Nikon D3X vs Sony Alpha 850 – Autofocus
Autofocus is one area where the Nikon D3X really comes into its own. It uses the same 51-point AF system (with 15 of these being cross-type sensors) as the Nikon D3S, D700 and D300S, and it has a huge range of settings and custom options.
First of these is the ability to select the number of AF points being used, with 9, 21 or 51 being selectable.
All 51 points can be used with Nikon’s 3D Matrix tracking system. This uses the same module that is used by the metering and white balance system to track an object around a frame and
adjust the focus accordingly.
With the D3S, the 3D Matrix AF is ideal for documentary, sports and wildlife photographers who want to pan the camera and leave the AF to track the subject. Of course, the slower shooting rate of the D3x limits its usefulness for fast-moving sports photography, but for most wildlife subjects it is still usable, as I found out when photographing deer.
Of course, there is always the option to use just one of the 51 AF points and then to use continuous focus mode, but 3D tracking moves with the subject should it move away from that single AF point.
Fancy modes aside, when set to AF-S mode the D3x is fast and responsive, even in fairly low-light conditions. However, Sony’s Alpha 850 is no slouch when it comes to autofocus. It has only nine AF points, which are set in a diamond shape around the middle of the image frame. There are ten additional AF points, but these aren’t selectable or visible through the viewfinder. They are instead used internally by the camera to help focus with greater accuracy.
At the centre of the diamond layout the centre AF point is a cross sensor, and as such is more sensitive than the surrounding points. Although the Nikon D3x may outnumber the Sony Alpha 850 when it comes to AF points, I found that the 850 quickly locks on to focus, although the centre point is noticeably faster than the surrounding ones.
For the most part I’d recommend that photographers use the Sony Alpha 850 set to the Local Area AF setting. This allows any one of the main nine AF points to be quickly selected via the rear thumb control, which I found fast and easy to use.
I was a little surprised to see that EyeStart AF hasn’t been included in the Sony Alpha 850. EyeStart uses two sensors below the viewfinder to activate the AF as soon as the camera is moved towards your eye. I find it useful on Sony Alpha cameras when taking quick snapshots, but on the whole I can live without it because whenever the viewfinder knocked against me while I was walking, the EyeStart activated the AF and drained the batteries. However, the sensors are still in place on the Alpha 850, but this time they are used to turn the rear LCD off as soon as your eye (or anything else) is placed near the viewfinder, actually saving a little battery life. I just wonder whether Sony couldn’t have left the feature in, but left it turned off by default.
While the Nikon clearly has the better AF system, few photographers will ever really get the most out of it because fast, continuous AF tracking speed is quite low down on the requirements for the studio, landscape or candid photographer. Therefore, it would be difficult for an enthusiast to justify the extra money the Nikon D3X commands over the Sony Alpha 850.
