There’s more to image quality than just pixel count; color reproduction, dynamic range and low-light/high-ISO performance quickly come to mind. But the more pixels an image sensor contains, the more detail its images can reveal, and the larger you can blow them up before the pixels themselves become visible to the eye (or the more you can crop into an image). This doesn’t mean that more pixels are always better, but it does mean when you need to make huge prints with great detail, more pixels definitely help.
Medium Format Vs. DSLR: Advantages, Disadvantages And Key Features
Today’s highest-resolution, single-shot devices are the 80-megapixel digital backs from Leaf and Phase One. These use 53.7×40.4mm CCD sensors to deliver images measuring 10320×7752 pixels (or very close to that). There are also medium-format sensors delivering 60, 50, 40, 37.5, 31 and 22 megapixels, but currently the 80-megapixel units are the "kings," dwarfing in both area and pixel count the best of today’s DSLR sensors, the 35.9×24.0mm, 36.3-megapixel "full-frame" unit in the Nikon D800 and D800E, which deliver images measuring 7360×4912 pixels.
Besides the advantage in pixel count, the much larger area of the medium-format sensors gives them an advantage in image noise. A larger sensor area can collect more light (photons) than a smaller one when you use the same shutter speed and aperture. And the more light, the higher the signal-to-noise (S/N) ratio. That’s because photonic noise (the noise carried by light itself and the main source of noise in most digital images with current cameras) increases as the square root of the signal: If you have 4 photons of signal, you’ll get 2 photons of noise, for a photonic S/N ratio of 2:1. If you have 100 photons of signal, noise will be 10 photons, for a 10:1 S/N ratio. If you have 10,000 photons of signal, you get 100 photons of noise, for a photonic S/N ratio of 100:1. A better S/N ratio means fewer fine details and steps of dynamic range are lost to noise, and that means better image quality, including higher resolution for a given pixel count (because less detail is lost to noise). Of course, there are other sources of image noise, but from a photonic noise standpoint, bigger is better.
Nearly all medium-format digital cameras are 16-bit devices, while DSLRs are 14- or 12-bit. This means the medium-format cameras can provide a smoother range of tones (especially important in portrait work, but almost always a good thing), theoretically, 65,536 tones or color shades per channel versus 16,384 for 14-bit and 4,096 for 12-bit cameras. (JPEGs from any camera are 8-bit—just 256 tones from black to white—one of many reasons to shoot RAW.) Of course, some of these steps are lost to noise (if the difference from one step to the next is less than the noise level, you won’t be able to differentiate those steps), but still, 16-bit medium-format images can reproduce a smoother range of tones and colors than 14- and 12-bit DSLR images.
There’s also dynamic range. Most of today’s medium-format digital cameras and backs can deliver more than 12 stops of dynamic range without resorting to HDR techniques. While much medium-format work is in the studio, where the photographer can fully control the light, the wide dynamic range is a big benefit with a wide range of subject matter and in outdoor situations where lighting is harsh.