What is the primary effect of using a shallow depth of field in photography?

Using a shallow depth of field primarily allows photographers to emphasize the subject of their image while blurring the background. This technique isolates the subject, drawing the viewer's attention directly to it, which creates a more intimate and focused composition. When the background is rendered out of focus, it reduces distractions, making the subject stand out prominently against a soft, blurred backdrop. This approach is especially effective in portrait photography, where the aim is often to highlight the subject's features and expressions while diminishing elements that might compete for attention.

Other options do not pertain to the effects of shallow depth of field. Keeping all elements in focus relates to using a deep depth of field, which serves a different purpose. Creating a stark monochrome image is not influenced by depth of field but rather by how color or tone is used in the image. Increasing noise in an image is typically associated with high ISO settings or poor quality in image processing and is unrelated to the concept of depth of field in photography.