How do I set the ISO Speed (EOS 5D Mark III)
30-Jan-2012
8201362300
Solution
Set the ISO speed (image sensor's sensitivity to light) to suit the ambient light level. With the < 
> mode selected, the ISO speed will be set automatically.
> mode selected, the ISO speed will be set automatically.ISO Speed Guide
| ISO Speed | Shooting Situation (No flash) | Flash Range |
| L, 100 - 400 | Sunny outdoors | The higher the ISO speed, the farther the flash range will extend. |
| 400 - 1600 | Overcast skies or evening time | |
| 1600 - 25600, H1, H2 | Dark indoors or night |
CAUTION
- If [
: Highlight tone priority] has been set to [Enable], you cannot select "L" (ISO 50), ISO 100/125/160, "H1" (ISO 51200), and "H2" (ISO 102400). - Shooting in high temperatures may result in images that look grainier. Long exposures can also cause irregular colors in the image.
- When you shoot at high ISO speeds, noise (dots of light, banding, etc.) may become noticeable.
REFERENCE
Under [ 
: ISO speed settings], you can use [ISO speed range] to expand the settable ISO speed range from ISO 50 (L) to ISO 102400 (H2).
: ISO speed settings], you can use [ISO speed range] to expand the settable ISO speed range from ISO 50 (L) to ISO 102400 (H2).1. Set the power switch to <ON>.
2. Press the < 
>(ISO speed setting/Flash exposure compensation) button.
>(ISO speed setting/Flash exposure compensation) button.
3. While looking at the LCD panel or viewfinder, turn the <Main dial>.
ISO speed can be set within ISO 100 - 25600 in 1/3-stop increments.
With "A" selected, the ISO speed will be set automatically.
About "A" (Auto) ISO Speed
If the ISO speed is set to "A" (Auto), the actual ISO speed to be set will be displayed when you press the shutter button halfway. As indicated below, the ISO speed will be set automatically to suit the shooting mode.
| Shooting Mode | ISO Speed Setting |
| Automatically set within ISO 100 - 12800 |
 /  /  /  | Automatically set within ISO 100 - 25600(*1) |
 | Fixed at ISO 400(*1) |
| With flash | Fixed at ISO 400(*1)(*2)(*3) |
*2: If fill flash will result in overexposure, ISO 100 or a higher ISO will be set.
*3: When using bounce flash with an external Speedlite in the <
> and <> modes, the ISO speed will be set automatically within ISO 400 - 1600.