Fri 21-Jun-13 05:58 AM | edited Fri 21-Jun-13 06:06 AM by nrothschild
Here is a real world (uncropped) example of why one central AF point is far from ideal, in a wildlife situation.
Doing a focus/recompose on a small bird like this Ovenbird is extremely risky. If they aren't hopping along a branch they are bobbing their head around or otherwise moving slightly. In this case the DOF is a fraction of an inch and focusing on the breast could result in an out of focus eye.
If I had used the central sensor without a focus/recompose I would have been at risk of clipping the tail. And beside that, there is more to wildlife shooting than just getting a shot of bird. A little composition helps .
And that is usually not a bird square in the center of the image. That's boring. Ideally I would like to work the bird into its habitat with a tasteful composition. And I usually have only a couple of seconds to do all that.
The issue, with current Nikon lenses, is purely a TC issue because up until the past year or so Nikon could not make an AF lens slower than f/5.6 because if they did they would be making a lens they don't support on their own AF cameras (in AF mode).
This is why Nikon has never made something like the Sigma 500/6.3 zooms. They can't. Or couldn't until now. Sigma can, only because Sigma doesn't have to support the camera.
(Despite what I just said I have always thought there is some strangeness in what Sigma is doing, even if they do not have to directly support the camera. They are still making something that is, very technically, not fit for purpose).
Another example would be birds in flight because you don't necessarily want the bird centered in the frame, and there might be more than one bird. In that case it is impossible to focus/recompose.
Look at it another way... would you pay thousands of dollars for a camera with only one central focus point? Once you strap in a TC working smaller than f/5.6 then that is what you have effectively done. And you might have paid $10K for that lens, and now you have a body less capable than a $600 entry level camera model.
As a wildlife photographer who would like to make his TC17 work better with his f/4 long lens, I don't like the idea that any of the focus points of an "f/8 camera" are only capable of f/5.6. For the same reason that I consider all these cameras highly compromised simply because only a central cluster of AF sensors is cross type.
I often run into the situation where I want to use an outer sensor for a difficult very off center composition but then I have to pause and think about the fact that I am compromising AF by using a non-cross type sensor. The same would apply to f/8 - f/5.6 sensors.
I would never spend thousands on a camera that only has one cross type sensor. It is simply too limiting. And the D7100's single f/8 sensor is similarly very limiting.
You don't list any long primes in your profile. Life is not easy shooting subjects that can appear anywhere with a prime lens. The end result is you are almost always too long or too short. Rarely just right.
While it may be relatively rare to be too long, those are the rare, and often impossible to repeat, shots that simply cannot be blown. Those are the lifetime 5 star keepers. An easy way to blow a shot like that is to botch a focus/recompose because the subject moved.
The TC17 is a very controversial TC. I don't think anyone likes it as much as the TC14. Some don't consider it usable. Others, like me, use it sparingly, more or less in desperation.
It is my belief that the problems with the TC17 are more related to AF than optical quality. That because it does occasionally deliver brilliant results but consistency is always much poorer. That suggests focus problems because IQ can't occasionally get better - it is what it is and cannot vary.
Beyond all that, keep in mind that all AF engines work better at larger focal ratios. So if you have an f/5.6 sensor it will work better at f/4 than f/5.6. By extension, an f/8 sensor should work an f/5.6 configuration with more accuracy than an f/5.6 sensor because it is working well within its spec, and not at the edge at the absolute wall.
This is a lifer image for me. I've spent a couple of years looking for a clean shot of an Ovenbird, which spends most of its time scratching around the forest floor- not great habitat for a photographer. They are more often heard than even seen.
After all that searching I had exactly 5 seconds to get off 4 shots. The other 3 shots had the eye at least partially obstructed by the branch. In that case I don't want to be hobbled by a camera that can only focus properly with one sensor.