filter applies to the element itself

The filter property modifies the visual rendering of the element and everything inside it — text, images, children, borders, and backgrounds. If you blur a container with filter: blur(5px), all its children become blurry too. This is ideal for image effects, dimming elements, or creating disabled states. The element's box model and layout are not affected.

/* filter vs backdrop-filter — when to use each */

/* filter — applies to the ELEMENT itself */
.blurred-image {
  filter: blur(4px); /* blurs the image */
}

/* backdrop-filter — applies to what's BEHIND the element */
.glass-card {
  background: rgba(255, 255, 255, 0.2);
  backdrop-filter: blur(10px);        /* blurs the background */
  -webkit-backdrop-filter: blur(10px); /* Safari support */
  border: 1px solid rgba(255, 255, 255, 0.3);
}

/* Common backdrop-filter uses */
.frosted-nav    { backdrop-filter: blur(20px) saturate(180%); }
.modal-overlay  { backdrop-filter: blur(4px) brightness(0.5); }
.tooltip        { backdrop-filter: blur(8px); }

/* Key difference:
   filter: affects the element's own rendering
   backdrop-filter: affects what's visible behind the element */

backdrop-filter applies to what is behind the element

The backdrop-filter property applies effects to the area behind an element, not the element itself. The element must have a partially transparent or semi-transparent background for the effect to be visible. This is how you create frosted glass overlays, blurred navigation bars over scrolling content, and modal backdrops. Common use: background: rgba(255,255,255,0.3) with backdrop-filter: blur(10px).

Choosing the right property

Use filter when you want to modify how an element looks — image effects, grayscale, brightness adjustments, drop shadows that follow alpha channels. Use backdrop-filter when you want to modify what appears behind an element — frosted glass cards, blurred modal overlays, navigation bars with blur-through. If you need to blur a background image without blurring the text above it, backdrop-filter is the right choice.