Documentation on undocumented Canon flash feature (NEVEC)

Started Jan 4, 2013 | Discussions thread
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Documentation on undocumented Canon flash feature (NEVEC)
Jan 4, 2013

There's a fairly obscure feature in Canon's flash system called NEVEC (Negative Evaluative Exposure Compensation). It's been understood for about 10 years thanks to the online community. Even though Canon never published details, apparently they have acknowledged it is a real phenomenon. It's been discussed in this forum several times in the past, but it's almost 2 years since the last mention.

The reason I started this thread is that something reminded me of NEVEC today, and when I went to search for the adjustment chart, I was sad to find the only web site with details has gone offline! I thought it would be good to memorialize the information here before it's lost forever.

My quick summary of what NEVEC does follows. Canon cameras, in Av or Tv mode, basically run in a "fill flash" mode. As E-TTL flash aficionados know, the ambient light metering is identical whether your flash is on or off (meaning you get a really slow shutter or really wide aperture in a dark scene, even with flash). The catch is that this turns out not to be quite true all the time. The ambient metering actually *can* change up to 1 stop when you turn on your flash, if the scene is dark enough. So if you're shooting fill flash in Av/Tv and looking for a perfectly metered background, you need to be aware that NEVEC might kick in and add negative EC, giving you a darker background than you expected.

One workaround is manually boosting EC (but you need to somehow guess whether NEVEC is secretly applying 0, -1/2, or -1 stops of compensation). Other workarounds include changing to shooting or metering modes where NEVEC does not activate. Not much has changed in 10 years. My 7D definitely does NEVEC in Av/TV with evaluative metering. But it can be avoided in M mode (obviously, since there is no automatic EC). And even in Av/Tv, it does not do NEVEC if I use spot metering. I didn't test the other metering modes.

The original page at http://eosdoc.com/manuals/flash/NEVEC/ can currently be found cached at http://web.archive.org/web/20100721120744/http://eosdoc.com/manuals/flash/NEVEC/. And in case that disappears too, here's the chart with key information.

Hope this helps somebody. I think it will help me when I forget all about this and come searching for the info again a year from now

Canon EOS 7D
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