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It appears your aperture is still too open for the subject to be entirely in focus, at the current distance between the lens and the subject.

You could tackle this issue in three ways:

1) Close down the aperture

An open aperture will result in a narrow depth-of-field (DOF). The DOF entails the area in the image that is in acceptably sharp focus. Seeing as how almost the entire subject is in focus, I assume that -1 stop (so close the aperture by 1 stop) would do the trick. Note that lens diffraction becomes a significant issue when stopping down the lens to small apertures. Therefore, stopping down the aperture more than F/14 will increase the amount of subject in focus but will decrease image quality.

In short: open aperture -> little in focus // closed aperture -> a lot in focus

2) Apply the focus stacking technique

As per Romeo Ninov's answer. More work, but the best solution as a lens is usually at its sharpest 2-3 stops from its max. open position.

3) Increase the distance between your subject and the lens

Generally speaking, moving closer to your subject (as with macro photography) will get you a smaller DOF. This is especially the case if you use a camera with bellows. If you move the lens away from the subject your DOF will increase. To keep the subject the same size on the image, you will need to crop the image. Do not increase the focal length to compensate for the subject size change, as this will leave the DOF unchanged from the previous setup. Do note that moving away from the aperture will also affect compression, which may be an unwanted by-effect.

How to determine DOF

Many lenses feature a scale to roughly determine the DOF at a certain aperture and distance to the subject. For a more accurate measurement, you could use a DOF calculator such as this one. You could also manually calculate DOF with the formulas on this page.

Best of luck!

Based on the article posted here, it seems that this lens sharpness does not increase with aperture size reduction:

https://www.imaging-resource.com/lenses/sony/e-16-50mm-f3.5-5.6-pz-oss-selp1650/review/

Sharpness
At 16mm and f/3.5, the Sony E 16-50mm f/3.5-5.6 PZ OSS is fairly soft in the corners and across much of the frame, but the very center of the frame remains fairly sharp. As you stop down, f/5.6 and f/8 appear to be the sweet spot with the largest center area of sharpness; however, the far corners still remain relatively soft. Zoomed in to 35mm, overall sharpness improves, and at f/8, the corners start to look pretty good, although still not tack sharp. At 50mm, you'll see the best results at f/8. Based on the numbers, the best results overall are at 35mm at f/8.

Strangely, at 16mm at f/8, we saw the largest difference between sharpness at the center vs. the corners. The center of the frame was quite sharp, but the corners, conversely, were very soft. This is unusual in our experience; normally sharpness becomes more uniform across the frame as you stop down.

At all focal lengths, once you stop down to f/16 and beyond, diffraction limiting sets in, and you'll begin to see significant loss in image sharpness.

Based on your experiments, it seems that you have reached the optical peak performance of the lens.

The same behaviour was noticed by the people at DXOMark
https://www.dxomark.com/Lenses/Sony/Sony-E16-50mm-F35-56

Sharpness 6 P-Mpix

Chistopher Frost reached the same conclusion in his review video:

https://www.youtube.com/watch?v=bh7xbdKG-oc

Are you able to rent / borrow other non-kit lens to test them?

shutter speed 0.5 seconds

This is likely to be a bit of your problem. The shutter causes vibration of the camera. So, too, does your hand pushing the release button.

At faster speeds, this vibration does not affect the shot. Likewise, at very slow speeds (a few seconds +). But there’s a sweet spot somewhere between a second or two and ~1/30 where that vibration can ruin your shot.

If you have to use shutter speeds in that space, use mirror lock up and a remote release. If no remote, use MLU and the self timer.

Oh, and open up from f/14. That’s unnecessary. You should have plenty of DoF at f/8. Test with a DoF calculator to confirm.

To get the entirety of a subject in focus, you need to increase Depth of Field.

• Increase F-number (decrease aperture).


• Increase distance.


• Decrease focal length.

I would not use an aperture smaller than F8-11 because of diffraction. Once you have found a suitable distance, you will have to crop the image because the subject will be small within the frame.

Another technique you can consider using is tilt-shift. This allows you to align the focal plane with your subject. Since you are using mirrorless, you can buy a relatively inexpensive mount adapter with tilt function to use with a full-frame lens.

Try searching for "tilt shift lens mount adapter" on your favorite shopping sites.

Increasing the aperture will increase the effect of diffraction. To make photos with big DoF you need to apply technique as focus stacking.

Focus stacking (also known as focal plane merging and z-stacking or
focus blending) is a digital image processing technique which combines
multiple images taken at different focus distances to give a resulting
image with a greater depth of field (DOF) than any of the individual
source images. Focus stacking can be used in any situation where
individual images have a very shallow depth of field; macro
photography and optical microscopy are two typical examples. Focus
stacking can also be useful in landscape photography.

And also use some kind of remote shutter to minimize the camera movement

You could try applying a post-processing filter to make it sharper. Some manufactures automatically apply a post-sharpening filter (namely Nikon) to squeeze out even more sharpness. Here is what your image looks like after applying the Shake Reduction filter in Photoshop:

You could go crazy and add a lot of sharpening, but then the image starts to get a bit of artifacting (depends on how large the image can be displayed):

What I would do for this situation would be:

• A good tripod: this is essential for keeping the camera steady. Note that good tripods (a) cost some amount of money (you won't find one much below $100) and are heavy (anything less than 1.5 kg isn't good if we're talking about regular height tripods, excluding the high-end carbon fiber ones).


• Live view mode: this ensures the mirror is up already when the picture is taken, so no mirror movements => no mirror slap, no vibrations caused by it. Some good cameras also have a mirror lockup, but I have found live view mode an adequate substitute.


• 10 second selfie timer or remote shutter release: this ensures you either give the vibrations of the tripod chance to decay, or alternatively don't cause vibrations at all


• f/6.3 - f/8. The larger the F-number, the more diffraction you get. I'd say something between f/6.3 and f/8 is ideal. This of course depends on the camera: on full frame cameras, you can use higher F numbers. Of course there's a compromise between deep depth of field and low diffraction. Typically lenses are the sharpest between f/6.3 - f/8, but you might prefer f/8 due to the slight increase in depth of field.


• Consider also an off-camera flash (or multiple flashes) with suitable light modifiers such as umbrellas. This would allow using faster shutter speeds.