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The benefit of a head-up display is primarily in reducing the time required by the user to refocus their eyes from the head-down display to the head-up surroundings. In applications such as cars, the best focal length would be at infinity for driving on the highway. However for driving in the city, where the driver has to pay attention to events close to the wind-shield, a good location of the image would be closer than infinity. Is it possible to change the location of the virtual image in real-time for such head-up displays without using a moving lens?

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    $\begingroup$ You do not pay attention to something happening at wind-shield distance when driving in cities. Events happen at distance of 5 meters or more. Focusing at 5 meters is very close to focusing at infinity. The HUD at infinity will be most likely just fine. $\endgroup$
    – verdelite
    Mar 9, 2020 at 16:42
  • $\begingroup$ Head-up displays (HUDs), for instance an iPad, can also consist of projecting images at the distance of the dashboard. However, if the idea is to provide conformal augmented reality imagery, the distance might be higher (~ 7.5 m to 20 m) as in this white paper. The question is whether, if required, it is possible to change the location of the virtual image without using moving lenses (by changing the properties of the image). $\endgroup$
    – kbakshi314
    Mar 9, 2020 at 16:57
  • $\begingroup$ maybe you can use moving flat surface mirrors instead of moving lenses. $\endgroup$
    – verdelite
    Mar 9, 2020 at 22:56
  • $\begingroup$ I would appreciate a concise reference on using the movement of flat surface mirrors to change the virtual image location. I don't follow the statement 'In the optics of an AR HUD, the virtual image distance can be controlled by a small translation of the diffuser screen relative to the HUD optics. For example, the virtual image may be shifted from10 m to 20 m by a small adjustment of the screen position which could be less than12 mm.', on page 5 of the paper. $\endgroup$
    – kbakshi314
    Mar 9, 2020 at 23:03
  • $\begingroup$ Fig 1 in the paper does not shown where the diffuser screen is. Assume it is at where PGU is located. Then it is not hard to understand why a small movement of the screen can cause big movement of the virtual image. You can do an experiment by yourself. Find a curved mirror (very common for making ups; you can find them in stores selling makeups, in hair saloons, or Walmart), place an object near the mirror, and look at its image in the mirror. A small move of the object will cause a big move of the image. $\endgroup$
    – verdelite
    Mar 10, 2020 at 14:14

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