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Some info about Canon's flash system

ZenonZenon Member
edited February 2014 in home
As much as I like working with light I also really enjoy the technical side. Canon does not provide a lot of detail on what is exactly going on so these foot notes were created. These are not by Canon but are considered to be accurate in the Canon world. A lot of good info.


Besides how the flash determines exposures in both Evaluative and Average modes (flash settings, not camera) there are two other things happening behind the scenes that unless someone does some research would not know about.

AFR - auto fill reduction (or auto flash reduction) as I see no difference between the 2. Fill flash does not know it is filling in anything, it is just less flash power using a ratio based on EV- Exposure Value.

NEVEC - “Negative Evaluative Exposure Compensation” or Automatic Ambient Reduction. Basically when the EV drops below 10 the ambient exposure drops 1 stop. It appears the Canon engineers wanted to give your subject more punch. Canon does not provide a lot of documentation on NEVEC. This not in the footnotes but was discovered around 2003.

So first I'm going to get into fill flash. If you check out other forums the basic consensus is that he Canon system defaults to fill flash when in Aperture or Shutter Priority and not in Manual mode. If you read the footnotes it has nothing to do with the shooting mode at all. It is based on ambient light.

From the foot notes

"Unlike certain other camera systems (particularly Nikon), Canon EOS cameras always default to fill flash mode when the camera is in Tv, Av and M modes. They also perform fill flash in P mode if ambient light levels are high enough. There’s no separate switch or pushbutton to engage fill flash. For details have a look at the section on EOS flash photography confusion below".

The AFR portion of the graph was created from the following sections in the footnotes. Fill Flash Ratios and Auto Fill Reduction. NEVEC was added based on what it does. The graph was complied by a member at POTN.

Here is the what you camera and flash are doing based on EV. Personally I'm not very thrilled by it but I just do what I need to do to get the shot. I wish I could disable both but I guess Canon engineers want to have things happening in the background to enhance images to keep customers happy. You can disable AFR with the 1DX but I'm not sure about the other 1D lines. You can't with prosumer and intro lines. I know I can't with my 5D3.

Now you know all these things are happening in the background when you use a flash. At 10 EV AFR - is at -0.05 and NEVEC is the same giving you a ratio of 1:1



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