Canon EOS 5D Mark III Instruction Manual (en)

Set the ISO speed (image sensor’s sensitivity to light) to suit the 
ambient light level. With the <
A> mode selected, the ISO speed will 
be set automatically (p.127).
Regarding the ISO speed during movie shooting, see pages 223 and 226.
Press the <m> button. 
Set the ISO speed.
While looking at the LCD panel or 
viewfinder, turn the <6> dial.
ISO speed can be set within ISO 100 
- 25600 in 1/3-stop increments.
A” indicates ISO Auto. The ISO 
speed will be set automatically 
ISO Speed Guide
* Higher ISO speeds will result in grainier images.
: Setting the ISO SpeedN
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
If [z3: 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) (p.148).
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.
When shooting in conditions that produce an extreme amount of noise, 
such as a combination of high ISO speed, high temperature and long 
exposure, images may not be recorded properly.