The 1/250 FP high speed option in E1 allows the flash sync to be greater than 1/250 if that is the normal shutter speed when using P or A modes. For example if I set my ISO to say ISO 400 and want a narrow dof so I set the aperture to f1.8. If the normal shutter speed under these conditions are above 1/250 then that is what the flash sync will be set to. It could be any shutter speed up to the top speed of the camera.
If you choose the standard 1/250 sync speed that is the top speed that you can use when in M or S mode.
The answer to your question in it's simplest form is as flash sync increases faster than 1/250 second, the flash output decreases until at 1/8000 second the flash is extremely weak. The reason for this is the focal plane shutter is not fully open at one instant at speeds greater than 1/250. At the higher speeds the FP curtain slit moves across the film (sensor) plane so the light output from the flash has to be stretched so it uniformly exposes the plane as the slit moves. At the lower shutter speeds the flash fires in a single burst that increases in speed as required. Flash output is uniform.
If you set the maximum sync speed to 1/250th sec. the flash will sync at all shutter speeds up to a maximum of 1/250th sec.
If you set the maximum sync speed to 1/250th sec. FP mode, when using a compatible Speedlight it will sync at shutter speeds up to 1/8000th sec. though at significantly reduced power. FP mode is very useful when you need fill flash while shooting in bright daylight ambient conditions.
I would like to update/clarify this question. I do understand sync speed and FP mode on our speedlights. What my original inquiry was about was why is there the two settings of 1/250 and 1/250*FP mode? As I ponder this and read the replies here- I wonder if 1/250*FP mode instructs the flash to start the flash strobing. If so, then I would be curious as to why? If it syncs at 1/250 why is high speed sync needed at this setting?
If you set Custom Setting e1 Flash Sync Speed between 1/60th sec. and 1/250th sec. the maximum shutter speed the camera will use in P, or A exposure modes will be the speed you set in e1. FP high speed sync mode will be disabled.
If you set e1 to 1/250s (Auto FP) when using an SB-600, SB-700, SB-800, SB-900, SB-910, when the shutter speed is faster than 1/250th sec. Auto FP High Speed Sync mode will be activated.
Tue 18-Jun-13 12:51 AM | edited Tue 18-Jun-13 12:53 AM by jrp
The Focal Plane High Speed Sync Mode is for the flash to burst pulses inside -within- the set shutter speed sync, as many times as it may take to properly illuminate a subject at higher shutter speeds than the one set for normal sync.
Under regular flash shooting, say 1/250 (or 1/320 in more recent cameras), the flash will give you a single burst for any shutter speed at or under 1/250 (or 1/320) as selected.
But if you still need flash on an scene that calls for a thin depth of field and therefore higher shutter speeds (than 1/25 or 1/320), say... 1/1,000s, the speedlight will place as many bursts it takes to have the subject properly lit while the shutter remains open.
As Marty explained above, what you will be setting via CSM e1 is the slowest sync speed at which FP High Speed will kick in.
Thank you JRP- This info came in handy today. As I was looking at my strobe, SB900, and saw the FP I was wondering why when I was only 1/125 why it would be in FP mode and then I remembered I had put my camera in 1/250 FP mode.