Moment Of Force In Photography

The term " moment of force "used to bother me in high school physics. Actually, sitting down in the classroom and opening a Physics text used to bother me, and everything thereafter was just the icing on the urinal cake. But one day it changed.

I was in shop class and trying to free a rusted nut with a crescent wrench. I applied great force on a short hold - to control the application of the force - and got nowhere. Then I lengthened the hold on the wrench and applied mild force - still no movement. Then I applied a great deal of force to the outer end of the wrench handle and was rewarded with the thing turning rapidly and delivering my knuckles to the nearest sharp surface. A lesson writ in blood...

So, having understood moment of force by the most practical means, I was able to diagnose an equipment failure last Saturday night and remedy it in time to save the job.

My rig was the trusty Nikon D300s with the Stroboframe Press-T bracket attached and an SB 700 flash on top of this. My standard flash rig for the last few years when I need to move around at a bellydance show. I can flip the flash from landscape to portrait mode and still have it positioned over the lens axis.

This time I decided to gild the lily and add the Gary Fong Lightsphere II to the mix to soften the light blast. It was to be indoors so the top of the Lightsphere was also needed.

All worked well for a while as I played paparazzi at the party, but halfway through the flash started playing up. Nikon flashes don't play up - they are mega-reliable - so when the thing missed firing intermittently I looked to see if there was another explanation. Sure enough, I had over-egged the pudding with the Lightsphere. Out there on the front of the flash, it was perfectly balanced when in the landscape mode, but once it was headed sideways in the portrait mode on the Press-T, the extra weight of it pulled the contacts for the flash away from the corresponding spots on the hot shoe....no flash.

The moment of force was too great for the flash and shoe contacts.

Moral of this is that it is either one or the other - the Press-T and a bare flash for chasing stage events or the SB 700 mounted directly on the camera with the Lightsphere for interiors and people event shots. I quickly demounted the bracket, shifted the flash to the camera shoe, did the Fong Shuffle, and carried right on. My thanks to the decorator of the country club who decided upon white walls and ceiling.