It would be really nice if someone can download the photos I attached and comment on the level of sharpness.
I downloaded each of the files and opened them in Photoshop for closer inspection. Here's what I see:
- Image DSC_0485: The eye is definitely soft but the lichen on the perch and the belly feathers are pretty sharp. This looks mostly like missed focus or possibly a front focus issue where the camera focused slightly in front of the eye. This could have just been from the focus mode used (e.g. Group, Auto or one of the Dynamic Area instead of Single Point) or if a Single AF point was right on the bird's eye it could mean a slight forward focus in the AF system.
- DSC_0503: This looks good on my monitor, the highlights (brighter tones) are a bit hot which costs detail in the bright throat feathers but that detail is there and can be brought out with a bit of Highlight Recovery in a tool like Photoshop or Lightroom or with a Curves adjustment but that's a bit more advanced. I don't see any obvious lens softness issues with this image.
-DSC_0508: This image is soft all over, it also has hard clipped highlights that are over exposed but even a better exposure wouldn't have helped the sharpness in this image. It's hard to know exactly which Swallow you were focusing on but nothing appears sharp not even the branches. My best guess on this one is heat shimmer from shooting through too much turbulent air based on the strange patterns in some of the out of focus areas like out of focus branches to the right of the birds. Heat Shimmer is a common problem when shooting long distances anytime there are heat differences between where you are and where your subjects are. Common reasons are shooting out of a car or house window where the temperature indoors and outdoors is quite different. This can be a warm car shooting into a cold outdoors or the other way around or it can be all outdoors but shooting across something like a roadway that heats up and stirs up the air. The two things that can help reduce or eliminate heat shimmer are getting closer to your subjects (shoot through less air) or make sure you and your gear is at the same temperature as the outdoors or be outside yourself and try to avoid shooting across hot or cold surfaces when there's a lot of heat differences. With practice you can see heat shimmer with your naked eyes as a warbling look to far away objects. Best not to shoot long distance photos when you see that kind of shimmer.
-DSC_0594: Same as the previous image, soft all over. Hard to say if any of those leaves are crisp even at 100% zoom in Photoshop but the image is overall soft and I'd still suspect atmospheric effects and heat shimmer.
-DSC_0625: Looks like the focus missed on this one and grabbed the branch to the left of the bird's head. The branch is much sharper than the eyes. The highlights are also hard clipped which robs detail from the bright white throat feathers which is an exposure problem. Dialing in maybe -0.3 stops to -0.6 stops of exposure compensation would likely have saved the whites on this image. If you shoot in raw you might be able to recover those blown highlights during raw conversion but that didn't help from a detail standpoint. The lens focus itself seems to grab sharp focus on the branch. This is actually a hard shot to nail focus on given the position of the bird's head, the eyes make tiny targets at this subject distance and most camera AF systems would tend to grab the front of the beak which would lead to front focus but the best focus appears to be on the branch slightly behind the bird's eyes which looks to me more like a focus accuracy issue and either what focus area mode was used or how well the focus point was placed right on the bird.
-DSC_0804: The eyes are definitely soft but parts of the fence, especially the plank to the right of the bird is pretty sharp. Looks like a focus accuracy or focus point placement issue rather than a lens softness issue.
-DSC_0876: Image looks a bit low contrast overall and the eyes lack catchlight and sparkle but don't seem unusually soft. Again I wonder if there was a single AF point right on the eye as it looks like other areas on the bird's body and on the branch are a bit sharper.
A few big things jump out at me looking at this set:
- The birds are all very small in the frame even on a crop body camera with a 600mm lens. It takes time and practice but the single best way to get sharper images is to find ways to get closer to your subjects. Cropping ALWAYS robs image quality and sharpness and these images have to be cropped to almost a 1:1 pixel view (cropping to a 100% zoom level in a tool like Photoshop) to get reasonably sized subjects. Even with a very high resolution camera like a D850 it's best to avoid super deep crops like that. Yeah, I know it seems impossible at first to get that close to small birds but with practice it can be done and will go a long ways towards improving your photos. You really want to get as many pixels on your subjects as possible for best quality and you can't do that when the subject only occupies a tiny portion of the frame. Being so far away also has other downsides like making precise autofocus much more difficult as even when using a single AF point it will span a large portion of your subject making it hard to know exactly what the AF system is focusing on. Also when your subjects are that small with 600mm of lens on a crop body camera you're shooting through a lot of atmosphere which as discussed above is often a problem especially if you're shooting from inside to outside like from the house to the outdoors or out a car window.
- Your shutter speeds are mostly 1/800" with two shot at 1/1000". Not sure how you support your camera but I'd try to bump that up higher when the light allows when shooting 600mm of lens on a DX crop camera especially if you're hand holding your camera and lens. You have the field of view of a 900mm lens which is a lot of lens and even the smallest shake from releasing the shutter and the mirror slapping up can result in blurry images. For me the starting point for handholding a setup like that would be up around 1/1600" though when light levels drop it's normal to make tradeoffs to keep ISO from getting sky high but then I expect fewer razor sharp keepers.
- Probably not your immediate concern till you get the sharpness issues tamed but all of these images have the subjects bullseye centered and don't use very much of the frame. As you do find ways to get closer and fill more of the frame you might explore moving your focus points around and placing your subjects off center in visually interesting ways.
From what you've posted I'd put energy into field shooting techniques including: getting closer to your subjects, mastering exposure in varying light conditions, using the most precise focusing mode the situation allows for to get focus right on an eye before I worried about changing lenses. If all of that still results in too many soft images then you might look into the lens itself but I see enough sharp areas in some of these photos that I don't think the lens is really the problem.
Just my 2 cents (ok maybe 10 cents) worth but I don't think spending a lot of money on a different lens would have made a lot of difference in the photos above.
Stick with it, you're seeing great subjects in good settings and this stuff gets easier with practice. If you haven't done so already I highly recommend Steve's ebooks on Wildlife Photography, Exposure and Nikon AF Systems.
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