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I've found lots of information regarding inifinite focus/projection, and collimation with HUDs. However, I am unable to get a definitive answer to this question.

I understand that the HUD is in focus for the pilot even if the pilot is looking at a distant object (such as another aircraft in the pattern). The pilot does not have to focus on the combiner glass in order for the HUD to be in focus. But what if the pilot's eyes are focused on the glass right in front of them? Is the HUD still in focus even when focusing on something so close?

If there's a relevant difference, I'm looking for how this would work in a military HUD. Thanks in advance.

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The projector and the combiner create a virtual image on the other side of the combiner mirror. This image is perceived like a true image.

The eyes need to adapt the focal length to the distance of the virtual image. If the HUD is set to collimate at infinity, the eyes must accommodate at infinity to have a sharp image. If the eyes accommodate on the combiner frame, the virtual image is blurry. Without entering into the details of how the whole eye works (I ignore them) the overall principle is the focal length must be reduced (lens more convex, refracting more) when the object to accommodate on is closer.

Here is a drawing where the projector is not collimating at infinity, to facilitate the demonstration.

enter image description here

  • At the top, the eye accommodates on the combiner, it adjusts the focal length to form a sharp image on the retina.

  • In the middle, the eye, still focused on the combiner, forms a sharp image of the virtual image ahead of the retina, the image on the retina (not shown) is blurred.

  • At the bottom, to have a sharp image of the virtual image on the retina, the eye refocuses, increasing the focal length, now the combiner image (not shown) is blurred as the focal length is not adequate.

If the projector really collimated at infinity, then:

  • In the middle, the image of the virtual image would be sharp on the focal plane and still blurred on the retina.

  • At the bottom, the focal length would be increased

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    $\begingroup$ Awesome, this all makes sense and was beginning to be my hunch. Thanks for the explanation! $\endgroup$
    – Ben
    Commented Apr 18 at 11:23

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