I have tested the accuracy of the Sony a7RII autofocus for static subjects; the results were impressive. I have used the camera for many actual photographic people shots. On the whole, the autofocus systems (I say systems, because there are a dizzying number of modes and sub-modes) have performed their task with remarkable flexibility and precision. The combination of phase detection (to get close) and contrast detection (to trim up any residual errors) is powerful in concept, and generally well-executed in practice. Of course, there are always things I’d like to see changed in today’s cameras and I’ll get to my suggestions for improvement in a future post.
Rather than post an endless and unenlightening procession of “look, it focused that perfectly” images, today I’m going to concentrate on what happens when you take the camera out of its comfort zone. There are those who will call me a Cassandra. While I understand that paying more attention to the negative aspects rather than the positive ones does not give a balanced view of the camera, today’s cameras all do so many things so well that a review that uniformly emphasized what’s good and what’s bad would be very long, much the same from camera to camera, and not provide much content that would help people get the most out of the camera in question in proportion to the length of the review.
If a camera does some thing just fine, the photographer doesn’t have to think about that thing at all. If a camera has deficiencies — and they all do — the photographer is benefited by knowing about them, so that she can avoid the situations which bring the deficiencies into play, or mitigate them with workarounds.
Last month, I took a few images of my oldest granddaughter swimming butterfly. I used the Nikon D810 and the Nikon 80-400mm f/4.5-5.6 ED VR AF-S Nikkor (the name doth flow trippingly off the tongue, does it not?), with the lens set to 400mm, and the aperture set wide open, which, at that focal length, is f/5.6. I set the drive to continuous low, and the autofocus mode to continuous.
Here’s what resulted:
Yesterday, with Sony a7RII and Sony 70-200mm f//4 OSS FE in hand, I asked me granddaughter to swim some ‘fly. I set the aperture to f/5.6 and the focal length to 200mm. I experimented with letting the camera find the subject, with flexible spot mode, with single shot, with continuous low and continuous high, with pretty much everything I could think of. I never got a good shot.
I think I know a few reasons why.
When a person swims butterfly coming towards you, there are moments when she is under the water, and moments when she is out of the water. this confuses continuous autofocus, which works by predicting the subject’s rate of closure with the camera. This is true for both the Nikon’s all-phase AF system and the Sony’s hybrid system. Once continuous AF is confused, it must focus very quickly once the swimmer comes out of the water.
The Nikon setup I used is faster than the Sony setup. I don’t know how much of that is the camera, and how much is the lens. I have no way of testing, but I do know that autofocus speed is highly dependent on the lens. The older Nikon 80-400 was snail-slow, and the Nikon 400/2.8 is shockingly fast, both used on a D3 or D4. The new Nikon 80-400 is a lot better than the old one, but by no means in the territory of the 400/2.8. Still, it was fast enough to get the job done, and the Sony 70-200/4 wasn’t.
Autofocus speed may relate to battery capacity as well. The motors in lenses could benefit from a lot of current for a short period of time to increase focusing speed, and that current may not be available from cameras with small batteries, like the a7RII. That relationship could be fixed with the addition of a supercapacitor, but I don’t think Sony would want to incur the cost, size, and weight penalty for what has got to be a corner case.
I did have some success with the a7RII by the pool. Breaststroke turned out not to be quite as challenging.
There is another difficulty connected with using the a7RII on this kind of subject. I never could use single shot mode to pick the right moment to make the exposure. I think that is because of the latency introduced by the electronic viewfinder system in the a7RII. It seems to be even worse when the camera is operated in continuous low mode. If I’m right about the source of the problem, and it is not a subtle problem, the only solution for mirrorless cameras in this situation would be a faster viewfinder refresh rate.