After you have been taking photos for a while, you may advance to a point where you are trying to limit the depth of field or DOF to make the subject of your images more pronounced or really stand out in your images. When doing so, it can make your images soft and it is all about the math. The more light that the lens lets in or the larger aperture, the less of what is coming through the lens is in focus. More light means less overall detail in your image due to a smaller DOF or plain of focus. There is lots of info on the web about DOF and Cambridge in Color was one of my favorite places to read about photography to dig in deeper.
In most cases, better lenses let in more light and are more expensive. On a budget, and you don’t have a nifty 50, get you one to play with and really learn about DOF through practice. Download a free DOF phone app too and use it. The better your lenses the more you need a calculator or to memorize the chart.
Below are three shots: a sharp one, a close up of a missed shot and the full image of the missed shot. Look at the closeup of the missed shot. You can see the legs are in sharp focus, but the eye and face are soft. This is where many photographer’s cameras focal points are not on the right spot to get the eye in sharp focus, which is considered necessary in wildlife photography.
Sharp shot with controlled DOF, creative exposure to stop motion and highlight the bird’s splash and eye along with the reflection.