What is the difference between focal distance and image distance?

I would have thaught that image distance is how far away the thing is your wanting to photograph and focal distance would be how much focus is used... but don't hold me to it lol.

Focal distance: the distance from lens to focus or the distance between the optical center of a lens to its focal point. Image distance: distance between the lens and the image. The distance on the optical center joining the rear nodal point of a lens with its focused image.

This distance increases as the lens is focused to nearer objects and is at a minimum with the lens focused at infinity.

Both are measured from the centre of the lens or curved mirror. The focal distance (more usually focal length) is the distance from the centre of the lens (or mirror) to the focal point. The focal point is the point through which all rays of light parallel and close to the principal axis pass or appear to pass after refraction (lens) or reflection (mirror).

The image distance is the distance to the point where the image of the object is formed, or appears to be formed, after refraction or reflection. For an object a long way away the focal distance and image distance will be the same since the rays of light from it can be considered to be parallel to the principal axis. For near objects there will be a significant difference between the two.

Noun Optics . 1. The distance from a focal point of a lens or mirror to the corresponding principal plane.

Symbol: f 2. The distance between an object lens and its corresponding focal plane in a telescope. Also called focal distance.

--------------- image distance Definition: Separation between the lens and the image. * Strictly, the distance on the optical axis joining the rear nodal point of a lens with its focused image. * This distance increases as the lens is focused to nearer objects and is at a minimum with the lens focused at infinity.

============================ I hope this is helpful.

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