I think you got the focus just right on this, with the antennae and eyes in focus. (An alternative would be to have the eyes at the nearest focus distance and then some more in focus going backwards, with the antennae out of focus. I think either would be fine, although with finer detail much more evident in the antennae than - even were it in focus - behind the eyes, my personal preference would be to have it the way you have it here.)
And the exposure looks fine too.
So I'm not thinking of user error here.
Let's think this through. The D810 has a 36 megapixel sensor. I'm going to assume for now (and obviously correct me if I'm wrong, because much of what follows will be wrong too) that what we are seeing here is a crop, not a downsized version of the whole frame. The full size version over at Flickr is 2048 x 1367 (which coincidentally is around the output height that I use for my insect etc close-up/macros, so it is a size of image that I'm used to looking at).
Note: There are generally complications to do with the aperture set on the camera (the nominal aperture) and the effective aperture (the one you are actually using), with the effective aperture depending on the magnification. However, you are using a Nikon camera and I'm going to assume (again, correct me if I'm wrong) that you are using a lens and teleconverter which communicate with the camera, in which case, with this being a Nikon camera, what the camera shows as the aperture will be the effective aperture (what you see is what you get as far as aperture is concerned, whatever the magnification), and so this complication does not arise. I am therefore simply going to talk about "aperture" here.
If it is a crop, and if we are looking at is what is left after the crop, unresized, then this is the extent of the crop.
NOT MY IMAGE - Son_of_Thor - Ladybird, Extent of crop by
gardenersassistant, on Flickr
If my assumption is right then we are looking at a 100% crop, and this is a big crop. That will have two effects (at least):
- In the parts of the image that are most in focus (see next point), there will be less detail than if we were looking at the same framing with an uncropped or less cropped higher magnification image.
- The depth of field is greater than it would have been if we were looking at the same framing (with the same aperture) with an uncropped or less cropped image higher magnification image. So, more will be in focus, front to back, than would have been the case with a more magnified image, but none of it will be as detailed as what is most in focus in the more magnified image.
You might want to look at
this Crop for magnification post in my Journey thread which illustrates some aspects of this.
You used flash and a shutter speed of 1/250, so motion blur from movement of the subject and/or the camera should not have been an issue.
So, how to get more detail?
- Magnify more and get better fine detail, but less in focus front to back.
- Capture an image with less magnification and crop it more. I think you've probably already got to about the limit of that option
- Use a smaller aperture. You will get greater depth of field but lose detail from diffraction softening. With insects etc that is my approach. You can to some extent reverse, or at least hide, the softening effects of diffraction with post processing. You used f/16. I imagine the 105 macro you are using goes to f/22. With a 1.4X teleconverter this would be f/32, and with a 2X teleconverter it would be f/45. So with a 1.7X teleconverter presumably you have up to around f/38 available. You might want to try using it. (I use an aperture which would be f/45 on your system.)
- Focus stack. You will see stacks using many tens of images, especially with dead insects, using a mechanical and possibly automated rail that can move in tiny increments, but you do also see outdoors stacks which use quite a lot of images, achieved either by moving the camera, or having the camera stationary and moving the focus between shots, either by hand or with the camera doing it automatically (focus bracketing). If you camera does not do focus bracketing natively you might be able to get an app which does this. However, you can significantly increase the depth of field by using just a few shots, as few as two or three can make a big difference. As you know it is difficult to get the centre of focus exactly where you want it, it tends to wander around a bit, not just side to side, but front to back. With stacking you may be able to turn this to your advantage. Take a number of shots and then see if you have some that you can stack (this would exploit the front to back wandering - good stacking software will handle the side to side and up/down wandering). The more shots you take, the more likely you will be able to stack some of them without leaving gaps. You may be able, especially if you are using a tripod, be able to combine this with using a larger aperture which will give you more detail than f/16. The price of this is thinner depth of field for the individual shots and so more shots needed to cover the distance you want to cover without gaps.
That's all that comes to mind just now.