I find all your images sharp as expected given lens and aperture. Focusing is in place. Diffraction kicks in a bit due to narrow aperture but nothing disturbing to me. What really bothers me is sensor saturation. The second image suffers from being light deprived (not solely due to the clouds !).
On sensor saturation
Since the 2 photos require a similar shooting approach, let's try to analyze the EXIF metadata below.
On the photo you like
- Focal length : 40mm
- Aperture : f/7.1 - trade-off is below par. Best sharpness is at f/5.6 for this lens and large depth of field is not required here.
- Shutter speed : 1/30 - Handheld limit reached (IS may have been activated.)
- ISO : 400 - Minimum ISO for proper exposition given the aperture.
The maximum sensor saturation is reached as we cannot go below 1/30. ISO noise remains low at 400 but could have been even a bit lower at ISO 250 by opening at f/5.6. Overall, the settings are balanced enough to reach maximum sensor saturation. As the scene is low contrast, the colors are vivid and details pop. Also the highlights are near blown, ETTR is achieved.
On the photo you don't like
One of the second photos EXIF, they all share about the same characteristics.
- Focal length : 40mm
- Aperture : f/9.0 - justified as a compromise for maximum depth of field and sharpness.
- Shutter speed : 1/1250 - Handheld limit not reached yet (1/40 given the focal length, use a tripod to go below).
- ISO : 500 - Base ISO not reached (ISO 100).
If the ISO was dropped to 100, the camera would have given a Shutter speed of 1/250 for the same Exposure Value. The difference is that your sensor would have been exposed 4 times longer. Longer exposition will translate into more image details. Overall, the settings used here a far from optimal for maximum sensor saturation. It emphasizes the image flatness and fuzzy details. Also, highlights are nowhere near blown. ETTR is not achieved there, the sensor would also benefit of a Exposure Compensation bump.
Lacking sensor saturation translates into softness in area with similar tones (e.g. grass, lake, rocks, dead trees...) making the whole image look flatter. Sharpness and post production cannot do anything to this problem. Once acquired, sensor data cannot be changed. In the future, be sure to drop the shutter speed to the lowest possible to make full use of your sensor and lens. If you are shooting RAW, ETTR improves sensor saturation even further.
On sharpness
Another difference between the 2 photos is the angle between the subject and the plane of focus. As landscapes are generally a more horizontal subject than the jewelry panel. You might want to focus stack your landscapes to have a greater resulting depth of field. 3 images would suffice : focus at bottom of the frame then, focus at 1/3 of the frame, last focus on infinity or farthest object before sky. It also helps against diffraction as it requires less depth of field per take.
On gear
If after trying getting better sensor exposure, you are still unsatisfied about the lens rendition (sharpness, diffraction, fidelity...) it may be time to switch gear. Generally, prime lenses can be sharper and transmit more light nuances as they are built for a specific focal length and application only. The best ones can be multipurpose. Reading reviews of your lens, it seems that it is not appreciated for its sharpness but for its versatility (24-105 is indeed a steep range for a high quality trans-standard zoom; more than a 70-200 or 100-400 telephoto zoom IMHO).
Or you can deal with it for now. Learn your gear limitations and how to bypass them. You'll learn a lot of knowledge and it will make your next acquisition a wiser decision.
The positive point is that you are critic to your own images and trying to get the best from your gear. Keep that attitude going !
