Adjusting the f/stop on a SB-800

[technics100]

When using my SB-800 in manual mode I can change the following;

Power 1/1 - 1/2 etc etc
Zoom 14-105mm
and the F/stop, is this also adjusting flash power? if so what should I set it at? should it match my f/stop on the camera?

cheers

 

[HoppyUK]

If the flash is in manual mode, then you have to calculate the f/number and set it as appropriate. The camera can't do anything as you have pre-set the power and it has no idea what you're pointing at.

In auto (on the gun) the flash will match whatever f/number is set on the camera, by moderating the power output. It does this by issuing a pre-flash for exposure measurement, which goes out a fraction of a second before the shutter opens and the main flash is fired. It happens so fast you can't detect the two flashes by eye.

 

[chip]

and the F/stop, is this also adjusting flash power? if so what should I set it at? should it match my f/stop on the camera?

In manual, the flashgun will tell you the distance that will be correctly exposed if the camera is set on the aperture shown on the back of the flashgun.

You will see this distance change if you adjust the f-stop. So the flash power is not changing but the range will increase as you change from f4 to f2.8 and you must set your the aperture on the camera to match (unless you are after something creative of course).

Another way to calculate manual flash is using the guide number...

The guide number (GN) of the SB800 is 125ft at 100 iso at 35mm zoom.

To calculate your aperture divide the GN by the distance (flash to subject) eg if your subject is 7.5ft away 125/7.5= f16.6 with the flash on full power and camera 100 iso.

Change the power to 1/2 and your f-stop becomes f11.6 etc.....

....or just buy a meter

 

[EOS_JD]

Yip easiest way to get the aperture is use a lightmeter.