full"
sleep t
release "shoot_full"
release "shoot_half"
sleep 2000
press "timer"
sleep 200
release "timer"
sleep 500
press "timer"
sleep 200
release "timer"
end

***NOTE***

It was found during extensive testing that a dark-frame subtraction routine could happen at shutter speeds from 1 second up to 1/6th second! This occurs when the camera becomes overheated from overuse or from ambient temperature conditions (leaving your camera in the sun for example). Apparently this is built-in to many of the Canon cameras (as confirmed by GrAnd when questioned on this). As the temperature rises, so does the level of noise in the CCD. The cameras detect this rise in temperature and raise the shutter-speed limit to which a dark-frame-subtraction routine is applied for noise-reduction. If you find that your lengthy 1 second shutter-speed burst-mode session has slowed down greatly, and you see a busy being flashed in your EVF or LCD after each exposure you will have to cool down your camera to get the faster 1-sec burst speeds again. If your camera has an articulating LCD display it might help to swing it away from the camera so the back of the camera is exposed to the outside air to help keep the temperature lower in the body of the camera. (An interesting finding, I didn't know these cameras would do this. :-) )

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