Subjects not focused when they are too far away

Question

I recently bought a Nikkor 200-500 f/5.6 lens. I've been trying it out with several kinds of subjects and results have been a little mixed.

When the subjects are close the focus is sharp. This bird was around 5m away from me, and I was using a 1.7 teleconverter as well.
_{Click any for full size}

But when the subjects are far away, the image is very soft . This person was probably 200-300m away. 1.7x TC was on.

Another example without the TC, subject hundreds of meters away.

Does this mean that:

• My shutter speed is too slow for the distance?
• The air softens the image?
• The lens simply cannot focus that far?

I don't expect this to be a problem with the lens itself as the lens is brand new and it gives beautiful images when focusing "not-so-far-away". The teleconverter doesn't seem to soften the image significantly either.

Answer 1

I'm not really seeing that any distance is softer than another.
Using screenshots at 100% zoom, I've focussed in on one significant area of each.

From this distance, they each look as soft as the others. The first looks slightly back-focussed, the others you wouldn't be able to tell at such a focus distance.

Of the three, I think the rower is actually the sharpest, until you zoom further. The airplane is being masked by atmospherics so it's not easy to tell at this point.

Once you zoom further, you can see colour aberration on the first two but not the third. I'd have to assume this is being generated by the teleconverter.

I've ignored noise, because the bird pic does seem excessively noisy. Hard to say exactly why without the exposure details.

Answer 2

As I am not Nikon user my answer will be mostly speculation.

DSLRs usually have a minimum aperture they can focus. For example (from Nikon web site). And with 1.7 teleconverter F5.6 become F8 to F10 and the camera may have a problem to focus.

Nikon Advanced Multi-CAM 3500DX autofocus sensor module with TTL phase detection, fine-tuning, 51 focus points (including 15 cross-type sensors; the center point is available at apertures slower than f/5.6 and faster than f/8 or at f/8), and AF-assist illuminator (range approx. 0.5 to 3 m/1 ft 8 in. to 9 ft 10 in.)

Answer 3

In my experience, that’s how long lenses tend to work.

They are hard to focus perfectly and produce low contrast images in ordinary daylight at long range due to atmosphere and thin depth of field…in your case about 1.3m of equivalent full frame focal length.

With practice and persistence, your pictures will improve.

As a tip for small bird pictures, use a fill flash.