The photographer does not always strive to make the photograph as clear as possible. He can highlight some objects on it with good focus, others with poor focus. Also, the photographer can blur the entire photo in order to achieve an artistic effect.
If you need a significant depth of field (so that all objects come out equally in focus, but the whole image is a little blurry), use a lensless lens - the so-called stopper. Such lenses also exist for digital cameras (designed for interchangeable lenses).
By using a lens with a very deep depth of field, you can make certain objects stand out from others. Set the camera to manual focus, then adjust the focus so that the desired subject is highlighted. You can make the background blurry, and the subject sharp, or vice versa, it all depends on your artistic intent.
For black and white shots, try to achieve a predetermined blur using a special lens called soft focus. This technique is not suitable for color photography, since this device uses the phenomenon of chromatic aberration. Therefore, either turn on the black-and-white or sepia mode in the digital apparatus, or use black-and-white film in the film apparatus, or later use the computer to turn the finished picture into black and white.
When shooting, try placing sheets of slightly scratched glass, plexiglass between the lens and the object. A digital camera allows you to immediately see on the screen exactly what effect will be the result. It's even better if it is connected to a TV (most digital cameras have a video output) - this way you will even better see the result of applying the effect before shooting.
Check what kind of blur effect you get if you move the camera slightly while shooting.
To transform the finished image into a blurred one, use a raster graphics editor. It can be anything you are used to if it has a blur function. For example, in the GIMP editor, do this operation as follows: Filters - Blur - Gaussian Blur (IIR).
If you need to blur not the entire image, but only a part of it, select this part before performing this operation. This way you can select objects relative to each other after shooting.