Let’s Talk Photography Ep.70 – Lens Lingo

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• Bart Busschots (host) – @bbusschots – Flickr

In this solo show Bart tries to breakdown all the photographic jargon associated with lenses.

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Show Notes:

• On some cameras, the lens is permanently attached to the camera, but on more advanced cameras lenses are Interchangeable, i.e. lenses can be removed and multiple different lenses that will work with the camera can be purchased. There is no single standard for interchangeable lenses. Each incompatible standard is referred to as a lens system.
• While we refer to a camera lens as if it were a single optical lens, it’s actually a very complex optical device consisting of many parts. You may also hear a camera lens referred to as a photographic lens, or even as a photographic objective, though that latter terms has fallen out of fashion in recent decades.
  • From an optics point-of-view, a single camera lens contains many pieces of shaped optical glass, each referred to as a lens element.
  • Some lens elements consist of multiple optical lenses cemented together. The cement used is transparent, so think of it as being more like glue.
  • Many of these lens elements are covered with special lens coatings (chemicals).
  • There are often physical gaps between lens elements, and its very important lenses are well sealed to keep dust out of these internal voids.
  • Together, the lens elements are known as an optical train.
  • We refer to the outside lens element, the one you have to keep clean with your lens blower and lint-free lens cloth as the front element.
  • There are two reasons for these often complex optical designs:
    • They counter-act lens distortions or lens aberrations (more on these later)
    • They allow some properties of the lens to be altered, either by manual physical controls, or by electronically operated motors or servos
  • As well as their complex optical train lens can contain moving parts:
    • Aperture blades are overlapping sheets of thin matt-painted metal that can move in an out to change the size of the hole or aperture through which light can pass as it moves through the lens. These blades are arranged so the opening is always centred on the centre line of the lens’s optical train, and the opening is approximately (but never perfectly) round.
      • The exact number and shape of the aperture blades products visible effects in your photographs:
        • The number of blades will determine the amount of spikes in the star-like diffraction spikes you’ll see when you shoot a bright light source like the sun.
        • The closeness of the shape to being round will determine the smoothness of the out-of-focus areas. Less round will result in harsher (less blurred) details in the out of focus areas, while more round will result in smoother (more blurred) details.
        • The aesthetic quality a lens produces in the out of-focus areas of images is referred to as the bokeh. So many under-informed photographers wrongly believe bokeh is the name for the our of region area itself that it has lost all meaning and tends to make a lot of people very cranky 🙂
    • Most lenses allow the distance in front of the lens at which an object has to be to be in sharp focus to be altered. This involves physically moving lens elements within the lens train.
      • Historically, the movement of the lens elements was performed manually through some kind of hardware control on the outside of the lens that the photographer had to operate.
      • Modern lenses contain motors for moving the lens elements electronically.
      • The most common physical control for a lens’ focus is a so-called focus ring, a rotatable ring around the barrel of the lens. Many modern lenses still have a focus ring so the lens can be operated manually when desired.
      • Lenses with focus rings often have a focus scale somewhere on the lens barrel which show the distance in front of the camera where subjects are in focus. These scales will generally mark two important focus settings on the scale:
        • infinity
        • The so-called hyper-focal-distance (see LTA 53 for details), the distance at which the camera has the maximal depth of field — everything from the hyper-focal-distance out to infinity will be in focus.
      • Many lenses with auto-focus motors will have a physical switch for disabling that motor, allowing focus to be controlled manually with the focus ring.
• Every lens has a closest focus — the closest distance from the sensor-plane at which it can focus. This number will be listed in the lens’ specifications.
• The focal length of a lens is the distance from the centre of the lens to where light that enters the lens parallel will cross, and is usually measured in mm.
  • For a given sensor size, the focal length determines the lens’ field of view. The same focal length lens in front of different sized sensors will result in different fields of view.
  • The Effective Focal Length or EFL is the focal length needed to produce the same field of view on a 35mm sensor.
  • The simplest lenses have a single focal length, this means they have a single field of view. These lenses are referred to as, single focus, fixed focus, or prime lenses.
  • Different focal lengths magnify the world by differing amounts. This affects the perspective of the image. A lens with a focal length of about 50mm (not an EFL, and actual focal length) will not magnify the world, so it will not distort the perspective. This is called natural lens, or colloquially, a nifty fifty.
  • Lenses with a focal length below 50mm have a negative magnification, and increase the size of foreground objects relative to background objects.
    • We call focal lengths below 50mm wide angle.
