Let’s Talk Photography – Ep.50 – Aperture

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Bart continues his series explaining photographic terms and phrases in this solo show all about ‘Aperture’.

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Show Outline
  • In science an aperture is simply a hole in a surface.
  • In photography, the aperture is the width of the opening in the lens that the light passes through on its way to the focal plane where the image is recorded (on a sensor or on a piece of film)
  • Aperture is one of the three legs of the exposure triangle (along with exposure time and sensitivity, or ISO)
  • On many camera systems the size of the lens aperture can be controlled, but not on all
    • Smaller, simpler camera systems like those in phone cameras often have fixed apertures
    • Larger systems like Micro 4/3 and (D)SLR almost universally allow control of the lens aperture
    • Variable aperture is achieved with overlapping blades, and the number & shape of these blades has noticeable effects:
      • Determines a lens’s bokeh
      • Determines the arrangement of a lens’s diffraction spikes (sunbursts etc.)
  • Aperture size is one of the factors that determines depth of field
    • When light passes through an aperture it get deflected a little (Fresnel diffraction). The amount of deflection depends on the size of the aperture:
      • A pinhole has effectively zero diffraction
      • The bigger the aperture, the more light gets diffracted
      • The more diffraction, the quicker focus falls off around the item that’s in focus
      • The bigger the physical hole, the shallower the depth of field
  • In photography, aperture size is represented as a focal ratio or f-number, not as a radius, diameter, or circumference as you might expect
    • Why?
      • Physical size of aperture is meaningless across camera systems
        • A 3mm aperture is tiny on a full-frame DSLR but massive on a camera phone!
        • Even on the same camera, a 3mm aperture on a wide-angle lens will result in a much brighter image than a 3mm aperture on a telephoto lens
      • Focal ratio is more meaningful across more camera systems
    • The focal ratio is also known as the f-ratio, f-number and f-stop
    • The focal ratio is usually written as f/N, e.g. f/1.4
    • The focal ratio is defined as the focal length of the lens divided by the diameter of the aperture
    • Because the focal ratio is a ratio it’s a dimensionless number, so you can’t mess up the units 🙂
    • Notice that the bigger the aperture, the smaller the focal ratio!
  • Why would you want to control aperture from an artistic point of view?
    • To control DOF
      • Sometimes you want to isolate your subject by having a shallow DOF, e.g. a portrait
      • Sometimes you want a really deep DOF to get a lot of things into focus, e.g. a landscape
    • To let in as much light a possible in low-light extremes, e.g. astro photography
    • To let in as little light as possible to make longer exposures possible, e.g. trying to blur a waterfall or clouds moving across the sky

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