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Nikon D3 Exposure

Nikon D3 Matrix Metering Improvements

While the 1,005 pixel RGB sensor used for Nikon's 3D Matrix Metering has a new function in the D3 and D300 (assisting with autofocus tracking), the 3D Matrix Metering system itself has been enhanced in both cameras.

3D Matrix Metering is a Nikon innovation that's been a feature of Nikon cameras for some time. In describing technical features of digital cameras, we often find ourselves saying "the basic concept is simple." Not so with Matrix Metering: The underlying concept is anything but simple, so much so that it amazes us that it can be made to work at all.

Metering sensor. Nikon's unique 1,005 element RGB autoexposure sensor benefits from improved optics in the D3 and D300.

The basic idea behind Matrix Metering is that the camera holds an extensive database of "scene types" that it uses to guide its exposure determination. To develop the system, Nikon's engineers began with a library of thousands of different photographs, representing the most common types of images people might be interested in capturing. (What the reference images were, how they were originally selected and even the exact number of scene types themselves are closely-guarded Nikon secrets.)

Taking this library of thousands of representative scenes, the engineers ran them past a metering sensor and looked at the numbers that came out the other end. With appropriate manipulation (more Secret Sauce), the arrays of numbers from the exposure sensors became an index into the database of scene types. That is, when a camera using Matrix Metering sees a light blob here and dark areas there and there, that information tells it that the scene is similar to Reference Scene #13,257, which might be a portrait shot against a dark background. Following that example, the camera would know that it should place more emphasis on the exposure of the light blob (a Caucasian person's face, perhaps) than that of the darker background. Beyond the patterns of light and dark, Nikon's current version of 3D Color Matrix Metering takes into account distance information fed back to the camera by the lens and autofocus system, and color information from the RGB exposure sensor.

With the foregoing as background, we can talk a bit about the enhancements to the D3 and D300's exposure system. Most of the following is based on a conversation I (Dave) had with Mr. Tadao Kai, while in Tokyo attending the August 2007 press tour for the D3/D300. Mr. Kai is a Development Manager and senior engineer involved in the design of the autoexposure system used in both cameras, and so was able to discuss its operation in some detail. I apologize in advance for any errors I may have made in interpreting what Mr. Kai told me.

There were two important changes made in the D3/D300 autoexposure system, one involving the optical chain leading to the 1,005 element sensor, the other in the underlying scene database. To understand the changes in the AE optics, refer to the illustration below:

The first change relates to how subject images are resolved onto the 1,005 element RGB AE sensor. Let's start by taking a look at how the AE sensor is placed within the camera's optical system. The diagram above shows a schematic look at the viewfinder optics of a Nikon digital SLR. Light enters from the lens below left, is reflected by the main mirror, bounces around the pentaprism, and exits toward the rear through the viewfinder objective. A small prism between the pentaprism and the viewfinder eyepiece refracts a portion of the light upward, where it passes through a small lens and then falls onto the RGB autoexposure sensor.

The purpose of the small lens positioned below the RGB sensor is to bring the subject into focus on the surface of the sensor. It does so just as you'd expect, but that's not the end of the story. It turns out that the RGB sensor is a striped array, with successive columns of red, green, and blue elements. This means that the red, green, and blue images of the subject captured by the sensor aren't in precise spatial alignment, but rather are offset from each other by plus or minus the width of a pixel column. In the past, this has limited the ability of the array to accurately resolve objects.

The new innovation in the D3/D300 autoexposure system is the introduction of a small diffraction grating or Diffractive Optical Element (DOE for short) in front of the autoexposure lens. The DOE bends the light passing through it by an amount proportional to its wavelength, or color. The amount of this bending is precisely controlled, such that the red, green, and blue wavelengths are shifted by an amount equal to the spacing of the columns of pixels in the RGB sensor. The net result is that the position of colored objects can be much more precisely determined from the data captured by the 1,005 element RGB sensor.

