CHDK supports shutter speed overrides that will give you more precise control over the exposure of your image.

It can be a little confusing when you first use the override settings found in the Extra Photo Operations Menu. 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 aperture control. That is why depending on the 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 for me. 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 1/250th of a second set the override shutter speed to 4 and the value factor to 1/1000.

The shutter speed is calculated in the camera with CHDK as:
(override shutter speed) * (value factor) = real shutter speed
4 * 1/1000 = 0.004  (1/250th 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.

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 =    2 * 1/100
1/100 =    1 * 1/100
1/125 =      8 * 1/1000
1/250 =      4 * 1/1000
1/500 =    2 * 1/1000
1/1000 =   1 * 1/1000

Below is a table of CHDK  shutter speed overrides I computed. Some of the shutter speeds have been rounded to their nearest fraction.

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