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Optimum aperture

An ƒ-stop near the center of the lens's range gives the best quality image


Your lens's aperture setting has an effect on the quality of your picture.
Your lens's aperture setting has an effect on the quality of your picture.

Image quality is affected by the size of the aperture of the lens. Variations in picture quality at different apertures are usually barely noticeable, but they are there nonetheless. A lens's aberrations and the diffraction of light in a lens are the culprits. Their effects can be minimized by the size of the aperture used in taking a picture.

ABERRATIONS

A lens's aberrations (flaws) prevent light from being brought into sharp focus, reducing image quality. Aberration in simple lenses is sub-categorized into seven types:

  • Astigmatism - lines in some directions are focused less sharply than lines in other directions,
  • Chromatic aberration or Axial chromatic aberration - different wavelengths of light (colors) coming into focus in front of and behind the film plane, resulting in points of light exhibiting a rainbow-like halo and reduction in sharpness,
  • Coma - the image of a point source of light cannot be brought into focus, but has instead a comet shape,
  • Curvilinear distortion - distortion consisting of curved lines,
  • Field curvature - the image is incorrectly curved,
  • Lateral chromatic aberration also known as Transverse chromatic aberration - variation in the magnification at the sides of a lens (this aberration type used to be termed “lateral color”),
  • Spherical aberration - variation in focal length of a lens from center to edge due to its spherical shape - generally all parts of the image, including its center.

  • ALL LENSES HAVE ABERRATIONS - Some less noticeable than others

    All lenses have all these flaws to a degree. Minimizing them is known as correction. High-quality lenses have greater correction, and the effect of their aberrations is less evident.

    LARGER APERTURES MEAN GREATER EFFECTS OF ABERRATION

    The effects of lens aberration usually increase with increases in the size of the aperture. An image improves as the aperture is stopped down, or closed more. This reduction in the effects of aberrations occurs only up to a point. Selecting an aperture setting near the center of the lens's range of ƒ-stops - around ƒ/5.6 or ƒ/8 - generally results in the best image quality for that lens. As the aperture is further closed, the image begins to degrade slightly due to diffraction of the light passing through the lens.

    ƒ/8 is in the center of this lens's aperture range.
    ƒ/8 is in the center of this lens's aperture range.

    The arrow is pointing to the edge of the lens's diaphragm mechanism, which opens or closes to provide various apertures.
    The arrow is pointing to the edge of the lens's diaphragm mechanism, which opens or closes to provide various apertures.

    DIFFRACTION

    Diffraction occurs when a light ray contacts the edge of an opaque object, changing direction, just as water in a stream is deflected when meeting the edge of a rock. In a lens, diffraction happens where light rays meet the edges of the diaphragm mechanism.

    The degree of image degradation due to diffraction depends on the proportion of rays that pass through the center of the aperture and the proportion that meets the edges of the aperture. Light travels without diffracting through the center of the lens, away from the edges of the aperture. As the aperture is made smaller, the amount of diffracted light increases since a higher proportion of the light strikes the edges of a smaller aperture compared with a larger one.


    The effect is not as bad as it sounds, though, since the degradation caused by light diffraction in a lens, even stopped down to its smallest aperture (ƒ/22, for example), is almost unnoticeable to the eye.

    Image degradation due to diffraction is virtually undetectable at any aperture setting, even a very small one.
    Image degradation due to diffraction is virtually undetectable at any aperture setting, even a very small one.

    Where great depth of field is not a concern, the particular aperture you select is less critical. Choose an aperture in the middle of the range for optimum image quality.
    Where great depth of field is not a concern, the particular aperture you select is less critical. Choose an aperture in the middle of the range for optimum image quality.

    If you happen to be taking a photograph where aperture selection is not critical, such as the picture of the helicopter on the left where depth of field is basically immaterial, and you want the sharpest-possible picture, select an ƒ-stop in the middle of your lens's range of aperture settings.

    The critical setting in photographing a helicopter in flight is shutter speed, where a fast shutter speed will freeze the movement of the blades and a slow shutter speed will blur their movement.


    Related topics...

    How light behaves

    How much will be in focus?

    Hyperfocal distance

    Aperture

    Aperture selection guide