How to unlock a Flash Exposure Lock?

[Advnterrr]

My 5D Mark II is set in Manual Mode with an attached Speedlite (580EX II).  I've set the manual settings to 1/400-F8 for an outdoor portrait.  When I depress the shutter release the settings switch to 1/200-F8 and record the image. Overexposed. 

If I fire off 6-8 images, the camera will record 4 or 5 images at 1/200-F8, then it will record 2-4 images at 1/400-F8.

I have already examined every menu function and tried (unsuccessfully) to reset the camera's settings.

Does anyone have any explanation that could help me solve this issue.

Thank you,

Jim

[TCampbell]

1/200th is the camera's max "flash sync" speed. You can't take an exposure faster UNLESS you enable high-speed sync ,ode on your 580 Speedlite. Keep in mind that high speed sync causes the flash to pulse light rapidly as the shutter sweeps across the sensor. That means the flash can't fire at full power because it needs to conserve enough power in the capacitors to be able to use the required number of pulses. E.g. If it has to pulse 4 times than no single pulse can use more than 1/4 of the power. You can cluster multiple speedlights to get around that problem.

[Skirball]

I didn't know that the camera will reduce shutter speed to MSS when in Manual mode.  Interesting.

As Tim said, you're past your max sync speed without HSS enabled.  You eventually exceeded the Speedlites power buffer, so while it's recycling it didn't fire, hence the camera went back to shooting at 1/400.

[TCampbell]

On my 5D III and II it bumps it down to 1/200th if I power up a flash that doesn't have HSS enabled (I tested to see if it made a difference whether the flash was in manual vs. E-TTL mode, but it made no difference.)  

[Skirball]

That's interesting.  I would have thought that on Manual mode it simply wouldn't fire the flash, or possibly turn on HSS if available.