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Building off of this question, say I put a 1 m lens in front of a camera so I can see a virtual image at 500 m to focus to, when I place an object within the focal length (0.998 m), but I want a larger field of view for it. As an example, I want to see a 1 m wide chart in good focus within 1 m away, and be focused at or close to infinity. What lens would I use to achieve this?

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2 Answers 2

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You need to place your camera very close to the lens so that the lens subtends the desired field of view. The object the lens is focused on needs to be large enough to fill the desired field if the focusing lens is not there. As the virtual image you are focusing on is 500 meters away you will be able to focus. In my example of a 1 meter lens the virtual image will be 500 times larger than the real object.

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  • \$\begingroup\$ This setup can focus at infinity and see a 1 m wide chart in focus 250 mm away. How is that done? imatest.com/product/120-collimator-lens-for-wide-field-of-view \$\endgroup\$
    – user109880
    Jan 22, 2023 at 19:16
  • \$\begingroup\$ It is a concave lens that takes rays that were converging from a wide angle and makes them parallel. No magic about it. \$\endgroup\$ Jan 22, 2023 at 20:51
  • \$\begingroup\$ Ok, what lens sizes and distances should be used? \$\endgroup\$
    – user109880
    Jan 22, 2023 at 20:53
  • \$\begingroup\$ Or a similar example, that's what I'm asking \$\endgroup\$
    – user109880
    Jan 22, 2023 at 21:01
  • \$\begingroup\$ You need to define the requirements, then define a system that will do what you want. Here is a start \$\endgroup\$ Jan 22, 2023 at 21:05
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A close-up lens, sometimes called a close-up filter, is in common use. These are convex lenses akin to reading eyeglasses. The close-up supplemental lens allows the camera to focus to closer distances than the focusing mechanism normally allows. If you mount such a lens with a focal length of 1 meter, with the camera’s focus set to infinity, an object 1000mm forward of lens will be in focus. A close-up lens with 1-meter focal length is also called a 1 diopter close-up lens.

You desire to place a target that is 1 meter wide at a distance of 1 meter from the camera and image it in-focus. Further you want the camera’s focusing mechanism to be set to infinity.

By mounting a 1000mm supplemental close-up lens, your goal to have the camera set to infinity is achieved. This will happen regardless of the focal length lens in use.

Next you need to calculate what focal length lens must be mounted on the camera to perform this task.

You can draw imaginary lines from the left and right edge of the target, back to center of the camera lens. A triangle is formed. The base of the triangle is 1000mm. This is the width of the target. The height of this triangle is the distance target to lens. The height is also 1000mm.

The dimensions of this subject triangle have a specific height to base ratio. This ratio is 1000 ÷ 1000 = 1.

The camera lens projects an image that plays on film or sensor. The focal length of the lens is the height of an image triangle. The length of this projected image is the base of the image triangle.

If you choose to exactly fill the width of the camera frame, the film / sensor format length will be the base of the image triangle. If the camera is a full frame, this will be 36mm. If the camera is an APS-C, this will be 24mm.

In any case, the ratio focal length to image triangle for this set-up is 1. Using a full frame, you must mount a 36mm lens. Using an APS-C, you must mount a 24mm lens.

If you chose other camera formats and place the target at one meter with the camera set to infinity, you can work out the focal length using a ratio of 1, base to focal length. However, you must use a +1 close-up lens to keep the camera set to infinity focus and the subject distance 1000mm.

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