A little stumped on how to do this image...

Discussion in 'Beyond the Basics' started by fiveoboy01, Oct 8, 2009.

  1. fiveoboy01

    fiveoboy01 TPF Noob!

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    I apologize in advance for the length of this...

    Basically there is a building I want a shot of, it would be a nightime shot and bracketed, the building is brightly lit but the surroundings not so much, I want to bracket to burn in street lights, stoplights etc...

    The thing is that I will be setting my tripod up on the road and it's very steep, angled downward towards the building. It's blacktop with a double yellow centerline and I'd like to get the centerline and street illuminated. There is virtually no traffic at night, so I don't think cars will illuminate it, and though there are streetlights in the distance I don't believe there are any near my position. My GUESS is that due to the steepness, the road will fill the bottom 1/4 of the frame at the most.

    I'd like to use my flash below camera in front of the tripod to hit the street with light. So:

    1 - if this is a bracketed image, and I only use the flash during the last frame, will this screw up the merging process in Photomatix? I'm guessing it will try to balance the exposure on the street between the totally dark and the one frame that's lit, but I'm not sure. If it's an issue, I can flash each frame as the light will spill off the street and not affect the rest of the image. I just need to figure out how:mrgreen:

    2 - Not exactly sure how to trigger the flash, I'm not aware of a way to do conventional bracketing and trigger it with each frame. There is a flash bracketing function but I think that just varies the flash power to adjust the exposure rather than the shutter speed... I do have a PC cord, well a cybersync transmitter and reciever which are the same thing... Commander mode won't work here I don't think, as the flash sensor won't be anywhere near the spot it needs to be to see the flash. If I can get away with flashing one frame, I can just use the test button on the flash, but I'm not sure if that fires the SB900 at full power or not(not sure how much power I'll need). Nikon experts, what say you;)

    I guess another option would be to manually bracket and layer the images in PSE7. Thing is, I don't know how to do this, and I bet I'd be lost once I got them laid on top of each other.
     
  2. Derrel

    Derrel Mr. Rain Cloud

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    Back in the mid-1980's I did a small amount of night time tripod work using light painting techniques and Kodachrome 64 or Ektachrome 100 slide film, as well as Tri-X Pan, which was 400 ASA film. Burning in of the lights might be a big,big problem, since they are sometimes 1,000 times brighter than the facade of a building--you might just have to let the lights turn into bright blobs or point sources.

    Wearing dark clothes, and nothing like reflective-accented running shoes (!), you can walk brisky through a scene and use the open flash or test button on a speedlight and flash the facade of a building multiple times. It's best to tape over the readylight buttons on your flash and on your external battery pack. You need to keep the flash itself from illuminating your body with rimlighting, so keep the flash down near your belly, and kind of hunch up your shoulders if you are within the frame of the shot.

    If you shoot some flash pops to light an area, you need to overlap them a bit, and using lower ISO settings makes it easier because it gives you more time to work and a bit less critical sensitivity. An external battery pack works great for this. Also, using the flash at a lower power, like 1/2 to 1/4 power makes it easier on the flash's tube, gives you faster recycle times, and just in general makes it easier to "paint" the light on without accidentally putting on wayyyyy too much paint with one mis-placed full-power flash pop. Fast recycle times give you a mental picture of where the last pop was and where the next pop needs to be aimed.

    You need to compute the proper flash exposure for the f/stop at full power based on distance from the flash to the subject area, and then at 1/4 power, you need FOUR pops to get the equal of one full-power pop of flash. Be mindful of the tremendous amount of light fall-of there will be on the road if one edge of the flash beam hits 10 feet from you and the other hits 40 feet away from you....in such a case, you might find you'll want to run across the road to get the light evenly painted on the roadbed. It helps to squint your eyes a bit too, when you do this at night--squinting makes it easier to see where the edges of the flash kind of die off.

    Work from a sketch of the building/composition, and pencil in how many flash pops each area will get/got. Make sure an assistant watches that camera in the road where no traffic will be too! I used Vivitar 285 HV set to NORM and a Quantum Turbo or Battery 1 for this stuff. It takes a bit of practice to do this, but if you allow a 30-second exposure at f/8 at ISO 400 it's fairly easy to get a smoothly lighted scene. Longer exposures with the lens set to f/16 will tend to give nice,sharp-edged "starburst" effects on bare streetlamps, but the aperture is so small that it renders the flash's power rather low, so those would be best done at ISO 400 or thereabouts with a longer exposure. I would also suggest turning ON long exposure noise reduction.

    You can make a straight time exposure for the brightest areas,and then add light to the very darkest areas only. Or you can paint the entire scene with pops of light. It's all very situationally dependent. Dealing with mixed color temps on light sources is another aspect, but we won't go there.
     
    Last edited: Oct 9, 2009
  3. Garbz

    Garbz No longer a newbie, moving up!

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    I am going to take a guess and say Photomatix will crap itself if the flash exposure isn't constant with respect to the rest of the image. The key here is that auto bracketing is normally done in camera using the shutter speed. The shutter speed will have no effect on the brightness of what is illuminated by the flash due to a flash only being a tiny fraction of a normal exposure.

    There's 3 workarounds to this:
    1: Bracket using aperture - bad idea, depending on your composition it may change the depth of field of the shot between exposures.
    2: Bracket using ISO - less bad idea, photomatix is rather noise sensitive and the lowest denominators would be ISO 100 200 and 400. Not sure what your camera is like by nighttime HDRs at ISO400 are a write-off for me depending on the scene and how much photomatix needs to push from the shadows.
    3: Bracket using shutterspeed and compensating the flash output power - This would give you the best result for photomatix I think because the composition and noise remain unchanged. You'd need to fire the flash off in manual and do something like 1 second exposure 1/4 power flash, 2 second exposure 1/2 power flash, 4 second exposure full power flash. To do the bracketing.

    Whatever way you do it this is a classic case of when you need to let go of your automatic controls and setup your gear in full manual including flash output. None of the above will work if your flash is set to TTL metering.
     

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