The Sony a7 series of cameras has been wildly popular in the last few years and has certainly stirred the mirrorless versus DSLR debate, along with other popular cameras like the Fujifilm X-T1. I'm not here to reignite that debate, at least not intentionally. Most cameras these days, mirrorless or DSLR, are incredibly advanced and capable machines that will give excellent images in almost any situation when used correctly. Of course, some excel where others fall short, but nonetheless, camera performance is at a remarkably high level.
I've always been intrigued by mirrorless offerings; features like WYSIWYG (what you see is what you get) viewfinders and the beautiful colors of Sony and Fuji sensors were quite tempting, but I never sold off my Canon kit and took the plunge for two reasons: lens availability and AF speed. Sony's lens lineup hasn't been particularly well filled out for this system and while most standard lenses have begun to appear, specialty portrait lenses, super telephoto, and higher-end low light zooms have been lacking. This really limits the native abilities of the system. On the other hand, Fuji has been releasing lenses at a blistering rate, but unfortunately, the APS-C sensor takes it out of the running for me. That's not to say it's not a great sensor, but as someone whose work frequently requires high four-digit and occasional five-digit ISOs, I just couldn't sacrifice a full-frame sensor (though I will say the high ISO performance of the X-T1 is highly impressive for the sensor size).
The solution would be to use adapters to mount my Canon lenses to the Sony, right? Who can argue with Canon's vast library of glass paired to a sensor with some of the best color reproduction and dynamic range out there? Well, I can, and for one reason: AF speed. Early on, mirrorless cameras were notoriously slow at autofocusing, mostly due to the fact that they by and large used contrast detection AF, which is slower than phase detection AF for several reasons. In recent years, phase detection has been integrated onto mirrorless sensors and the autofocus gap has started to close. Nonetheless, when using adaptors, phase detection autofocus was unavailable. Until now.
Phase Detection Autofocus
The Sony a7RII can use phase detection autofocus when using an adaptor for Canon EF lenses. Sony has trumpeted the improved autofocus speed as being on par with native body performance (including continuous autofocus and tracking) and though unbiased hands-on examples are scarce at the moment, those that are out there have been quite promising. This is the feature that can potentially represent a fundamental paradigm shift in the world of imaging.
The Sony a7RII can use phase detection autofocus when using an adaptor for Canon EF lenses. Sony has trumpeted the improved autofocus speed as being on par with native body performance (including continuous autofocus and tracking) and though unbiased hands-on examples are scarce at the moment, those that are out there have been quite promising. This is the feature that can potentially represent a fundamental paradigm shift in the world of imaging.
Until now, cameras and lenses were essentially all or nothing affairs; if you bought into one brand's lenses, you also bought into their bodies and vice versa. Some systems allowed adaptors, but there were always compromises, namely reduced AF performance. This was of course acceptable for certain applications, namely where accuracy, but not speed was key. However, in high performance situations, this just wasn't good enough.
I like autofocus. Many mirrorless cameras, including the a7 line, have excellent manual focus features, but at the end of the day, in high performance or high stress situations, having a reliable AF system removes one large factor to worry about. Preliminarily speaking, the AF system on the a7RII looks excellent. With 399 on-sensor phase detect autofocus points (and 25 contrast detect) that cover a larger portion of the frame with a greater density than any current full-frame system, the a7RII looks to have both precise and wide-ranging tracking abilities, assuming performance is up to par.
If the AF is really everything the early reports say it is, this is groundbreaking. Sony will have obviated the need to use native lenses for decent autofocus performance and opened up the a7RII to an entire library of lenses, without the compromise in AF performance. Granted, AF is not guaranteed to work on Canon lenses made before 1996, but essentially every current version of a standard or specialty Canon EF lens was made after 1995. Furthermore, on-sensor phase detection autofocus points have traditionally been small, meaning that in low light, the system reverts to contrast detection and the dreaded slowness returns. However, Sony has rated the phase detection system onboard the a7RII to -2 EV, equivalent to the 5D Mark III, which has a stellar AF system itself. This potentially means that the Sony system truly does cover all situations.
A further benefit of on-sensor phase detection autofocus is that because the measurements are made on the imaging sensor and not by a separate AF module, differences in calibration between the AF sensor and imaging sensor will be nonexistent, meaning focus accuracy might actually be higher than most DSLRs. The only drawback seen so far is the 5 FPS maximum shooting speed; it's decent, but for a camera with such a potentially amazing autofocus and tracking system, 8–10 FPS could make it a real monster. However, I've yet to see any full-frame above 25 MP (the a7RII is 42.4 MP) come close to those numbers.
Other Features
On the topic of the number of megapixels, another feature has me excited. When I heard the a7RII was 42.4 MP, I was disappointed as I expected noise performance not to be up to today's leading high noise performers, which typically have lower MP counts. The curve ball here, though, is that the sensor is the world's first full-frame back-illuminated sensor. Traditional sensors actually don't gather all of the light hitting them; the wiring and electronics reflect a lot of the incoming light back. Back-illuminated sensors move the wiring behind the photodiodes, reducing reflections and increasing the ability of the sensor to capture incoming light by up to 30 percent in some cases. This means higher sensitivity and better low light performance. Sony has capped the ISO range of the a7RII at 25,600 (expandable to 102,400), and while those numbers are impressive given the MP count, we'll have to wait to see actual image samples, but nonetheless, the inclusion of this technology is promising.
On the topic of the number of megapixels, another feature has me excited. When I heard the a7RII was 42.4 MP, I was disappointed as I expected noise performance not to be up to today's leading high noise performers, which typically have lower MP counts. The curve ball here, though, is that the sensor is the world's first full-frame back-illuminated sensor. Traditional sensors actually don't gather all of the light hitting them; the wiring and electronics reflect a lot of the incoming light back. Back-illuminated sensors move the wiring behind the photodiodes, reducing reflections and increasing the ability of the sensor to capture incoming light by up to 30 percent in some cases. This means higher sensitivity and better low light performance. Sony has capped the ISO range of the a7RII at 25,600 (expandable to 102,400), and while those numbers are impressive given the MP count, we'll have to wait to see actual image samples, but nonetheless, the inclusion of this technology is promising.
Add in features like 5-axis in-body image stabilization, reduced shutter vibration, and a silent mode with a fully electronic shutter, it seems as if the a7RII might be a monster of a camera. For me, though, the ability to use phase detection autofocus with nonnative lenses might be the feature that shifts the paradigm and opens the door to a new way of building a camera kit without compromise. Of course, we'll have to wait until the a7RII is actually released to see if the performance is up to par, but if early tests are shown to be true, it could be a groundbreaking camera.
Source: https://fstoppers.com/critiques/why-sony-a7rii-could-be-most-important-camera-years-76556
