I went to the pond again today but got held up at home so I didn't make it until around 11 AM, resulting in a good of distortion - or what I think was - but not as bad as the last time I went. I did get some decent shots, but there also a LOT of poor ones and it seemed like the issue was going in and out seemingly unpredictably.
However in lookin over the results at home I did notice something I've not seen before that has me questioning. Take a look at this seagull shot, which is relatively sharp but is in fact a bit soft if you look closely.
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This was not the peak level of sharpness I saw today, but it was one I'll keep. HOWEVER, look at the following shot, which I've cropped into the head for a reason that will soon be clear:
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These were part of a burst, so I just had the AF-on button held down and subject detection was focusing on the eye. I was shooting at 10fps. These were two shots apart in the sequence, so 0.2 seconds separated them. The first thing one might notice is that this is much softer, but more than that look at the catch light. Here's the first, better photo cropped in the same way for comparison:
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That's what the catch light is "supposed" to look like, even if still a tad soft. In that other shot, though, notice that the catch light seems to have a ghost image. It's doubled up, and if we look at the overall photo it's not unreasonable to think that the softness we're seeing here is basically just a ghosted image slightly misaligned from the first. I took the first image into Photoshop, copied the image to a second layer, moved it slightly, and changed the opacity to 50%. The result looks a bit like the bad shot from that sequence, though not entirely. For instance, the real bad shot does not have ghosting on the beak as my experiment does, but it does on the eye.
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So this raises for me a host of questions!
Is this effect consistent with what some kind of atmospheric may do? I suppose I can imagine water vapor in the air diffracting light to cause something like this, and we do have snow evaporating today.
Or, is this more consistent with some kind of misalignment of the lens elements? If it is, what would explain that the effect comes and goes, even 0.2 seconds apart during a burst of shots when nothing about the position of the various lens elements has changed?
It is related to the topic discussed above of light reflecting around inside the lens assembly?
Or, might this just be the result of incorrect focus? I don't want to post them all due to image limits, but I have sequences from 10fps bursts which should be very low effort for the AF system of for instance a duck perfectly still, literally sleeping on the ground, with a relatively sharp shot followed by one that is badly out of focus where you can see the plane of focus has literally shifted several inches to the front of the subject before shifting back in the very next shot. The focus didn't shift just a little, but several inches. - AND I can say that when taking these bursts the green AF-eye detect box stayed solidly on the bird's eye. Here is the in between image. Look at the snow that is in focus several inches in front of the duck. In the shot in the sequence immediately before and immediately after this one, the focus is on the duck's face, so all without really... anything at all changing, including the location of the AF box which was locked on the eye as subject detection, the focus shifted a few inches forward and back again in a few tenths of a second.
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So more fundamentally, are most of my soft/"thermal distortion" shots representative of this issue? Or these issues?
To be clear, I got sharpness like this today:
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So the lens is capable of that. I don't doubt its sharpness anymore. I do however have no idea whether what I am seeing in terms of how it is focusing is normal or a consequence of some factor like the atmospherics or if it's indicative of some kind of problem with the lens or the camera or both.