Sunday, November 8, 2015

The SB-400 Does Something Useful, Part 2

I presented the theory in my last post that when (some) Nikon cameras are set to manual exposure, the Exposure Compensation Dial can be used to adjust a shoe-mounted iTTL speedlight. The goal was to provide a level of control over the speedlight when used as a fill light.

The Two Exposure Theory: Before proceeding, I'd like to present a model often used to explain the difference between ambient exposure and flash exposure. The ambient exposure, or the "available light", is set using a combination of exposure duration (shutter speed) and aperture size (F-Stop).  The Flash exposure is determined by the F-Stop only, assuming that the shutter speed you are using is within the normal synchronization range of the camera. In most Nikon DSLRS, it is 1/200 or 1/250 of a second. On the Nikon D50, D70/D70s, and D40, it is 1/500 of a second. However, the P7700 has a leaf shutter mounted inside of the non-removable lens, and can therefore synchronize at any speed, theoretically. There is a caveat to that, as we shall see.
Exposure 1 - Ambient : To test the theory presented in my last post, I mounted the P7700 on a tripod and positioned it in the shadow of a tree.With the ISO set to 200 and the aperture set to F 5.6, four exposures were made.
  • Top left: 1/250 of a second
  • Top right: 1/500 of a second
  • Lower left: 1/1000 of a second
  • Lower right: 1/2000 of a second
Exposure 2 - Flash : The shoe mounted SB-400 speedlight does not have a built-in exposure compensation dial. Instead, I'll be using the dial on the top of the P7700, as shown in my last post. 

In this series, the Exposure Compensation Dial was set to, form left to right, -3 stops, -2 stops, and -1 stop of underexposure. Notice the the sky stays the same, and the flash only illuminates the tree.

Now this is all TTL stuff, so the exposure represents the camera's best guess at what you want. It makes a judgement based on the relative brightness of the regions identified by the Nikon Matrix Metering algorithm, and it's usually pretty close.

In this next sequence, the Exposure (flash) Compensation was set, from left to right: 0 stops, +1 stop, and +2 stops. You may notice that the difference between the +1 exposure and the +2 is not as pronounced as one might guess. This may (emphasize the word "may") be cause by the flash exposure getting clipped at the exposure time of 1/2000 of a second. This is something that must be considered when using leaf shutters in these super-sync situations.

Depth Of Flash - Again: Remember that distance plays a part in the intensity of the flash. Nearer objects are "brighter" than those more distant. The shot above was made at 1/250 at 5.6, ISO 200, without flash. The concrete sidewalk is obviously darker than the vertical retaining wall.

For this shot, the flash was engaged. You can see that the shadows nearest the camera (lower left) are brighter than those farther away (upper right). Distance decreases intensity, as you can plainly see here.

What this proves is that there are some controls available when using the Nikon P7700/SB400 combination. Not that this is an ideal combination, but I think this may come in handy someday.