KAP UAV Exposure Control Script

This page desribes a script designed for use in KAP (Kite Aerial Photography) & UAV (unmanned aerial vehicle) applications. It may also be useful for balloon photography.

Current Release : v3.2

Download Link :   KAP & UAV Script Download Link

Forum Thread :    CHDK Forum Thread for KAP_UAV script

Overview

The KAP Exposure Control Lua script automatically controls shutter speed, aperture, ND filter, and ISO settings so as to maintain the fast shutter speeds needed in kite aerial photography (KAP) and unmanned aerial vehicle photography (UAV). With these applications, the camera is almost always moving, and resulting motion blur is often a big obstacle to getting sharp pictures. Simply setting the shutter speed to a fixed fast value may not allow enough flexibility when lighting conditions are variable. This script actively manages all exposure inputs in order to keep the shutter speed fast, but will allow slower speeds if necessary to get proper exposure. 

Features

• interleave video and still images  (new in version 2)
• user defined preferred values for Tv, Sv, Av
• user defined ranges of Tv, Sv, Av to use
• ND filter can be enabled to engage on high brightness
• sophisticated exposure calculations
• all input and output values displayed in "real" camera terms rather than APEX96 values
• intervalometer
• startup delay
• camera power off after shooting completes
• exposure compensation
• focus lock at infinity
• preset zoom position
• turn off display backlight
• various USB remote control options including intervalometer enable/disable, single shot per pulse, or PWM control (via gentWIRE-USB2 or equivalent)
• configurable for cameras with or without adjustable iris and ND filter
• logging

On Screen Display

Script startup showing initial values selected.

First two shots - note metered vs calculated values used.

Calculation Algorithm

For a camera with no ND filter and no adjustable iris (the "Tv-only" case), the script works as follows :

• the script meters the scene and determines if the picture can be properly exposed using your target Tv (1/1000 second) with ISO somewhere in the range of Min (100) to Max2 (800). If that's possible, it takes the picture at the appropriate ISO.

• if the scene brightness is too low with ISO at Max2 (800), it slows the shutter speed from target, enough to get a good exposure.

• if the scene brightness is too high with ISO at Min (100), it raises the shutter speed enough to get a good exposure, which may be above your maximum setting (1/2000).

For a camera with an ND filter but no adjustable iris :

• the script performs the TV-only calculation described in A above

• if the resulting shutter speed is above your Max setting (1/2000), it inserts the ND filter, lowers the meter reading by three stops, then does the Tv-only calculation again.

For a camera with an adjustable iris, but no ND filter, the script works as follows:

• The script meters the scene, and determines if the picture can be properly exposed using your target Tv (1/1000 second) and your target Av (f/4.0), with ISO somewhere in the range of Min (100) to Max1 (400). If that's possible, it takes the picture at the appropriate ISO.

• If scene brightness is too low with ISO at Max1 (400), it lowers the f-stop setting as needed, down to your Min setting (f/2.8).

- If still more exposure is needed, it raises the ISO, up to your Max2 setting (800).

- If still more exposure is needed, it slows the shutter speed below target to get a good exposure.

• If the scene brightness is too high with ISO at Min (100), it raises the shutter speed, up to your Max setting (1/2000).

- If exposure is still too high, it raises the f-stop, up to your Max setting (f/8.0).

- If exposure is still too high, it raises shutter speed above your Max setting to get a good exposure.

For a camera with both an adjustable iris and an ND filter, the script works as follows:

• The script withdraws the ND filter, then performs the iris-only calculation described in C above.

• If the resulting shutter speed is above your Max setting (1/2000), it inserts the ND filter, lowers the meter reading by three stops, then does the iris-only calculation again.

Note that in very high scene brightness, the algorithm may yield a shutter speed higher than your Max setting, and even higher than your camera's highest nominal setting. Under CHDK such fast shutter speeds may indeed work, but may not be reflected in the EXIF data for the picture (which may only go up to about 1/2000 second). If that occurs, the log data will reflect the actual settings at which the picture was taken.

