You may have noticed that as you stop down your lens, your images become sharper at first, but after a certain point, they actually become softer. Here is why no matter what lens you use, eventually, you will experience softness issues as you use narrower apertures.
Coming to you from ZY Productions, this great video will show you the concept of diffraction and why it causes softness issues at narrow apertures. The thing to remember about diffraction is that it is a physical phenomenon, and thus, there is no way for manufacturers to get around it. Thus, it is important to be aware of it when you work at narrower apertures. You might be tempted to stop down to something like f/22 for better depth of field across your image or a similar f-stop so you can use a lengthier shutter speed for a long exposure. Instead of dropping your aperture to such an extreme f-stop, consider using a technique like focus stacking or calculating the hyperfocal distance or adding an ND filter if you need to extend your shutter speed, particularly if sharpness is the most important aspect of your images for you. Check out the video above for the full rundown.