![]() but very quickly, the Airy Disk is representative of circle of confusion. When we look at the interference patterns they are also very very small.ĭepth of field is often bound to the concept of Circle of Confusion. Really really small, with visible light wavelengths in the range of 4000 to 7000 Angstroms, or 0.4 to 0.7 micrometres. Remember that light wavelengths are measured in micrometres or Angstroms. We have to remember the power of scale though. The smaller the aperture, the greater the lack of sharpness. However the other bright areas are also impacting the sensor, creating a lack of sharpness. What is sharp is the bright dot at the centre. This Airy Disk shows the area of falling sharpness. What you would see on the face of the sensor is called an Airy Disk. Imagine that you are super tiny and the sensor is enormously huge. To do this, we bring our observation point to be viewing the incoming wave as it strikes the sensor. So far we have looked at images of the wavefronts from the side as it were, but not as they would impact the sensor. Diffraction is ALWAYS happening regardless of the size of the aperture, however the point at which it becomes impactful will always be more apparent as the size of the aperture decreases. If we use a larger aperture, there is still diffraction happening. It’s not moving in the still image, but your amazing eyeball is trying to make sense of the interference pattern. Some people even sense movement in the wave image on the right side. The smaller the aperture, the more the change of the wave is apparent. This is evidenced in the bending of the wavefront in general and that the edges of the aperture also create reflections such that we see wave portions that are darker and portions that are lighter, yet none have the intensity of the centre portion. Part of the wave passes through the aperture of course but we see that the aperture itself creates interference to the passage of the wave. That wave hits the tiny opening (our lens aperture. Think of the wave on the left as coming from such a distant source that the wavefronts are effectively parallel to each other. As photographers we are not imaging based on structured sources. ![]() This image is using a laser as the source which is much more constrained than a typical light wave, for the purpose of illustration. Here is a simple example of the wave pattern as light passes through a small opening. If you ever did experiments with ripple tanks in High School physics this is probably coming back to you now. The interference patterns now create intersections where the crests and troughs interfere with each other. Those that do have a primary crest and trough, but the opening creates an interference whereby the opening creates a reflection of the original wave that passes through. Now introduce a barrier with a very small opening in the path of these waves. ![]() To make a simple allegory to this, drop a pebble into a circular birdbath. When light waves must pass through a small slit, think small aperture, some never make it through, but since light waves are not all parallel, when constrained by a small opening, we are more likely to see a loss of sharpness as the waves passing through the slit tend to bend around the edges of the slit. This principle is critical to lens design. Light can be reflected When light passes through a medium change, such as from air to water, the medium change can cause a change in the direction of the light ray. Light rays, if you will pardon the simplifcation, move in straight lines unless interfered with. Why? As this is not a physics classroom, my cheap answer is Because Physics. While it may seem odd that something can in fact behave as two very different things, light does. We have heard the phraseology light wave and the term photon before. What we think of as a ray of light is both a wave and a stream of particles. Diffraction DefinitionĪs noted in the podcast, light is very special. I determined that a podcast is not the optimum vehicle for this topic, so an article it is. We had considered doing this as a podcast but as I reviewed the materials. Hello neighbours! Gordon and I recently did a podcast discussing the whys of depth of field reality and once area that we did not touch on was the subject of lens diffraction at small apertures as this is sometimes incorrectly aligned with depth of field.
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