FP Sync to Freeze Motion
Prepared 2007-02-11 (154/20960) by Bill Claff

When the D70 was my main body I used to use the rear contact trick to occasionally do flash photography at high speeds.
I frequently used 1/500s for outdoor fill-flash when shooting at 420mm.

Now, with the D200 I can only go to 1/250s with any real power for outdoor fill flash.
See SB-800: Power versus Shutter Speed (D70 and D200 for some details.

To go faster than 1/250s on the D200 you must use FP Sync, the rear contact trick will not work due to a lack of electronic shutter.

The attached image is a mosaic of 6 shots.
All are D200 50mm f/1.8D f/8 ISO 100 SB-800 in FP Sync
The target was a rapidly spinning fan in a totally darkened room.
The images were adjusted slightly for uniform brightness.
The shutter speeds were:

1/8000s	1/2000s
1/500s	1/400s
1/320s	1/250s

Note that we see increasing smearing of the image as we slow the shutter down from 1/8000s until we reach the non-FP Sync speed of 1/250s.
At 1/250s the fan blades are "perfectly" frozen; frozen better than the 1/8000s image although it's hard for you to see with the size of image I have provided

So, FP Sync does not seem to be of any value to freeze motion when flash is the primary source of light.

However, if you're freezing ambient light motion with high shutter speeds then FP Sync certainly makes sense as fill-flash.
Since the flash illuminated part of the image may smear, if you must go into FP Sync territory, it looks like going as fast (without raising ISO beyond reason) as possible is the best policy.

Edited to add D70 images at the same shutter speeds.
D70 50mm f/1.8D f/8 ISO 200 SB-800 with rear two contacts covered.
Note that none of the D70 images is smeared.