    • Focal lengths below about 28mm EFL are often referred to as ultra-wide
    • Extremely wide lenses (10mm and lower) create very pronounced distortions, and are generally referred to as fish-eye lenses
  • Lenses with a focal length greater than 50mm have a positive magnification, and will reduce the size of foreground objects relative to background objects.
    • We call lenses with a focal length greater than 50mm telephoto.
  • Lenses with a variable focal length are known as zoom lenses.
    • The range of the zoom can vary greatly. Zoom lenses which go all the way from a very wide angle to a very strong zoom (e.g. 18mm to 300mm) are often referred to as super zooms.
  • A common mistake is to confuse zoom and telephoto.
    • It is true that a zoom lens that zooms from 70mm to 250mm is both a zoom lens and a telephoto lens, those two words are not synonymous!
    • A zoom lens that zooms from 10mm to 20mm is a wide angle lens, and not a telephoto!
    • Similarly, a 70mm fixed-focus lens is a telephoto, but not a zoom lens!
  • Increasing the focal length on a zoom lens increases the magnification, and is referred to as zooming in.
  • Decreasing the focal length on a zoom lens decreases the magnification, and is referred to as zooming out.
  • With a fixed-focus lens the only way to increase or decrease the size of the subject in the frame is to move closer and further away from the subject, an act sometimes referred to as sneaker zoom, or zooming with your feet.
  • When a subject comes close to the edges of the field of view the zoom is said to be tight. E.g. ‘wow, you managed to zoom in really tight on that little butterfly’
• Aperture-related
  • The focal ratio is also known as the f number or f-stop (for details see TLP 50).
  • Small numbers mean a large opening to let light into the camera, and large numbers a small opening.
  • Small numbers mean a shallow depth of field, large numbers a deep one (for details see LTP 53).
  • Like focus, the aperture setting was traditionally controlled by an aperture ring around the outside of lenses that could be turned to adjust the f-number. Modern lenses generally don’t have a hardware control for aperture anymore.
  • Every lens has a minimum and a maximum possible aperture, and for zoom lenses the minima and maxima may vary with focal length.
    • While the spec sheet for a lens will list both the minima and maxima, photographers generally only refer to lenses by their minima. And f/1.4 lens is a lens with a minimum aperture of f/1.4.
    • When only one number is used that usually means the lens has a fixed focal length, or, the same minimum aperture is available at all zoom levels.
    • When two numbers are mentioned the first is the minimum f number when the lens is zoomed all the way out, and the second when the lens is zoomed all the way in.
  • A lens with a low minimum aperture is referred to as bright
  • At the point you take a photo the camera’s aperture setting will be set somewhere in the range between the minimum and maximum possible f number.
    • Adjusting the focal ratio upwards (bigger f number, smaller physical opening) can be referred to as stopping down or closing down the lens.
    • Adjust the focal ratio downwards (smaller f number, bigger physical opening) can be referred to as opening up the lens.
• Perspective Effects
  • A lens which keeps straight lines straight and preserves angles is known as a rectilinear lens
  • Lenses that are not rectilinear will cause parallel lines to bend relative to each other.
    • Wide angle lenses will cause parallel lines to bend outward from each other at the centre of the image, we call this barrel distortion or barrelling. Barrel distortion looks like the centre of the image is bulging out like the middle of a barrel.
    • Telephoto lenses will cause parallel lines to bend inwards towards each other at the centre of the image, we call this pincushion distortion, or Pincushioning. Pincushioning cause the centre of the image to be pinched in, like someone pulling a stretched rubber sheet down from below by its centre.
  • When a wide-angle lens is angled upwards it distorts vertical lines so that parallel lines appear to converge. This is known as keystoning and is a big problem in architectural photography.
  • With normal lenses, the centre of the lens is always aligned with the centre of the sensor, and the lens is always parallel to the sensor. Some lenses allow the lens to be moved (shifted) relative to the centre-line of the sensor, and to be angled (tilted) relative to the sensor. Such lenses are referred to as tilt-shift lenses.
    • Tilt-shift lenses can be tilted and shifted to counter-act keystoning, allowing wide-angle architectural shots that are angled upwards where the edges of the buildings remain parallel.
    • Tilt-shift lenses can also be used to move the plane of focus to be parallel to the subject rather than the sensor. This can allow for much more of a slanted surface to be in focus than would otherwise be possible.
    • The same ability to tilt the focal plane relative to the sensor can be used to minimise the depth of field, giving real-world scenes such a shallow DOF that they appear like models shot up-close. This effect can be simulated in software where it’s often referred to as the tilt-shift effect.
• Chromatic Aberration or fringing is an effect that occurs because lenses bend different colours of light by different amounts. The result is coloured fringes along edges where dark regions meet light regions within the photo. Cheaper lenses are more likely to suffer more strongly from fringing.
• While we now think of it more as an artistic effect, vignetting is an effect caused by a lens that is not big enough to project a bright image over the entire sensor. The intensity of the light falls off from the centre out, resulting in a circular fade.