Also remember that for cameras with an iris, if you use the zoom settings to zoom in to any amount, your camera's minimum f-stop will go up. The script will automatically adjust your minimum and target Av settings if needed. This may be the explanation if your EXIF aperture data shows higher f-stops than your settings call for and you are using zoom.

Setting Focus at Infinity

The script offers several methods to automatically focus at infinity using the :

• Focus @ Infinity Mode

user parameter.  

Not all methods work with every camera so some experimentation may be required to find the one that works for yours.   Release 2 of the script automatically selects the best focus method for CHDK release 1.2.0 or 1.3.0 versions.

Listed below is a description of the various included method,  Even with those. you may have to edit the script to customize one of the methods to work with your camera.

None : tells the script not to attempt to set the focus at infinity

@shot : tells the script to  focus at infinity prior to each shot.  Some cameras do not require the use of Canon AFL or MF moded to focus.

AFL :  sets the camera into Canon AFL ( auto focus lock ) mode and sets the focus to infinity.

MF :  sets the camera into Canon MF ( manual focus ) mode and sets the focus to infinity.

Note :  if this parameter is selected,  the script onscreen log will display an warning message in red & white if the focus does not lock at infinity.

Application Notes

1. A loader program is available for cameras with low memory.  The loader script will load the kap_uav.lua script in approximately 1/2 of the memory needed to load the script directly. See the links below.
2. The script assumes the cameras ND filter effect is the equivalent of three f-stops.  Change the value of the variable nd96offset in line 93 if this is not the case.
3. While the script is running,  you can stop it "cleanly" by pressing the MENU key.  Pressing any other key will cause the display/backlight to activate for about 30 seconds,  allowing you to check the script status. Pressing anything other than MENU again will increment the amount of time that the display/backlight stays enabled.
4. This script requires CHDK 1.2.0 (or higher) and build 3149 (or higher)
5. As always,  CHDK is experimental and while very stable,  it is not perfect.  Use at your own risk.

Parameter Setup

Shot Interval

Specifies the time between shots in seconds.

• values 2 - 120 seconds
• default  15 seconds

Total Shots

Defines how many shots the script will take before returning the camera to playback mode.

• values 1 - 10,000  ( 0 = infinite )
• default  0 (infinite)

Power off when done?

Causes the camera to completely power off when the total shot count is reached. If not selected,  the camera will go into playback mode when the total shot count is reached.

• default  Disabled

Exposure  Compensation

Works like the built-in Canon exposure compensation,  allowing the exposure reading to be offset by up to two f-stops.

• values -2 to +2 f-stops ( in 1/3 f-stop increments)
• default  0.0 

Start Delay Time

Allows a delay at script start before shooting starts - giving time for the kite or UAV to reach a desired altitude.

• values 0 - 10000 seconds
• default  0 

Tv Min

Defines the lowest shutter speed that will be used. Note that the algorithm will not try to go below this value to get the correct exposure, possibly resulting in underexposed images that will need to be fixed in post processing.  Used to prevent motion blurr when the illumination is poor.

• values  None, 1/60 to 1/640 sec
• default  None

Target Tv

Defines the desired shutter speed - usually set to a fast shutter speed like 1/1000 sec

• values 1/100 sec to 1/5000 sec
• default  1/1000 sec

Note : selecting a Tv setting higher than your camera can achieve could  result in  over exposed pictures.

Tv Max

Defines the maximum shutter speed that will be used before the ND filter (if present) is inserted. Note that the algorithm may try to go above this value to get the correct exposure as a last resort, depending on CHDK's ability to extend the shutter speed.

• values 1/1000 to 1/10000
• default  1/2000 sec

Note : selecting a Tv setting higher than your camera can achieve could  result in  over exposed pictures.

Lowest Av

Used to define the lowest aperture setting to be used.

