Gaussian blur
Gaussian Blur softens your image with an even, natural blurriness. A single radius slider controls how strong the blur is. It is the most-used blur filter in Photoshop.
Choose Filter, Blur, Gaussian Blur and set the radius to the effect you want. Use it to calm down a background, soften a mask or build the low frequency in frequency separation. With a layer mask you limit the blur to exactly the area you want. Tip: apply it to a smart object, so the radius stays editable and you can bring back sharpness locally.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.