Auto-Focus (AF) Assist Beam

Speedlite Auto-Focus (AF) Assist Beam

In low light conditions your camera’s auto-focus system may have problems obtaining a sharp in-focus image. Your camera’s ability to focus will vary between models and depend on a combination of factors including type of lens, aperture setting, camera processor, number of auto-focus points, type and distribution of auto-focus points, focus point/s used, and the auto-focus working range. You can find these details in your camera’s specifications section usually located at the back of your camera manual. Most of the newer Canon cameras will be able to focus in light levels ranging from 18EV right down to -3EV which basically equates to midday summer sun on a ski slope brightens to practically shooting in the dark. However the camera also needs something contrasty for auto-focus system to lock on to and in low light conditions you often loose any contrast particularly if try to focus on a persons face.

To help you with focussing in low lighting conditions many speedlites have an auto-focus (AF) assist beam built into the speedlite. When the camera detects low lighting conditions or is unable to achieve good focus then it send an instruction to the speedlite to fire it’s AF assist beam. This is usually a grid of infra-red light that illuminates the subject and covers one of the camera’s auto-focus points. From experience in very low lighting it’s always best to use the centre focus point to achieve the best and most reliable results.

In the Canon 600EX ii-RT the infrared AF-assist beam emitter sits behind the red panel. The beam can be disabled using one of the the Custom Functions buttons.

Shooting in low lighting is very important for wedding photographers either in the church during the ceremony or at the wedding reception. Accurate focus is obviously critical and the AF assist beam can be invaluable.

The AF assist beam can also be used to achieve accurate focus in low light even when flash isn’t being used. Natural light photographers can turn the flash off but still benefit for the feature, examples being in boudoir photography.

Auto-Focus assist beams are not built equally and with this feature Canon speedlites are much more reliable and consistent when compared to third party manufacturers such as Yongnuo and Godox.

The infrared AF-assist lamp sits behind the small red window at the front of the Godox XPro-C controller
The Yongnuo YN-E3-RT and YN-E3-RT II both have powerful IR autofocus assist beams to help focus in low-lighting or low-contrast situations. The Canon ST-E3-RT unfortunately doesn’t have one.

Canon camera auto-focus systems:

Canon EOS 80D

AF System/ Points
45 cross-type AF points 
(45 f/5.6 cross-type AF points, 27 f/8 points [9 cross-type], centre point is f/2.8 and f/5.6 dual cross-type)

AF working range
EV -3 – 18 (at 23°C & ISO100)

Canon EOS 6D MkII

AF System/ Points
Via optical viewfinder: 
45 cross-type AF points 
(45 f/5.6 cross-type AF points, up to 27 f/8 (9 cross-type) points,  centre point is f/2.8 and f/5.6 dual cross-type and sensitive to -3 EV) 
The number of available cross-type AF points will differ depending on the lens. 
Via liveview on LCD screen: 
Maximum 63 AF points (Fixed location on 9×7 grid) via camera automatic selection

AF working range
Via optical viewfinder: 
EV -3 – 18 (at 23 °C & ISO 100) with centre AF point & f/2.8 or brighter lenses 
Via liveview on LCD screen: 
EV -2.5- 18 (at 23 °C & ISO 100) with One-Shot AF

Canon EOS 7D MkII

AF System/ Points
65 cross-type AF points (Centre point is an extra sensitive dual-cross-type point at f/2.8, cross-type at f/8 and sensitive to -3EV)

AF Working Range
EV -3 – 18 (at 23°C & ISO100)

Canon EOS 5D MkIV

AF System/ Points
61 Point / max of 41 cross-type AF points inc 5 dual cross type at f/2.8 and 61 points / 21 cross-type AF points at f/8
The number of cross-type AF points will differ depending on the lens.

AF working range
EV -3 – 18 (at 23°C & ISO100)

Canon EOS 200D

AF System/ Points
Via optical viewfinder: 
9 points 
Center AF point is AF cross-type supporting f/5.6. 
Center AF point is vertical line-sensitive supporting f/2.8. 
Other AF points is vertical line-sensitive or horizontal line-sensitive AF supporting f/5.6
Via Live View on LCD screen: 
Maximum 49 AF points (Fixed location on 7×7 grid) via camera automatic selection
Freely position 1 AF point/ 1 AF Zone (9 points, 3×3 grid) via manual selection

AF working range
Via optical viewfinder: 
EV -0.5 – 18 (center AF point)
EV 0.5 – 18 (other AF points)
Via Live View on LCD screen: 
EV -2 – 18 (at 23 °C & ISO 100) with One-Shot AF

← Previous:

Radio Triggers

→ Next:

Battery Packs

Latest Posts