NIKON D7000 FEATURES (continued).
Usually a pro-body feature, a 100% coverage viewfinder allows precise in-camera framing, potentially eliminating the need for future cropping. Underwater, this feature is relevant only when using an external viewfinder, like Aquatica’s Aqua View (discussed below), since it’s nearly impossible to see the entire frame through any housing manufacturer’s stock viewfinders.
1/320s flash sync speed:
A disappointing aspect of digital sensors is their inability to capture highlights in the same way that film does.
Sensors are comprised of an array of pixels, which collect and record photons. These pixels and photons can be thought of as buckets and water droplets, respectively. Nothing prevents water from entering the buckets, so they can get filled as fast as water is poured into them.
Now, consider slide film, which is comprised of a plastic substrate coated with grains of light-sensitive silver-halide. Each grain can still be thought of as a bucket, but with a small funnel attached that the water must first travel through. Average quantities of water can pass through the funnel without restriction, but, when faced with large amounts, the funnel will overflow, only letting a portion of the water into the bucket.
In other words, digital sensors are much more sensitive to high-intensity light than film.
What this means for underwater photographers is that shooting into the sun will often result in an unsightly stark-white ball in your images, usually encircled by an equally unattractive cyan ring.
To expose the sun more pleasingly, a very high shutter speed, small aperture, and low ISO are typically required, but this usually results in extreme underexposure of the foreground, which isn’t always desirable (unless shooting silhouettes). Ideally, you’d just turn on your strobes, crank up their power and illuminate the underexposed area, but DSLRs are limited by something called the “maximum flash synchronization speed” which prevents strobes from being used past a particular shutter speed. The higher this limiting shutter speed, the more these sunballs can be kept under control, so having a camera with a high sync speed is very desirable. The D7000 can sync up to 1/320s, which is the fastest in Nikon’s DSLR range, other than the lower grade D70/s and D50 which sync up to 1/500s.
For those interested, here is a brief explanation of why this limitation exists: Focal plane shutters in DSLRs consist of 2 “curtains” – front and rear. The front curtain blocks the sensor until the shutter release is triggered, and then moves fully out of the way. The sensor remains exposed for the selected duration (i.e. the shutter speed), then the rear curtain quickly springs into place, ending the exposure. At a high enough shutter speed the second curtain cannot move quickly enough to provide the correct exposure. This mechanical limitation of independently-moving curtains is overcome by having the curtains move together in the form of a slit that pans across the sensor. The shutter speed is now determined by the slit’s fixed travel time and its width —-the narrower the slit, the faster the effective shutter speed. This mode of shutter operation presents a problem for flash photography. If a short strobe burst (typically 1/1000-1/50000s) occurs while the slit is passing over the sensor, only that part of the image under the slit will “see” the flash, and the rest of the frame will be dark. The flash must therefore be triggered while the shutter curtains are moving independently. The maximum shutter speed at which this occurs is the camera’s sync speed. [Note: Some newer cameras extend the sync speed to higher values by rapid pulsing of the dedicated flash while the slit is travelling, but this significantly diminishes the flash power].
In addition to being essential for suppressing sunballs, high sync speeds are also a key element in creating jet-black backgrounds. When shooting against bright backgrounds, like sand, seagrass, or coral, it can be difficult to eliminate all of the ambient light, so the higher the camera’s sync speed, the better.