The Diffraction Limit: RF 24-70mm f/2.8L IS USM Past f/11
Every lens on every sensor has a diffraction limit — the aperture beyond which stopping down no longer increases sharpness but instead decreases it. Light diffracts around the aperture blades, spreading the point spread function across adjacent pixels. On a high-resolution sensor like the R5’s 45-megapixel chip, diffraction becomes measurable at around f/11 and visible at f/16. By f/22, you have traded resolution for depth of field at a poor exchange rate.
The RF 24-70mm f/2.8L IS USM reaches peak sharpness between f/5.6 and f/8 across the zoom range. At f/11, sharpness drops by a small but measurable amount. At f/16, it is visible in direct comparison. This is not a defect of the lens — it is a physical property of all lenses on high-pixel-density sensors. The R5 at 45 megapixels is more diffraction-sensitive than the R6’s 20-megapixel chip because the pixel pitch is smaller; diffraction spreading covers proportionally more pixels.
The practical consequence: if you are shooting landscapes with a foreground element at two meters and a distant background, f/16 gives you the depth of field that f/8 does not. You accept a diffraction penalty in exchange for spatial coverage. That trade is often correct. If you are shooting a flat subject — a painting, a document, a product on a table — f/8 is the answer, and stopping down further is damaging.
The technique is knowing what you are trading and making the trade deliberately. Use Canon’s depth of field preview button to assess what stops are actually covered at f/8 versus f/16 in your specific shot. The difference is frequently smaller than it appears. Modern focus stacking — shooting two or three frames at different focus distances and blending them in Photoshop or Helicon Focus — delivers better overall sharpness than a single frame at f/16 and eliminates the diffraction penalty entirely.
f/22 is available. It is not always useful. Understanding why is more valuable than the aperture ring itself.