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Have light from a very distant object directed through the lens onto an opaque, white surface. Change the distance from the lens to the projection surface until the distant object shows a sharp image on the surface. The distance from the lens to the surface at that point is the focal length.

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14y ago
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14y ago

For most purchased telescopes, the focal length of the objective lens will be printed on

the lens' mounting ring, and it will definitely be stated somewhere on the packing box

and in the literature that comes with it.

The focal length of any unknown lens can be measured by using the sun, the moon, or a

distant street-light at night.

Focus an image of the distant light, through the lens, and onto a piece of white paper or tissue.

The distance between the lens and the sharpest image is the focal length of the lens.

Technically, the object must be infinitely far away. So the farther the light source is, the more

accurate the measurement is. That's why it's good to use the sun or the moon as the source.

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11y ago

For a lens ...

1/image distance + 1/object distance = 1/focal length of the lens

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10y ago

To get the focal length when the magnification is given, simply divide it by the focal length of the eyepiece.

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16y ago

you could use a lensometer.

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Q: How would you find the focal length of lens?
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Related questions

How can we find focus of a lens?

Focus or focal length?


If a telescope has an objective lens of 3 inches what is the focal length?

The size (diameter) of a lens does not determine its focal length. The amount of curvature of the lens does. Citing a diameter for a lens doesn't help us find the focal length. Lenses are ground to specifications that allow short or long focal length. The more curved the lens, the shorter the focal length. You can see this if we specify a given curvature and then start to "flatten" the lens. The focal length will get longer and longer as the lens is flattened. When the lens is flat (has to curvature) the lense has an infinite focal length, just like a piece of flat glass.


How do you find the new focal length when the lence is put in to the water?

to find the new focal length when the lens is put into water it becomes the 4 times the focal length in air.


Whats the focal length of a lens?

The focal length of a lens is the distance from the principal foci to the center of the lens.


Explain how the focal length changes as a convex lens becomes flatter?

The focal length of a lens is the distance from the center of the lens to the point at which it focuses light rays. The bigger the focal length, the more powerful the lens. ChaCha!


How do you find the power of a lens?

The power of a lens is 1/focal length (measured in meters).


Length of a lens is the distance from the center of the lens to its principal focus?

It is called the focal length. It is equal to 1/2 times r, and is positive on concave mirrors and negative on convex mirrors.


Does the focal lens length have three classifications?

yes, focal lens length has three classifications


What is defined as the distance between a lens and its focal point?

focal length of the lens


What is the focal length of the convex lens?

The distance from the centre of the lens to the focal point.


What will happen to the focal length of a lens if it is cut into half vertically?

When the lens is cut vertically then the focal length of the lens will increase.the focal length will become approx double.


What is focal length of a door peephole's lens and is it concave or convex?

The lens in a door peephole is a convex lens with a very short focal length, giving it a wide field of view. If it were on a camera, it would be called a "Fisheye lens".