NUMBERS GAME
Let’s kick off with a little lesson about sensor resolution and pixel size. It’s pretty simple really; at any given sensor size, if you keep adding pixels they’re going to be smaller, right? When a photodiode converts photons into electrons, it creates both signal and noise (a.k.a. interference) which is measured by the signal-to-noise ratio. The noise is a constant value irrespective of the photodiode’s size and, having a smaller surface area, smaller photodiodes deliver a smaller signal charge. Consequently, the signal-to-noise ratio is lower and, as a result, the noise reduction processing will have to work harder. NR processing compromises sharpness… so you can see where we’re going with this. It’s feasible – theoretically, at least – that a sensor with a higher pixel count could deliver less image sharpness than one with fewer, but bigger, pixels.
And we haven’t touched on the issues relating to the pixel density which can also compromise sharpness due to the need to more effectively eliminate even the tiniest of vibrations, both internally and externally.
There are already challenges with 50 megapixels sensors in the full-35mm format, so how about 61 MP? It looks good on paper to have trumped everybody else – in both mirrorless cameras and D-SLRs – but what are the practical implications? Is the Sony A7R IV flattering to deceive? Compared to the A7R III – which has 42.4 MP on tap – the A7R IV’s sensor represents an increase in resolution of nearly 45 percent… which is quite substantial and will, all other things considered, make a difference in terms of image quality. And we are talking Sony here; the company that has nailed image sensor design more comprehensively than any other (you’d be surprised how many cameras have a Sony-made sensor under the bonnet). It also knows a thing or two about processing the raw data off the sensor and, in particular, creating highly effective noise-reduction algorithms. Don’t be surprised then, if the A7R IV’s high ISO performance is still pretty decent too. We shall see.
TURNING SIXTY
Not surprisingly, the sensor is a backside-illuminated (BSI) CMOS in Sony’s ‘Exmor R’ series to make the most of the available space on the front surface. It has an imaging area of 23.8x35.7 mm with a total pixel count of 62.5 million and there’s no optical low-pass filter because you want every bit of defi nition that’s on offer here.
The sensor is paired with Sony’s current ‘Bionx X’ processor and an additional front-end LSI – a configuration we’ve already
