Flash GN for APS sensor

[IanMcBean]

Can anyone tell me how the range of a flash is affected when using a camera with an APS sensor? If a 105m zoom is fitted to the camera is the flash range the same regardless of whether it is a full frame or APS-C sensor?

[greenwing]

The Guide Number of the flash is fixed according to the settings on the flash for any particular ISO. If you can manually zoom the flash to 160mm when you're using a crop-sensor camera and the 105mm lens, that will get you the extra GN of a tighter flash beam.

Nikon's SB-900 and SB-910 select DX or FX mode automatically, so you get the extra that way.

Chris