Focal Length and Focus Point

[Dredd123]

I have never quite been able to get my head around whether or not these two things are linked, and if one is dependent on the other.

I think I'm correct in saying that the focal length of a lens is the distance from the lens to the film or digital sensor in the camera (i.e. when a zoom lens is operated, the lens moves closer to or further away from the film/sensor as the focal length changes).

How are focal points effected for a given focal length (if it all?)

 

[DonnaM]

Not quite sure what you mean by focal point. As all camera lenses are compounds and some of the compound elements move and some do not, going back to basic physics of a single lens to appreciate modern camera lenses may not be the right idea. All I will say is that on modern "zoom" lenses, the focus point of a true zoom lens remains the same wherever you are in the zoom range. Many of the cheaper "varifocal" zooms the focus point changes over the zoom range. Does that help at all?

 

[Kaouthia]

All I will say is that on modern "zoom" lenses, the focus point of a true zoom lens remains the same wherever you are in the zoom range.

That may be true of video cameras, where you would traditionally zoom in, focus, then pull back, but not at all on most lenses for SLRs.

Most SLR zooms that I've experienced are not Parfocal lenses (a lens that stays focused at the same distance as you zoom), even if they claim to be "true zoom" lenses.

http://en.wikipedia.org/wiki/Parfocal_lens

 

[Dredd123]

What I mean by focal point is, for example, the ability to choose what object within a scene you want to focus on, using the little red "focus squares" visible through the view finder. In other words, if I have a prime lens of a fixed focal length (say 50mm), I am still able to choose to focus on different objects within the scene I am looking at through the view finder, either closer or further away from the camera, irrespective of the fact that the lens is a fixed 50mm lens. I can't quite get my head around the relationship between focal length, and focusing on different objects which are different distances away from the camera.

 

[Kaouthia]

I can't quite get my head around the relationship between focal length, and focusing on different objects which are different distances away from the camera.

If I'm understanding your last post correctly, in respect to what you're saying, there isn't one.

Any AF lens that's compatible with your camera can utilise all of your camera's AF points.

 

[DonnaM]

Looking again at your OP, the focal length of a lens (distance from sensor) is always measured with the lens focussed on infinity. I think you are asking if the focal length changes as you focus down from infinity towards the camera and the answer (certainly for some zoom lenses) is yes it does.

 

[Arkady]

Not quite - the 'focal point' is the plane in the camera where the light focussed by the lens comes into focus - it used to be called the film-or focal plane, but now it's where the sensor is located.
Tech-speak: it's the point at which initially collimated rays of light meet after passing through a convex or compound lens, or reflecting from a concave mirror.

The other thing you're talking about is subject distance.

The lens's focal length (i.e. the angle of view of the scene shown or if you like, the amount of magnification) has no bearing on what you can focus on within that scene, whether it be close or far away: it will either appear smaller (wide-angle lens), roughly the same size (standard lens) or magnified (telephoto lens).
As long as you operate the camera correctly, it will always be in sharp focus.