With an impressive image quality of 40 megapixels via a high-performance image sensor, Pentax’s 645D assures sharp and quality images. Despite its large sensor size, the camera feels lighter than expected 
Medium format
A full-frame DSLR is the dream camera for many enthusiasts but expert explains that beyond this is the medium format’s up to par quality 

For the longest time, medium format cameras are the tool of choice for working professionals. The arrival of digital cameras led to smaller sensor formats, so it took a while for full-frame SLRs to arrive. These days, the medium format digital is gaining ground and full-frame DSLRs are far from being the ultimate professional tool.

 But what exactly is the medium format? Back in the days of film, it referred to 120mm roll film, which had a height of 60mm and could be used in cameras shooting 6x6cm negatives, 6x4.5mm, or 6x7. The height remained constant, but the different cameras used varying widths, and hence took different numbers of shots on the single roll of film. Medium-format cameras couldn’t actually use the full 60-millimeter width of film. The actual size of the old square medium format negative was around 58x58mm, but the size of the slightly smaller 6x4.5cm rectangular format was actually 56x41.5mm.

 Digital medium-format is rather different. The makers use a broader definition of medium format (sometimes called “middle format”) to include any sensor larger than the 36 x 24mm of full-frame SLRs. 

"The Pentax 645D sensor almost mimics the 3:2 

aspect ratio of full-frame DSLRs, but is slightly 

greater in the height."

 The Pentax 645D is not as bulky as other models, and is only a little larger than a professional DSLR like the Canon 1D Mark III. The 645D can be easily used handheld, but it is likely to be more at home on a tripod. It does have the sensor with 1.6x the area of full-frame cameras.
 Generally, the larger sensors give you better quality, but this will also depend on how up-to-date the sensor design is, what it’s designed for, and the resolution. Resolutions range anywhere from 21 megapixels, such as the early Phase One P25 that backs up to 60 megapixels of the new Phase One P65 or Hasselblad H4D models. In between these resolution values would be the Pentax 645D at 40 megapixels. Higher pixel counts are the inevitable trend, but these may reach the same saturation point seen in smaller formats; an increase in resolution could be offset by noise issues. 

 The newer sensors may have much higher resolutions, but they’re also benefiting from the latest sensor technology, so it’s probably unwise to start making any assumptions about high ISO performance and dynamic range on the basis of megapixels alone. The real debate about megapixels and quality boils down to the size of the individual photosites (pixels) on the sensor. Bigger photosites capture more light, a wider brightness range, and less noise.

 It’s not just the sensors that are different. Medium-format cameras use different lenses from regular SLRs, and this was also the case back in the days of film. Focal factors aren’t just a product of the digital age, and anyone moving up from a smaller format will have to get used to a different set of focal lengths. With medium format digital, you need to apply a factor of approximately 0.7. In other words, a 50mm lens on a medium-format camera would actually be equivalent to 35mm on a full-frame SLR.

 But there’s more to it than just focal factors. Medium-format cameras primarily use fixed focal length lenses rather than zooms. They’re not as flexible to use, but they banish the two big headaches with zooms: distortion and chromatic aberration. When talking about medium-format, sensors are only part of the quality equation—lenses are important too.

 “Medium format digital cameras could take your photography to the next level, but it’s also about changing the way you think and work when you’re taking pictures”

 Medium-format lenses also have other tricks. While camera bodies have focal plane shutters just like SLRs, some lenses include internal leaf shutters. Leaf shutters have a number of advantages. One is reduced vibration; the other is high-speed flash synchronization.

 As regular SLR users would know, focal plane shutters achieve higher speeds only by releasing the second curtain while the first one is still moving. This means the sensor is exposed by a narrow moving “slit”, but this is no good for the near-instant burst of light you get from flash photography. Plus when the flash fires, the sensor is fully exposed, so you’ll have to stick to slower shutter speeds.

 But leaf shutters expose the whole sensor at all shutter speeds. This means you can use flash even in daylight, when the intensity of the light forces you to use fast shutter speeds. This exposure is perfect for top-quality outdoor fashion shots, portraiture, and weddings.

 To sum up, medium-format cameras may be more expensive and a bit more cumbersome, but they offer a clear and decisive step up in image quality of regular SLRs. There are obvious technical reasons for this, including the much larger sensor size and the higher resolutions, but the fact that most medium-format lenses are high quality prime lenses play a part too.

 But there are also some less obvious pictorial benefits. The larger format brings shallower depth of field. While this can be an annoyance, it does lend portraits, product shots, and many other kinds of photos a very attractive sense of depth.

With smaller formats, it can be difficult to achieve this sense, even at maximum aperture. Larger formats offer a smoothness and subtlety of tone that’s very difficult to explain technically, but the quality is clear once you’ve seen the results. It was the same during the days of film, when you could have a sharp 35mm, but the details would not look as good as medium-format transparencies.

The final factor is the photographer. Medium-format cameras demand a level of skill, attention, and patience that smaller formats don’t: they make you think about what you’re doing. This is perhaps the biggest difference between professionals and amateurs: professionals will spend much longer getting the shot right before pressing the shutter button.

 Medium format digital format cameras could take your photography to the next level, but it’s also about changing the way you think and work when you’re taking pictures.