The photographic distance affects the depth of field.

Depth of field is actually the sensitivity of focus to distance.

You can review the physical optics and mathematics parts of middle school. The formula of convex lens is: 1/u (object distance)+1/v (image distance) = 1/f (lens focal length). Convert this formula into v=uf/(u-f). When the focal length f is variable, u is fixed.

If the above formula v=uf/(u-f), F is certain, and U is a variable value, we can also find that U is also a function of V, like a parabola, which is the relationship of the third point. The closer the distance, the more obvious the change of image distance.

A point on an object is imaged on the negative, and when it is focused, the image is also a point. Everything at a certain distance before and after the object is almost out of focus, so people's eyes are actually invisible. This allowed range is the focal length.

The optical path of this point, from the lens to the basement membrane, is a cone with a perfect image at the apex and a lens area surrounded by an aperture at the bottom. The larger the aperture, the larger the bottom diameter and the larger the cone angle. Without a perfectly imaged object, the image spreads along the cone angle to form a larger virtual spot, so the focal length is smaller.