Photographing horses with flash can be cause for concern... anytime you are doing something around a 2000 lb animal that spooks easily you'll want to make sure the animal will remain calm. If the horse isn't accustomed to it... things could work out poorly.
The 6D deals very nicely with low light so with a low focal ratio lens, you may not need a flash.
As for which flash... the 600EX-RT (or the new 600EX-RT II) is going to be the "best". It's pricey.
If you omit the final "0" on any Canon flash, the numbers that remain are the "guide number" of the flash in meters. In other words a 600EX-RT II becomes "60". A 580EX II becomes "58". A 430EX-III RT becomes "43".
That number represents the distance in meters that the flash can adequately illuiminate a subject assuming the camera is at ISO 100 _and_ the lens (this is the important part) has a focal ratio of f/1.0.
No lens made today offers f/1.0 (Canon used to sell a 50mm f/1.0 lens but they haven't produced that in years.) But the reason for the f/1.0 value is because it makes the math very easy to determine the effective flash distance.
All you do is divide the "guide number" value by the f-stop that you'll be using, and the result is the effective distance.
In other words if your flash has a guide number of 60 meters and you are shooting at f/4 then you divide 60 ÷ 4 = 15. That means your effective distance (at ISO 100 & f/4) is 15 meters ... or just about 49 feet. If you were shooting at ISO 400 then that would work out to about 98'.
Tim Campbell
5D III, 5D IV, 60Da
