lens focal length question

Can someone explain how the lens' local length is measured. I read somewhere it is from the center of the lens, when it is focused on infinity, to the flim or sensor plane.

What is the center of the lens? How do you set the camera on infinity since it seems to be dependent on f/stop--higher f/stops would yield focus to infinity "quicker".

Thanks for reading.

Kkamin

The focal length of a simple lens is the distance from the center of the lens to the point of convergence (the sensor plane) when the lens is focused at infinity. This distance is both an optical and a physical distance.

Camera lenses, however, are not simple lenses.

The focal length is the optical, not physical, distance from the rear node of the lens to the sensor plane when the lens is focused at infinity.

Two things to keep in mind. First, don't confuse infinity focus with depth of field. They are entirely different. The lens is focused at infinity at only one position of the focus adjustment. Aperture has nothing to do with it.

Second, a complex camera lens has no optical center. Instead it has a front node and a rear node. The front node is where the "center" would be when viewed from the front of the lens, and is usually behind the rear node. Similarly, the rear node is where the "center" would be when viewed from the rear of the lens, and is usually in front of the front node. In fact, the rear node may not be within the body of the lens at all, but somewhere in front of the front element.

The focal length of a lens is a physical property of the lens itself, and has nothing whatsoever to do with the sensor size. The so-called "crop factor" affects only the angle of view, not the focal length.

If as a child you ever used a simple magnifying glass to focus the sun onto ants, the distance from the lens to the focused image of the sun is the Focal Length. What 20droger described is with a complex lens, and for the dSLR the long FL lens is designed to have a shorter-than-FL distance so it is called 'telephoto' design, and the short FL wide angle lens is designed to have a longer-than-FL distance so it is called a 'retrofocus' design.

20droger wrote in post #9200168
The focal length is the optical, not physical, distance from the rear node of the lens to the sensor plane when the lens is focused at infinity.

Two things to keep in mind. First, don't confuse infinity focus with depth of field. They are entirely different. The lens is focused at infinity at only one position of the focus adjustment. Aperture has nothing to do with it.

Is that the focus ring twisted all the way in the "far focus" direction?

Maybe not. Sometimes a lens will have the focus extend beyond infinity to allow for variations due to temperature.

PhotosGuy's right. Most lenses can focus past infinity.

When a lens is focused at infinity, all the light rays from an extremely distant point source, like a star, enter the lens as parallel rays. These are then focused to a point at the focus distance (ignoring lens aberration, which is another subject [and why camera lenses are not simple lenses]).

When a point source is relatively close to the lens, the light rays are diverging (getting farther apart) as they enter the lens. They will be focused to a point at a distance farther than the focal length. This means that to focus on a object closer than infinity, the lens must be moved away from the sensor (optically).

Most lenses can be adjusted to cover a range of minimum focus to infinity and beyond! Focusing past infinity means the lens is optically closer to the sensor than the focus distance, and light rays entering the lens must be converging (getting closer together) as they enter the lens. This cannot occur naturally.

The reasons for being able to focus past infinity are to compensate for thermal changes and manufacturing tolerances in the lens.

Frank—nice car, but it's no Brownie.

kkamin wrote in post #9205075
Is that the focus ring twisted all the way in the "far focus" direction?

PhotosGuy wrote in post #9206351
Maybe not. Sometimes a lens will have the focus extend beyond infinity to allow for variations due to temperature.

In the "olden days" when all camera lenses were simpler manual-focus lenses, twisting the focus ring to one stop usually was the correct position for "infinity" focus.

With today's autofocus lenses (including all three "L" series Canon zoom lenses that I have), there is often a little overtravel at the "infinity" end of the focus ring travel. I am fairly certain that there is one reason why this is true, and it is not related to temperature. The reason for overtravel is so that the autofocus drive system won't slam the focus system against a hard stop every time you aim the camera at a distant subject and activate autofocus. There's a little bit of room for overtravel in the servo system that drives the focus mechanism. If this overtravel were not allowed, I suspect that you'd be sending your autofocus lenses in for repair rather frequently.

SkipD wrote in post #9207831
In the "olden days" when all camera lenses were simpler manual-focus lenses, twisting the focus ring to one stop usually was the correct position for "infinity" focus.

With today's autofocus lenses (including all three "L" series Canon zoom lenses that I have), there is often a little overtravel at the "infinity" end of the focus ring travel. I am fairly certain that there is one reason why this is true, and it is not related to temperature. The reason for overtravel is so that the autofocus drive system won't slam the focus system against a hard stop every time you aim the camera at a distant subject and activate autofocus. There's a little bit of room for overtravel in the servo system that drives the focus mechanism. If this overtravel were not allowed, I suspect that you'd be sending your autofocus lenses in for repair rather frequently.

I represent that remark!!!

http://en.wikipedia.or​g/wiki/Focal_length

SkipD wrote in post #9207831
In the "olden days"....

20droger wrote in post #9208086
I represent that remark!!!

So do I.

20droger wrote in post #9206680
Frank—nice car, but it's no Brownie.

The Brownie will be back after Christmas.

SkipD wrote in post #9207831
IThe reason for overtravel is so that the autofocus drive system won't slam the focus system against a hard stop every time you aim the camera at a distant subject and activate autofocus. There's a little bit of room for overtravel in the servo system that drives the focus mechanism. If this overtravel were not allowed, I suspect that you'd be sending your autofocus lenses in for repair rather frequently.

Good point!