Infra red assist

[RKGPHOTO]

I purchased a New 680 EX and the manual says the IR assist will not work with a lens below 28 MM Is this correct?

My main Camera is a 5D Mark II and a 7D back up as well as Two Mark II 1DNs I put the flash on the 7D with a 

24x105 F4 and the assist light works. What might I be doing wrong as it works? 

[TCampbell]

You may be thinking of the 600EX-RT... there is no 680EX (there's a 580EX II).  

I've tested with a 5D II and a 14mm lens with both a 580EX II and 600EX-RT flash.

The flash projects the assist beam (it's not really IR, btw... it's visible light but just happens to be red.)  The beam has a specific pattern.  But the beam is designed to work with any and every AF point on the camera.

When you use a particularly wide lens, the lens has the effect of squeezing that pattern into the middle of the frame so that the pattern is no longer visible out near the sides of your frame.

If you focus on the center AF point, the beam still fires and the camera can still lock focus... this works even with my 14mm f/2.8L.

BUT (and here's the rub)... suppose you manually set the AF point to the far right or far left point (not the center).  The lens has the effect of squeezing the pattern so that it's only visible in the middle area and the pattern isn't projected out wide enough for those left or right AF points to pick up the pattern -- and the focus fails.

In testing with my 5D II and 14mm f/2.8L, if I pick the center AF point, it works every time.  If I pick the far right point, it never works (and I can visibly see through the viewfinder that the pattern isn't being projected out where that point is located.

Hope this helps.

Regards,

Tim