Panasonic FZ1000: Flash FP high speed syncs, 1 / 16000s mechanical shutter?

Because the Panasonic LUMIX FZ1000 uses a sheet shutter, there is no maximum sync speed; The flash can be used in the entire range of mechanical shutter speeds, up to 1 / 4000s (1 / 3200s unless stopped at 32mm equivalent to 135 or more). Typically, this means that FP high-speed sync is normally not required. However, FP sync can be enabled on a hot shoe-mounted TTL Four Thirds flash (such as the Panasonic DMW-FL360L) and the flash will fire continuously throughout the entire exposure, just as it would on a camera with a focal plane shutter. . This is evident in the flash that it no longer freezes motion very fast when it would in normal mode.

However, I have noticed a number of oddities with the flash when it is in FP sync mode (all photos taken in manual exposure mode):

  • The flash may not sync properly when the aperture is fully open at focal lengths of 33mm (135 equivalent) and less; While the flash fires, the image may be exposed as if the flash had not fired during exposure. This could have to do with 1/3200 mechanical shutter speed fully open at 32mm or more. (This seems to indicate that the flash is operating as if the camera had a focal plane shutter with a sync speed of 1 / 125s, possibly a dummy value reported to the flash for compatibility purposes.)
  • More significantly, with the Shutter Type (Rec menu on page 4) set to Automatic, the camera allows the use of the mechanical shutter at speeds as high as 1 / 16000s, speeds that normally cannot be achieved without using the electronic shutter. The flash fires, and shots taken in this mode are in fact correctly exposed as long as the subject is within range of the flash. These shutter speeds appear to be real: if the subject is out of the flash range, the image becomes progressively underexposed as the shutter speed increases. Also, stopping the aperture affects exposure and depth of field as it normally does; Faster shutter speeds do not result in an increase in depth of field (which would be expected if the shutter cannot be fully opened at high speeds).

If the mechanical shutter can only operate at 1 / 4000s, how can it possibly reach 1 / 16000s when using a flash in FP high-speed sync mode? Is this the result of some kind of firmware failure? (It's worth noting that the camera always uses the sheet shutter in conjunction with an "electronic first curtain"; there is no audible sound when long exposure starts, only when exposure ends, and shutter immediately reopens to resume viewing live unless long exposure noise reduction is enabled.)