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Small apertures can be a gift to photographers. On bright sunny days, they prevent an image from being washed out, and they save the physical film from being burned by intense sunlight. But there’s a downside to smaller apertures when it comes to achieving an optimally sharp image: as apertures get smaller, images get less sharp and can even end up with undesirable blurring effects. This is known as lens diffraction.

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What Is Lens Diffraction?

Diffraction is a term that describes light waves interfering with one another. Lens diffraction is an optical interference that occurs when light passes through a tiny opening, such as an aperture with a small value f-number. Lens diffraction occurs when both the wavelength of light and the opening itself are roughly the same sizes.

As a result, the effects of lens diffraction are most pronounced on shots taken through lenses with small focal lengths. This means that a subject in the foreground of a photograph will look sharpest with the largest possible aperture, assuming that all other factors are neutral.

What Causes Lens Diffraction?

Lens diffraction is caused by overlapping wavelengths of light forced through the same tiny opening in a lens. The overlapping waves interfere with each other by canceling out visual wavelengths. Lens diffraction is almost never so intense as to completely obscure an image, but there is no doubt that highly diffracted images lack the sharpness of images captured by higher-resolution, wider-aperture lenses.

Physicists can partially explain lens diffraction via the Rayleigh criterion, which specifies the needed separation between two sources of light such that they might be identified as distinct objects. The criterion is applied to consumer products like cameras, but also to serious professional pursuits like astronomy and optical microscopy. Theoretically, any optical system is subject to the effects of lens diffraction.

How Does Diffraction Affect Photography?

In digital photography, the effects of diffraction will depend on the number of pixels in your camera’s sensor. When a diffraction pattern hits a camera sensor, it will produce an image known as an Airy disk. The diameter of an Airy disk depends on the diameter of the camera’s aperture. As the aperture of a lens gets smaller, the Airy disk gets larger. The opposite is also true: as the aperture widens, the Airy disk’s diameter shrinks.

As a general rule, the larger the pixel size of a camera’s sensor, the more likely that an entire Airy disk of diffracted light will be confined to a single sensor. Therefore:

  • In a high-resolution camera sensor (one with large megapixels), diffraction is noticeable at wider aperture settings.
  • In less sensitive cameras (ones with smaller megapixels and lower overall resolution), diffraction doesn’t become noticeable until you reach much smaller apertures.

This stands to reason because large-megapixel cameras are more sensitive across the board. They show more of your triumphs but also more of your mistakes—including less-than-sharp, semi-distorted images.