If you are looking for manual exposure controls on your Canon Powershot camera you should check out CHDK shutter speed overrides. The override settings are controlled in the Extra Photo Operations menu.

It can be a little confusing when you first try use the CHDK override settings due to the naming of the user interface controls. Besides controlling shutter speed there are also controls for bracketing in the Extra Photo Operations Menu.

Some but not all Canon Point & Shoot cameras also support aperture overrides. The Canon Powershot SD780IS for example lacks manual aperture control. That is why depending on the Canon camera you have you might not be able to find the override aperture menu item mentioned in the CHDK user guide.

With time and experience the Extra Photo Operations Menu can become easier to understand and control.

Check out this CHDK Wiki article for a detailed explanation of shutter overrides.
http://chdk.wikia.com/wiki/CHDK_User_Manual#Extra_Photo_Operations

I must admit I find the wording for the options in the Extra Photo operations menu a bit confusing to say the least. I come from a DSLR background so I will try my best to explain how to get the controls to work.

Open the Extra Photo operations menu.

Change the Disable overrides from "disable" to "off" to engage the custom shutter speed overrides.

Change the Shutterspeed enum type to Factor. This allows the table below to be used for override values. For an exposure of 1/4000th of a second set the override shutter speed to 25 and the value factor to 1/100K.

The shutter speed is calculated in the camera with CHDK as:
(override shutter speed) * (value factor) = real shutter speed
25 * 1/100K = 0.00025  (1/4000th of a second)

The override shutter speed is adjustable in integer values from 0-100. The value factor is adjustable to Off, 100, 10, 1, 1/10, 1/100, 1/100, 1/1000, 1/10000, 1/100K. When you are in photo mode you will notice at the top left of the screen the text "TV:0.00400" displayed in red. This is your shutter speed.

If you are doing an activity like aerial photography and want the brightest image possible with the fastest shutter speed it is a good idea to set the ND Filter state to OUT. This will disable the ND Filter and make sure the most light possible arrives on the image sensor.

For handy reference you can save this table below to your /CHDK/BOOKS folder as a text file and reference this from your camera when you are on the go.

Here are a few pre-calculated shutter speed overrides that I use often:

1 sec = 1  * 1
1/2 sec = 5  * 1/10
1/4 sec = 25 * 1/100
1/50 sec = 2 * 1/100
1/100 sec = 1 * 1/100
1/125 sec = 8 * 1/1000
1/250 sec = 4 * 1/1000
1/500 sec = 2 * 1/1000
1/1000 sec = 1 * 1/1000
1/2000 sec = 5 * 1/10k
1/4000 sec = 25 * 1/100k

Long Exposure Photography

With CHDK you can also use a shutter speed override to take long exposure photos. This is great for capturing interesting pictures of stars at night, light streaks from car headlights, or anything else you can think of.

It is possible to extend the duration of a long exposure in CHDK up to the camera's hardware limits. If you shoot an exposure over 30 seconds or so you may experience a delay when saving the photo due to the camera enabling its built-in 'dark frame subtraction' feature on long exposures to reduce sensor noise.

The same controls used for regular CHDK shutter speed overrides also apply to long exposure photography:
(override shutter speed) * (value factor) = real shutter speed
20 * 1 = 20 second exposure

The possible long exposure related values for value factor are: 100, 10, 1

The override shutter speed numerical range is 0 to 100.

By changing these two settings you can take long exposures. Because you can change both the value factor and override shutter speed settings there are multiple ways to achieve the same exposure setting. I usually go with the setting that takes the least amount of button pressing to set in the CHDK controls.

For example you could take a 30 second exposure by setting:
Override shutter speed= 30
Value factor = 1

or you could also change the Value factor to 10 and achieve the same 30 second exposure using:
Override shutter speed = 3
Value factor = 10

Here are a few pre-calculated long exposure shutter speed overrides:

30 seconds = 30  * 1
60 seconds = 60 * 1
60 seconds = 6  * 10
120 seconds (2 minutes) = 12  * 10
300 seconds (5 minutes) = 30 * 10
600 seconds (10 minutes) = 6 * 100

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4 Responses to “CHDK Shutter Speed Overrides Explained”

1. Teamman says:

   August 19, 2010 at 8:13 am

   Your work is interesting.
   Allbest 50 Instructions are way out of date, CHDK no longer has branches such as Allbest,
   it is known as CHDK.
   The latest guide is:-
   http://chdk.wikia.com/wiki/CHDK_User_Manual#Extra_Photo_Operations
2. Required Name says:

   September 15, 2010 at 9:09 am

   Here's the original chart that was on the Allbest documentation at the Wiki. But apparently someone went through and "prettied up" all the old documentation and lost about 50% of what was once there on all 3 of the early manuals for each build. Losing lots of valuable information that was never carried over to the final manual. That's what they get for letting idiots on the loose.

   1/3rd stop shutter-speeds, > = full stops
   # = not easily rounded to whole value,
   use (alternate) setting for accuracy.

   1/640 = 16 * 1/10,000
   1/800 = # 13 * 1/10,000
   > 1/1000 = 10 * 1/10,000 ( 1 * 1/1000)
   1/1,250 = 8 * 1/10,000
   1/1,600 = 6 * 1/10,000 (62 * 1/100K)
   > 1/2,000 = 5 * 1/10,000
   1/2,500 = 4 * 1/10,000
   1/3,200 = 3 * 1/10,000 (30 * 1/100K)
   > 1/4,000 = # 2.5 * 1/10,000 (25 * 1/100K)
   1/5,000 = 2 * 1/10,000 (20 * 1/100K)
   1/6,400 = # 1.56 * 1/10,000 (16 * 1/100K)
   > 1/8,000 = # 1.25 * 1/10,000 (12 * 1/100K)
   1/10,000 = 1 * 1/10,000 (10 * 1/100K)
   1/12,500 = 8 * 1/100K
   >1/16,000 = 6 * 1/100K
   1/20,000 = 5 * 1/100K
   1/25,000 = 4 * 1/100K
   >1/32,000 = 3 * 1/100K
   1/40,000 = # 2.5 * 1/100K
   1/50,000 = 2 * 1/100K
   >1/64,000 = 1.56 * 1/100K (1 = top limit)
   1/80,000 = 1.25 * 1/100K (1 = top limit)
3. Required Name says:

   September 15, 2010 at 9:15 am

   (correction: "prettied up"' = "dumbed down" for all the ignorant newbies who couldn't understand how to use CHDK.)
4. Andrew says:

   September 15, 2010 at 9:41 am

   Hi. I actually had originally listed links to the Allbest shutter speed chart before I later wrote a program that created a longer table of values without the rounded values.

   Andrew