Basics of Flash Photography, emphasis on Nikon....

This page was started in response to a common question on
Can someone explain the common features available on flashes and why one would want them....?

I'll try.

First, some basic concepts - Light from a point souce (e.g., your flash) gets dimmer the farther away it is from the source, this is the reason you can see in the dark with a flashlight out to 10 or 15 feet, but shine it 100 feet away and it's pretty 'weak'; shine it at something in your hands and your eyes hurt as they try to adjust. The light fall-off obeys something called "the inverse square law"; this says something like "light intensity is proportional to the inverse of the square of the distance from the source". In photography, we use "f-stops" as one measure of light intensity. For a flash unit, we've developed a standard we call 'Guide Number' (GN) that is an expression of the power of the flash; it's simply the f-stop times the effective distance of the flash for a given ISO and coverage. As an example, for an ISO of 100, a typical full-power shoe-mounted flash has a GN of 110 for a 35mm lens coverage, expressed in feet. This would give some thoretical maximimum exposures / distances:

If you multiply any of these number pairs, they'll equal 110, which is what a 'Guide Number' is all about.

This defines 'the physics' of flashes, and why those of us who understand it laugh at others who use P&S flash cameras whale watching, at major sporting events from hundreds of feet away, and at the rim of the Grand Canyon.

Features that are desirable on most flashes, regardless of 'dedication':

Next, a discussion of 'dedicated' flash features for Nikon cameras. The original flash 'hot shoe' standard for 35mm cameras standardized the size, and the location of the central 'trigger' contact (the ground or return is in the hot shoe 'rail'. As each camera manufacturer added features to 'their' flash systems, they added additional contacts. If you stick with the flashes marketed by the camera manufacturer, you generally get the advanced features working properly. Many '3rd party' flash manufacturers like Vivitar, Sunpak and Metz use 'dedication modules' to enable the specific features for a given camera. I refer to all of the above as 'dedicated flashes'.

As you can probably guess, the subject of Nikon flashes and how they interact with various Nikon cameras can 'fill a book'. It does; I recommend for 'advanced study' Thom Hogan's book on the subject, you can find it at

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