Nikon's new D4 has no shortage of spec sheet hyperbole, in fact the huge numbers fall over themselves: ISO 204,800 support, 51-point AF, 91k pixel color metering. However it's the 16.2-megapixel sensor that has many talking, the number - on paper at least - looking low when you can grab a cheap(er) T3i with 18-megapixels. The megapixel race has been most obvious of late in the smartphone world, but DSLRs aren't immune to the lust for bigger numbers. If anything proves how little pure megapixels matter, it's the D4.
It comes down to sensor size, initially, and then all the accompanying aspects that make a decent camera. The D4 has a full-frame CMOS sensor - measuring 36.0 x 23.9 mm - a more expensive, and larger, sensor than you'd find in most cameras out there. Cheaper models use APS-C sized sensors, considerably smaller, where fitting more pixels into the array means each pixel is smaller.
Smaller pixels adds up to less light making it through, while the cropped down frame means the field of view is also reduced. In effect, you get less in-picture from your lens than you would with a full-frame sensor. APS-C sensors also suffer more noise, especially when you get into the low-light situations Nikon is positioning the D4 as ideal for.
It takes a lot of processing power to crunch a full-frame image, and even more when you're delivering 10fps continuous shooting like the D4 can manage (11fps if you lock off AF/AE). That's when things like your image processor comes in - in Nikon's case the EXPEED 3 - but it's also about how quickly the sensor can churn out data to that processor. Nikon reckons the new D4 sensor spits out info faster than the D3S, despite packing more pixels.
Megapixel chasing came around because shoppers prefer simplicity. Rather than balancing the merits of a half-dozen different specs, it's far easier and quicker to cast your eye down a list and say "oh, this has more, so it's obviously better." In smartphones - where most people probably play the spec-sheet bingo game today - that's resulted in phones with cameras that promise plenty but actually deliver mediocre stills and video, whereas rival devices with lower pixel counts but better performance are overlooked.
We'll have to wait for the first reviews to see exactly how well the Nikon D4 performs in the real world. The company's own sample images certainly look impressive. What it's useful for today, though, is hammering home how just one number tells you so very little about a device's capabilities in general. Whether you're shopping for a $6,000 DSLR or a cheap smartphone, that's something worth bearing in mind.