Canon Mark III AF while in low light is not focusing anymore

[lafh0001]

My camera was great at focusing in low light, and now its acting too unstable.

Im having a hard time getting to focus on any part of a subject.

[TCampbell]

Ability to focus in low light is impacted not just by the camera, but by which auto-focus mode you chooose (e.g. spot focus, singple point focus, expanded point focus, surround point focus, etc.) and also by which AF point (or set of points you choose.

If you're trying to focus in very low light you'll be better off picking the "surround AF" mode (it's the cluster of 9 boxes -- it's actulaly primarily using the center box but it is "borrowing" the other boxes to help it achieve focus since it's struggle to get enough contrast.  Avoid "spot" focus mode.    Also, the focus points at the center will likely do better than focus points at the sides.

As if that's not enough... it will also vary by which lens you choose.

If focusing in low light is something you have to do often, consider picking up a flash with an AF assist beam.  I use a couple of Canon 600EX-RT speedlights.  These (and several other models) will project a red pattern that the camera can use to very quickly lock focus in the dark.

[lafh0001]

I will try that and let you know, its is not my first mark III its just this one is acting up

[ebiggs1]

Actually we need to know your definition of "low light"?  Plus you said at one time it was better at low light AF?  If the latter is the case you changed some setting or lens.  But first what is low light?

[lafh0001]

its a modeling under a modeling light from my strobes

[ebiggs1]

"Modeling" is not exactly low light.  It is artifical light.  This is the plce to look, "If the latter is the case you changed some setting or lens."

[lafh0001]

all my canon lenses are doing it, how can I check the settings?

[TCampbell]

Check out this video by Canon USA's Rudy Winston explaining how to control the AF area selection choices.

The "spot" AF mode uses only part of hte AF point (it is a reduced size point) and this mode does not work well if subjects don't have enough contrast.  It works well with high-detail / high-contrast subjects.  If you're in this mode and have low light on a subject with poor detail then it could easily explain why the camera is struggling.

There's also "single point" AF (uses a single AF point... but it uses all of it, it's not a reduced size).  This mode is usually good in most situations but may struggle with moving subjects or subjects that really lack detail or contrast.

The expanded point AF modes (4 extra points or 8 extra points) "borrow" the AF points surrounding your single point to pull in more detail and contrast to help the camera lock focus when it's having trouble using the normal single-point AF.  In these modes the center point really is THE point that it wants to use... it's just "borrowing" the surrounding points to help out.

All the other modes (zone AF or full 61 point AF) let the camera pick the AF point automatically (you don't get to tell the camera to stick to a specific AF point... it use any point it wants even if that point isn't exactly on your subject of interest.  It biases it's decision by picking the point that can lock focus at the closest distance to the camera.  Usually that would be what most people want... but obviously not always.  So use these modes with care.)