Hi! I'm somewhat new to flash photography but am trying to learn how everything works. I have a D80 and a SB800 mounted on the camera. The other evening I was out in my back yard about 6:00 P.M. (75 degrees, sorry) taking flash pictures and using ambient light. My camera flash mode was slow/rear. My ISO was 400. I was in Aperture Priority with f3.5 setting on my 18-135 lens. My SB800 was set at TTL BL FP. This worked great. My pictures were just what I wanted. My question is why did the flash fire twice? It was my understanding that in slow/rear the front curtain will raise, the sensor will gather ambient light, the flash will fire and then the rear curtain will close. It seems that the flash is firing after the front curtain opens and again before the rear curtain closed. Any help in understanding how this works would be greatly appreciated. Alex
#2. "RE: Why double flash?" In response to Reply # 0
With the rear shutter synch, the camera determines the correct flash exposure at the half press with CLS pre-flashes, and then at the full shutter release the shutter opens and just before the shutter closes the Speedlight flashes. This is causing a longer delay between the Speedlights preflash and the exposure flash than you are accustomed to and long enough for you to see the pre-flashes and the exposure flash as 2 very distinct flashes.
#6. "RE: Why double flash?" In response to Reply # 2 Mon 09-Mar-09 03:12 AM by PAStime
>With the rear shutter synch, the camera determines the >correct flash exposure at the half press with CLS pre-flashes, >and then at the full shutter release the shutter opens and >just before the shutter closes the Speedlight flashes.
Hi. If I'm understanding you, you are saying there are preflashes on half-press of the shutter release button. I certainly don't see any on my D90 and SB-600. The pre-flash occurs on *full* press of the shutter release button. At that moment, the camera:
1. sends a pre-flash 2. meters the scene using the sensor in the viewfinder 3. flashes some more to send power/exposure information to remote slave flashes, if any 3. opens the shutter 4. then fires the flash(es) for purposes of capturing the scene (either front or read curtain)
#7. "RE: Why double flash?" In response to Reply # 6
Richmond Hill, GA (Savannah), US
>Hi. If I'm understanding you, you are saying there are >preflashes on half-press of the shutter release button.
There are no preflashes on the half-press.
When you are using rear or slow sync, the preflashes occur immediately when the shutter is fully depressed, but before the shutter opens. Then, the shutter opens, and the main flash occurs right before the shutter closes.
So, if you are using a long shutter, you will see the preflash followed by the main flash.
#4. "RE: Why double flash?" In response to Reply # 3
Powder Springs, US
In fact, you can use rear curtain sync to meter your flash. With a slow shutter speed, you can wait until after the pre-flash to push the button on your meter prior to the final flash. That way your true flash level will be metered by not reading the pre-flash.
Scott Chapin Powder Springs, GA, USA Nikonians Team Member