I am learning to take macro images using Nikor 105 mm macro lens and D750 body. The below images were shot using the same gears. The spider and the fly are about an inch in length. I shot them in the 1:1 magnification with the closest focal distance of 6 inches.
Is there any way to shot sharper images/images with more details? I am using f/20 and shutter speed of 1/160 with built-in flash and a self-made paper diffuser.
You seem to be bumping up against the limits of depth of field when shooting at macro distances. Since depth of field is proportional to focus distance, the shorter the focus distance is the narrower the depth of field will be for the same f-number.
With such critical distances the camera needs to be locked down on a stable mount, preferably one that will allow you to control subject distance when the lens is at minimum focus distance/maximum magnification. Tripod heads with macro rails are designed to do exactly that.
The first image of the fly seems to be focused behind the fly's head and centered on the torso. The second image of the spider is focused a little in front of the spider's head. The area of sharpest focus is about halfway between the head and the end of the two front legs. Using a macro rail to adjust focus while viewing via magnified Live View might have allowed you to get the focus squarely on the faces of the fly and spider.
To get more in focus you probably are going to need to start doing some focus stacking. This is taking multiple shots with slightly different focus distances and combining the sharpest parts of each image.
There are a number of existing questions and answers here regarding focus stacking with macro photography:
Image Stacking for macrophotography
Is there a way to overcome focus breathing in deep macro focus stacking?
Are there lenses that exhibit absolutely no focus breathing?
Macro photography stacking: focus shift vs. camera movement
What are the pros and cons of using macro rail vs lens focus?
How do I calculate how far a macro rail needs to travel between shots for focus stacking?
Why does my focus stack result in a blotchy background?
How to get rid of halo effect when focus stacking with helicon focus?
Google for "focus stacking" (Yes I know a bit cheap answer, but it would be a long boring text if I write it by myself and also worse than the great tutorials you can find out there.)
