Is there something wrong with my 600 EX-RT (or myself)?

[ksagomonyants]

OK guys, I know I'm quite ignorant in photography, so I'd appreciate your help with my issue. I was taking some pictures of my friend, who was sitting on the sofa next to the Christmas tree with some lightning. So, when I was far away (~1 foot) from the tree, flash was working just fine. However, when I was approaching the tree (~ few inches) and pressing the shutter button half-way, the flash got active for 1-2 seconds. I could literally see the light during these 1-2 seconds and it was making some noise (like the one you can sometimes hear from light bulbs). I was not touching the lights with my camera/flash. However, when the Christmas tree lights were off, everything was fine, even when I was close to them. Any thoughts on why it could happen? Is there something wrong with what I was doing or with the flash itself? I was using Canon 5d with 600 EX-RT flash.

Thanks

 

[Loswr]

Sounds like the modeling light (high speed pulse lasting 1 second). Could you have been (accidentally) pressing the DoF Preview button?

 

[ksagomonyants]

Sounds like the modeling light (high speed pulse lasting 1 second). Could you have been (accidentally) pressing the DoF Preview button?

You're 100% correct. I just tried to take the picture next to the lights without and with pressing the DoF Preview button. In the former case it behaves just fine, in the latter one it does exactly as I described before. Thanks so much for your help so much yet to learn…