Sunday, February 19, 2017

Flash Versus Ambient: By The Numbers

Original You Tube video can be seen here.
Warning: Potentially Disturbing Images. I look pretty awful in these photos. Some days I look worse. Can't explain my Conan O'Brian hair.

I made this series of photos for my Outdoor Flash class to illustrate the role existing (ambient) light plays in the foreground - background relationship so far as exposure is concerned. There are two important points to be made:
  • The exposure setting for your background may not be the same as that of your foreground. 
  • The background and foreground settings may not be mutually exclusive.
  • Your camera's internal meter can only meter what it sees, which may be different from what you want.

Photo #1 Exposure: 1/8, F 5.6, ISO 200
I am standing in front of a closed exit door in our main meeting room (Photo #1). I am being lit only by the overhead fluorescent lights. The camera is mounted on a tripod with a subject-camera distance of about 8 feet. Setting the camera's exposure mode to Aperture Priority, I made this photo at F 5.6, my usual starting point. Exposure compensation was set to zero.

For the record the shots were made with an APS sensor Nikon D80 with a zoom lens set to 135mm. This was to insure that the background in all subsequent photos would completely fill the frame. This will be important in determining a suitable background exposure.

Photo #2 Exposure: 1/20, F 5.6, ISO 200
 For this shot (Photo #2), I opened the door to reveal the solar panels in the parking area adjacent to the building. Again shooting in Aperture Priority, I got this image. The Matrix Metering system attempted to reconcile the extreme differences between the background and the foreground, resulting in an exposure where neither background nor foreground were rendered properly, the two extremes being "averaged" mathematically. Clearly, this is not a satisfactory photograph by anyone's standards, but unfortunately, similar to so many I see when indoor subjects are photographed against an  outdoor background.

Photo #3 Exposure: 1/250, F 5.6, ISO 200
To get a handle on a more suitable background exposure, I simply stepped aside and let the camera evaluate the background minus the foreground (me). Without changing the aperture, the camera selected an exposure duration of 1/250 of a second, which is a full 3 1/3 F-Stop difference (Photo #3).

Now we have a slight problem. The preferred exposure time for the background at F 5.6 is 1/250th of a second. Since the D80 can't synchronize a flash at this speed, we need to find some equivalent exposure (aperture/exposure time) combination where the flash will work (maximum exposure 1/200th of a second).

Two factors to consider:
  • Can your speedlight deliver enough light at your chosen aperture?
  • Will the corresponding exposure time introduce noticeable camera shake?
Ideally, I would, in the Manual Exposure Mode, choose a combination from a series of equivalent exposure settings, one that would allow for proper flash synchronization. Any of the following might do:

1/200 @ F 6.3
1/160 @ F 7.1
1/125 @ F 8.0
1/100 @ F 9.0

Photo #4 Exposure: 1/200, F 8.0, ISO 200
As you can see from the caption in Photo #4, my choice of exposure settings underexposed the background by 2/3 of a stop. Notice that the blue sky is bit more saturated (true blue) in Photo #4 than it is in Photo #3, a happy outcome.

Reminder:Your camera needs to be in the Manual Mode, using the aperture and exposure settings you like best. If you leave your camera in Aperture Priority or Shutter Priority, you may wind up with an overexposed background like the one in Photo #2.


Photo #5 Exposure: 1/200, F 8.0, ISO 200, plus speedlight on camera
Adding Flash: For Photo #5, I attached an SB-900 (it was handy) to the camera's hot shoe and made a shot. The background remains exactly the same as it was in Photo #4, but now the foreground is illuminated by the speedlight. Not great lighting by any means, but a simple photo made by exposing the background with a manually selected F-Stop and shutter speed, and then allowing the iTTL speedlight to simply do its thing. As I warned, another in a series of very disturbing photographs.

Photo #6 Exposure: 1/200, F 8.0, ISO 200, plus speedlight off camera
Getting The Flash Off-Camera: By using a light stand and an extension cable, I was able to raise my light source to a height of about 8 feet off the ground, but since I'm 5' 6" tall, this translates to only  2' 6" above the center of my face.

Both Photos #5 and #6 were made with direct flash, which is to say that no modifiers were used. I only wanted to illustrate the interplay between the background and your foreground subject. So long as they the foreground illumination is not affected by the ambient or background illumination, you are free to adjust them both to achieve the effect you want.

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