What is the relation between zooming (in/out) and focal length?

Question

What happens when I zoom in/out and change the focal length (manual focus)

Answer 1

A Zoom lens is a variable focal length lens. in other words it a combination of different focal length lenses,  for example a 18 - 55 mm lens is a lens which got the focal length from 18mm to 55 mm,

at any point u stop that is ur focal length,

when ur zooming In the focal length is more (tele) and when u zoom out the focal length is decreeing (wide angle)

and   the angle of coverage decreases when u zoom in and the angle of covarage increases when u zoom out..

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Answer 2

When you zoom, you change the focal length of the lens. Larger numbers bring distant objects closer. Smaller numbers approach wide angle points of view.

Manual focus (or automatic, for that matter) is an unrelated issue. It does not affect the focal length of the lens/zoom setting. Rather, the focal "point" is the location in your visual field at which objects are in focus.

For example, you could be zoomed out to 200 mm, but be focused on an object five meters away. Alternatively, you could be zoomed down to 35mm, yet focused on something at the horizon.  

Answer 3

When you "zoom" in you are using the lenses longer focal length

When you "zoom" out, you are using the lenses shorter focal length.

That is the relationship.

Manually focusing is used when 1) the lens you are using is not an auto-focus lens or 2) you need to bypass the auto-focus in order to selectively focus a specific subject within a composition.