In my conversations with the designers, they didn't (or wouldn't) say exactly how the improved resolution would help the AE system, but it's possible to speculate a little. Specifically, with better resolution, it's not hard to imagine the new AE system being able to find and respond to smaller objects in a scene -- A face that occupies a smaller part of the frame, for instance.

Along with the increased resolution (and perhaps in large part enabled by it), the underlying scene database has been greatly expanded. As always, Nikon is a little coy with the exact numbers, but one engineer said that the total database size has been nearly doubled. Given that the approximate size they've given in the past was 30,000 reference images, this suggests that the new reference scene database is now on the order of 60,000 images.

Obviously, the proof of all this is how the camera performs in real-world shooting situations. We can certainly say that the D3's exposure accuracy seemed better than average in our testing of it: It seemed less prone to being fooled by contrasty or backlit subjects than most cameras we test. Ultimately though the subtleties of an exposure system like this can only be appreciated through long experience shooting with the cameras, rather than in the immediate outcomes of laboratory tests. The Nikon engineers I spoke with were quite enthusiastic over what they'd achieved with the new exposure system. Our own (limited) laboratory and real-world shooting with the D3 suggested that they had indeed achieved better exposure accuracy than we'd seen in prior cameras. At the end of the day though, it's our readers who'll ultimately be best positioned to determine the value of this new AE system.

Active D-Lighting

Nikon's "D-Lighting" has been around for quite a while, having first appeared on the Coolpix 7900 back in 2005. It's a dynamic-range enhancement approach that lightens the shadows without blowing out the highlights, while leaving the overall image contrast looking more or less normal. It's been a feature of most Nikon cameras since, and has been further refined with each passing generation.

D-Lighting Examples
With Without (as-shot)
The images at right (lifted from our original Coolpix 7900 review) show the effect of D-Lighting. It's been refined considerably since those shots were taken. In all cameras up until now, D-Lighting was available as an option in Playback mode: You'd select an image with too-dark shadows and apply D-Lighting to it. The result would be a new image on the memory card, the original being left undisturbed.

In the Nikon D3 and D300, D-Lighting has been taken to a new level, with the camera now able to make D-Lighting corrections on the fly, while you're shooting. Called Active D-Lighting, this is still an optional setting, made via the menu system, so you can turn it on and off and adjust the strength of the effect as desired. It also affects only JPEG images: if you're shooting RAW + JPEG, the RAW file will be undisturbed, in keeping with the overriding philosophy RAW files as "digital negatives." As with the chromatic aberration correction we discuss on the optics page of this review, D-Lighting can be applied to data from RAW files in Nikon Capture, an optional (added-cost) software package.

Noise is always an issue with any sort of shadow-enhancement technology like D-Lighting: When you brighten the shadows, the noise gets brightened right along with the subject detail you're looking for. This is where the D3's (and to a lesser extent the D300's) excellent image noise performance and 14-bit A/D conversion come in. With less noise to begin with, and very high-resolution digitization, the camera has much better shadow information to work with, and much finer control over how it applies gradation adjustments. Here again, we won't be able to judge the effectiveness of the new cameras' dynamic D-Lighting until we can test production samples, but the underlying hardware capability and increasing sophistication of the D-Lighting algorithms are encouraging.

Expanded Retouch Menu

This is a feature that's been brought over from Nikon's consumer cameras, and one that has been the subject of ongoing enhancement. The items on this menu all represent image modifications that can be done after capture, while in Playback mode, and without the use of a computer. In all cases, applying any of the modifications to an image results in the creation of a new image; your original is always left untouched.

Practicing professionals using the D3 or D300 will be more likely to rely on workstation-based software than in-camera one-at-a-time image tweaks like these, but it's conceivable that things like red-eye correction and D-Lighting might be useful to apply on a one-off basis. So this probably isn't a big feature for most users of these cameras, but given the negligible incremental expense of adding this menu to the camera, there's no reason not to provide this toolset.