D80 and SB-800 high speed flash sync

This modern piece of electronics is sure packed with a ton of features. Add a hot shoe flash and the feature set just got expanded by another 50 different options, or maybe more.

When the SB-800 was shoed on the D80 in the Aperture Priority shooting mode, I noticed that the shutter speed was always at a constant 1/60 seconds regardless of what aperture setting I dialed. Could there not be a way to increase the shutter speed? There is.

Last night, my mind got really curious. I could not understand why I could not shoot faster than 1/60 seconds with flash. 1/60 seconds is fast and the flash would freeze your subject but 1/60 seconds is still slow if your subject is a fluttering hummingbird. Well, I did some testing and so far this is what I found out.

First, the maximum flash sync speed, which is the fastest shutter speed possible where the whole sensor is exposed to light, of the D80 is 1/200 seconds. That speed is how fast a Focal Plane shutter curtain can move across the sensor surface. Faster than this shutter speed, the two shutter curtains will move in tandem with a small gap equal to the shutter speed between them. Without the SB-800 speedlight, the fastest shutter speed when flash is used is 1/200 seconds.

With the built-in flash, in Aperture Priority mode, for some reason I cannot understand yet, the fastest shutter speed is 1/60 seconds. In Manual and Shutter Speed Priority modes, the fastest speed can be set to 1/200 seconds. So, if you want to shoot at speeds faster than 1/60 seconds with the built-in flash, you need to switch over to Manual or Shutter Speed Priority.

With the SB-800, it is possible to go beyond the 1/200 seconds limit. On the Nikon D80, Custom Menu item #25 is called 'Auto FP'. FP stands for Focal Plane, referring to the Focal Plane shutter curtains. Auto FP should be turned on to enable automatic High Speed Sync between the D80 shutter and the SB-800. This will allow the camera to shoot at speeds up to 1/4000. For an explanation of how the Focal Plane shutter works, I highly recommend this article by Derek K. Miller (the best explanation, albeit a little long, of how the shutter curtains work I have read so far) and this video of the Nikon D3 shutter.

Once you have turned on 'Auto FP', it is still not possible to use speeds faster than 1/60 seconds in Aperture Priority mode. You need to switch over to Manual or Shutter Speed Priority to crank the shutter speed all the way to 1/4000 seconds. This is very cool but I wonder if it is any useful. The fact is, at speeds past the maximum sync speed, say 1/500 seconds, the second Focal Plane shutter curtain trails the first curtain by 1/500 seconds, i.e. exposing the sensor to light by a slit that lasts only that much time. When the flash pops (not a single flash but multiple flashes, i.e. strobes) the sensor is exposed to the reflected light through a moving slit. The slit moves down the sensor surface in 1/200 seconds. I wonder if shooting a fluttering hummingbird at a shutter speed of 1/500 this way is better (strobing light) than simply keeping the shutter speed within the maximum flash sync range (one flash pop).