You must first find out whether your overall clarity refers to a plane or an overall three-dimensional. If it is a plane, it is the problem of the optical axis; If it's three-dimensional, it's the depth of field.
2. At close range, the depth of field is shallow, which is affected by aperture, focal length and shooting distance. The larger the aperture, the longer the focal length, the closer the distance and the shallower the depth of field. That's pretty much your situation. ?
Therefore, if you can't change the focal length and don't want to change the distance, there is only one way, which is to narrow the aperture. Of course, it is estimated that the exposure time will be longer and a tripod will be needed.
Extended data
SLR cameras (SLR cameras) are also called SLR cameras. Refers to a camera that uses a single lens, through which light shines on the reflector and takes a view through reflection.
The so-called "single lens" means that the photographic exposure light path and the framing light path * * * use one lens, unlike the paraxial camera or the double-reflex camera, the framing light path has an independent lens. "Reflection" means that a plane mirror in the camera separates the two light paths: when taking a view, the mirror falls, reflecting the light of the lens to the pentaprism and then to the viewing window; When shooting, lift the reflector quickly, and the light can shine on the film or photosensitive element CMOS or CCD.
It can be seen from the structural diagram of a single-lens reflex camera that the light reaches the reflector through the lens and is reflected to the focusing screen above to form an image. Through the eyepiece and pentaprism, we can see the scenery outside in the observation window. The light passes through the lens and is reflected by the reflector to the frosted viewfinder. Through the convex lens and reflected in the pentaprism, the final image appears in the viewfinder. When the shutter is pressed, the mirror moves in the direction shown by the arrow, and the mirror is lifted, and the image is taken on CCD or CMOS, which is consistent with what is seen on the viewfinder screen.
Compared with paraxial cameras, SLR cameras have the advantage that what you see is what you get, and the imaging angle in the viewfinder is the same as the final film. However, compared with paraxial camera, the back focus of SLR camera lens should be able to image at the focal plane of reflector and photosensitive element at the same time, and false focus must be generated before imaging at the focal plane, which makes the structure of SLR camera optical lens more complicated and larger, and the imaging effect is not as direct and transparent as paraxial camera. At the same time, the volume of the "reflector" should be large.
References:
Baidu encyclopedia-SLR imaging principle