Let’s Talk Photography – Ep.53 – Focus

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This month Bart continues his on-going series on photographic terms. This time it’s all about focus. Not focus in the art of arting sense, but in the physical optical sense.

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Outline Shownotes
  • Focus Basics
    • You’ll often hear it said that a lens focuses light onto a plane, but that’s not quite correct, it focuses light onto many planes! The distance behind the lens where light comes to a focus depends on the distance in front of the lens the light came from.
      • For every plane in front of the lens there’s a plane behind the lens where light from the front plane is in focus, but it’s not one plane behind the lens, it’s infinitely many!
    • To get a sharp image of something, the light from that thing must be in focus as it strikes the light-sensitive medium. In other words, the focal plane and the film/sensor plane have to line up
    • When you focus your camera you’re adjusting the distance between lens and the sensor/film plane so that light from your subject comes into focus on the sensor/film plane.
  • A Note on Lenses
    • when I talk about lenses in this podcast I’ll be imagining a theoretically perfect ideal lens.
    • You can imagine an ideal lens as being like a magnifying glass with no optical imperfections
    • In reality, no lens is perfect, they all have problems where their actual properties vary from the centre out to the edge, or where they bend different colours of light a little differently
    • The reason a physical camera lens you buy is a big long tube containing many pieces of glass is that you need all that glass to come close to achieving a perfect lens when you’re stuck in the real world!
  • Focal Length -v- Focus Distance
    • Focal length is an inherent property of a lens. It does not change. It’s a measure of how much the lens bends light.
      • If you build your lens from multiple lenses that can move relative to each other then you can change the focal length of the system as a whole, and that’s what happens when you zoom a zoom lens
    • Focal length is defined as the distance behind the lens that rays of light that entered the front of the lens parallel to each other will meet.
    • For every position of the sensor/film plane behind the lens there is a distance in front of the lens to the plane that’s focused on the sensor/film. That distance in front of the lens if the focus distance.
  • Infinite focus — when something is infinitely far away, light rays coming form it arrive in parallel.
    • Nothing is actually at infinite focus at a strict mathematical level
    • In the real world though, light coming from objects just a few miles away is indistinguishable from light coming from infinity.
    • Once something is far enough away from the lens that the light is indistinguishably close to parallel, it is at infinite focus, and no matter how much further you move that thing away from the lens, it will remain in focus.
  • Real-world limitations — In the real world a camera’s lens can only be moved forward and backwards so much, so that means there are limits to the distances any lens can be focused on.
    • General purpose lenses are engineered such that they can reach infinite focus, so that means there is only one limit on the distance they can focus — they have a closest plane which they can get sharp, or, a closest focus distance.
    • If you want a lens that can focus in really close, you’ll have to sacrifice it reaching all the way to infinite focus, and then you get a lens with a nearest and further focus distance. Generally, the only kinds of lenses for which we can tolerate an inability to reach infinite focus are macro lenses.
  • Depth of Field
    • Mathematically speaking focus plane of any lens is infinitely thin. It is literally a 2D slice of the world that is in focus.
    • In reality, we can’t see infinitesimally small amounts of blur, so there region that appears to be in focus is always thicker than an infinitely thin plane (not hard!)
    • We call the distance between the nearest and furthers things (relative to the sensor/film plane) that appear to be in focus the depth of field (or DOF).
    • There is actually a way of mathematically calculating DOF, but it really lives up to its name — the circle of confusion!
    • DOF depends on a number of factors
      • The size of your sensor/film — the smaller your sensor, the deeper your DOF, hence it being really hard to get nice bokeh on a smartphone without resorting to optical and/or mathematical trickery!
      • The focus distance — the closer in you focus, the shallower your DOF gets (which is why macro photography is so difficult!)
      • The lens aperture — the bigger the hole through which light enters your lens, shallower the DOF. (see episode 50 for much more on aperture)
  • The Hyper-Focal Distance
    • For every lens there is a distance you can focus on where the edge of the DOF just reaches infinite focus. This is the focus distance where the DOF is at its absolute maximum because everything from infinity to the near-side of the DOF is in focus
    • The Hyper-focal distance for a lens depends on the sensor size and the focal length of the lens. There are lots of apps out there that allow you to enter you camera and lens combination and then give you the hyper-focal distance for that combination.
    • Manually setting your lens’s focus to the hyper-focal distance can be very useful, particularly in landscape photography where you want as many layers of interest as you can get.

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