Megapixel count is the most talked about digital camera spec by manufacturers and users alike. Megapixels are to cameras what horsepower is to cars: a simple specification that one can compare across all models and that's easy to wrap one's mind around. It's the spec that's impossible to get away from. While there's much more to a car's performance than horsepower, there's also an old saying that there's no substitute for horsepower. There's much more to a camera's performance than megapixels, but just like horsepower, there's no substitute for megapixels.
No matter where a camera falls in a manufacturer's lineup, megapixels come into play. Get two photographers in a room talking about their gear and you're going to get the question, how many megapixels does it have? Even entry-level DSLRs are offering 16 to 24 megapixels today. Megapixels do matter, and a high pixel count does offer some decided advantages—along with a drawback.
Higher pixel count doesn't always equate with higher image quality. There are compact digital cameras available with 20 megapixels, but none can touch any APS-C camera—much less a full-frame one—in image quality because their tiny sensors can't collect nearly as much light. But within a given sensor-format category, and contemporary technology levels, higher-pixel-count sensors can deliver better image quality than lower-pixel-count sensors.
A higher-pixel-count sensor can record finer detail than a lower-pixel-count one. This means more detailed landscapes and finer texture in fur and feathers in wildlife shots.
Of course, the lens plays a part in detail, too. A better lens will deliver sharper images with a given sensor than a less sharp lens. But a higher-pixel-count sensor will deliver more detail with a given lens than a lower-pixel-count sensor. If you want to get the most out of your high-megapixel camera, you'll need good lenses, but the higher-resolution camera will get more out of any lens you put on it.
Another factor in resolution is the sensor's AA (anti-aliasing) filter, or OLPF (optical low-pass filter). This minimizes the moiré and artifacts that occur when patterns in the scene conflict with the sensor's pixel grid, and are made worse by the fact that conventional Bayer-array sensors record just one primary color at each pixel site (see the "Sigma Foveon Sensors" sidebar). The AA filter slightly blurs the image at the subpixel level to minimize or eliminate these artifacts. The finer a sensor's pixel pitch (i.e., the greater the megapixel count), the less likely you are to encounter subjects that will produce moiré. A recent trend has been for manufacturers to eliminate the AA filter because eliminating that blurring filter produces sharper images. DSLRs without AA filters include Nikon's D3300, D5300 and D7100, Pentax's K-3 and K-5 IIs, and all Sigma DSLRs with Foveon sensors—all APS-C models. Nikon's D800E has an AA filter whose effect has been cancelled. Mirrorless cameras without an AA filter include Sony's full-frame a7R and Fujifilm's APS-C models with X-Trans sensors. Medium-format digital cameras don't have AA filters.
Pixel Count Vs. Sensor Size
For a given exposure (for example, 1⁄500 sec. at ƒ/8), a larger sensor will collect more photons (light) than a smaller one. And the more photons a sensor collects, the better the signal-to-noise ratio since photonic noise increases by the square root of the photon count: If the sensor collects 100 photons, there will be 10 photons of noise for a photonic S/N ratio of 10:1; if the sensor collects 10,000 photons, you'll have 100 photons of noise for a S/N ratio of 100:1. This tends to give larger sensors better low-light/high-ISO performance. In DxOMark.com's camera sensor ratings, the top 25 sensors in low-light ISO performance are all full-frame ones; the highest-rated APS-C sensor ranks 26th; the highest-ranking Micro Four Thirds sensor is 68th.
However, to produce the same angle of view and perspective, you have to use a longer lens on the larger sensor, and that will decrease depth of field. To get the same depth of field, you'd then have to stop down the longer lens on the larger sensor, which will reduce the exposure (amount of photons collected), unless you also lengthen the exposure time, which, of course, could cause image blur due to subject or camera movement. Bottom line: Larger sensors can collect more photons than smaller ones, but to take advantage of this, you have to give up some depth of field or shutter speed. (Google "Joseph James Equivalence" for a very thorough explanation of the effects of sensor size on images.)
Likewise, larger pixels can collect more photons than smaller ones. But, of course, if you have smaller pixels on a given-size sensor, you have more of them. So the total light-collecting capability is about the same for a given-size sensor, regardless of pixel count. Going back to DxOMark.com, the highest-pixel-count full-frame sensors (36.3 megapixels) rank 5th, 8th and 10th in low-light ISO performance. The top-five sensors in the category are 16-, 12-, 16-, 24- and 36-megapixel full-frame ones. Pixel size has much less effect on low-light capability than sensor size.
Another consideration is that some sensors are "tweaked" for low-light/high-ISO performance, others for low-ISO performance. For example, the 16.2-megapixel sensors in Nikon's D4S and Df were developed with an emphasis on low-light capability (and fast shooting rates), while the D800/D800E's 36.3-megapixel sensors were designed for low-ISO excellence—although all of these sensors deliver very good images at low and high ISO settings. Note that medium-format CCD sensors were developed for low-ISO excellence and don't have the high-ISO capability of the CMOS sensors used in current DSLRs—although the new Phase One IQ250, Hasselblad H5D-50c and Pentax 645Z feature 50-megapixel CMOS sensors that do much better at higher ISO settings than the medium-format CCD sensors.
The Nikon D800, for example, features a 36.3-megapixel sensor that delivers images measuring 7360x4912 pixels. But you can crop one of those images (in the computer or let the camera do it in DX mode) to APS-C format and still have a 15.4-megapixel image (4800x3200 pixels). Conversely, if you had a 12.1-megapixel Nikon D3S full-frame camera, in DX crop mode, your image would be only 5.1 megapixels—not a lot of "crop-ability." Incidentally, current Nikon and Sony full-frame digital cameras automatically switch to crop mode if you attach a DX (Nikon) or DT (Sony) lens that was designed for the APS-C format, but would vignette if used full-frame.
The big drawback to high-pixel-count files is that they're quite large. This means they take up more space on memory cards and hard drives, they require lots of processing power in-camera and in-computer, and they cause slower shooting rates and longer image-editing times. You'll note that Canon's and Nikon's fastest pro action cameras—the EOS-1D X and D4S—have sensors with 18.1 and 16.2 megapixels, respectively. The 36.3-megapixel Nikon D800 and D800E top out at 4 fps, compared with 12 fps and 11 fps for the EOS-1D X and D4S.
But today, memory is inexpensive—both memory cards and external hard drives—and computers are fast. And the high-pixel-count cameras also offer lower-pixel-count settings, which is handy when you don't need the sensor's full resolution.
Sigma Foveon Sensors
So, while the Foveon Merrill sensors don't meet the 20-plus-megapixel, horizontal-by-vertical image size criteria for this article (and they're APS-C, not full-frame), they do deliver lines-per-millimeter resolution competitive with the Bayer-array full-frame sensors, and are worth some consideration when selecting an ultimate image-quality camera.
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Here's a brief rundown of the highest-pixel-count, full-frame, interchangeable-lens cameras—DSLR, mirrorless and rangefinder. We've also highlighted the three new medium-format cameras featuring CMOS sensors; although their 50-megapixel count isn't the highest in medium-format (that's currently 80 megapixels, and those units are included in the accompanying chart on page 83), 50 megapixels is a bunch of pixels, and the CMOS sensors provide much better high-ISO capability, important to outdoor photographers who like to work at dawn and dusk. Note that we've listed only full-frame and medium-format cameras; while there are a number of 20-plus-megapixel APS-C cameras that deliver outstanding image quality, by and large, full-frame cameras deliver better image quality.
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