• values f1.8 to f8.0
• default  f2.8
• only used for cameras with adjustable apertures

Target Av

Defines the desired Av setting (usually the lens sharpness "sweet spot")

• values f1.8 to f8.0
• default  f4.0
• only used for cameras with adjustable apertures

Highest Av

Defines the maximum usable Av setting

• values f1.8 to f8.0
• default  f8.0
• only used for cameras with adjustable apertures

ISO Min

Defines the lowest ISO sensitivity value to use

• values  80 100 200 400 800 1250 1600
• default  100

ISO Max1

Defines the maximum ISO to be used during "normal" shooting.

• values 100 200 400 800 1250 1600
• default  400

ISO Max2

Defines the ISO value to be used when the ISO Max1 value cannot be used to achieve correct exposure

• values 100 200 400 800 1250 1600
• default  800

Allow use of ND filter?

Enabled / disables the script's ability to use the ND filter (if available).

• values  No Yes
• default  Yes

Zoom position

Specifies the position to set the zoom lens at the start of shooting.

• range  Off, 0-100% in 10% steps
• default Off

Focus Lock

Several modes to lock focus at infinity.  Some of these will not work with all cameras - some experimentation may be necessary

• values  Off  @Shot  AFL  MF
• default  Off

See the Setting Focus at Infinity section above.

Video Interleave (shots)

When enabled, tells the script to switch modes every time the defined number of shots have occurred and record a video segment.

• values Off 1 5 10 25 50 100
• default Off

Video Duration (sec)

Sets the length of the video recorded in seconds when the Video Interleave option (above) has been selected.

• values 5 - 300
• default 10

USB Shot Control

Enables the camera's USB port to be used for shooting control.   On/Off mode enables & disables intervalometer shooting when USB power is applied/removed. OneShot mode takes one picture each time the USB power is pulsed. PWM takes different actions depending on input pulsed width. Defaults actions are shoot, enter shooting mode, enter playback mode (lens retract), zoom to 100%, zoom to 50% or default, zoom to O%. Script can be edited to perform different actions for each pulse width

• values   None / On/Of / OneShot / PWM
• default  Off

Backlight Off

Disables the LCD backlight after a 20 second delay.

• values  Off/On
• default  On

Logging

Specifies where log message go. 

• values  Off   Screen   SDCard  Both
• default  Both

Release History :

• v1.1 on Oct 10 2013 - initial release
• v1.2 on Nov 21 2013 - minor changes to USB mode and focus at infinity operations
• v1.3 on Dec 08 2013 - fixed handling of ND filter on cameras without adjustable apertures
• v1.4 on Dec 15 2013 - adds a minimum Tv setting to limit the slowest shutter speed that the script can use. Also adds an option to power the camera off when the specified shot count is complete.
• v1.5 on Dec 21 2013 - converted from set_backlight() to set_lcd_display() to turn off the display
• v1.6 on Jan 25 2014 - camera placed in playback mode when USB control is enabled and USB power is not presen
• v2.2 on Mar 22 2014 - adds video recording,  more robust and simplified focus at infinity, autodetect of ND filter and adjustable aperture, ND filter enable/disable,
• v2.3 on Mar 30 2014 - adds focus unlock & relock when switching to video mode and back
• v2.4 on Apr 19 2014 - changed timing for more precise shot intervals
• v3.0 on May 30 2014 - added new USB control modes for intervalometer,  single shot per pulse, and PWM input control mode
• v3.1 on June 14, 2014 - small change to increase reliability of focus at infinity (after setting zoom) and fix for text string error is Ev comp script parameter.
• v3.2 on July 27, 2014 - allow Tv target to be set higher  (useful for UAV's with vibration problems)

Links

link > kap_uav v3.0 Loader for Cameras with Low Memory

link > KAP Forum Thread

link > DIY Drones

link > Ecology Drones

link > KAP & UAV Exposure Control Script V1