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Old Jan 15th 2018, 01:11 AM   #1
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Wink Focal lenses

What are focal lenses?
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Old Jan 15th 2018, 03:15 AM   #2
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I think you mean 'what is the focal length of a lens'....

Take a (convex) lens on a sunny day , you can focus the sun to a very small ,intensely hot spot ...

The distance of this spot from the lens is the focal length of that particular lens.
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Old Jan 15th 2018, 03:50 AM   #3
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no. i mean focal lenses like bi-focal
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Old Jan 15th 2018, 11:53 AM   #4
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Uhh- "focal lenses" are "lenses that focus"!

Since light will bend as it pass from air to glass, using curved glass, you can shape a glass lens so that all the light, from a given point, passing through it will "focus" on a certain point. That is, all of those light rays wind up at the same point, the "focus" or "focal point".

If you have good vision, the lens in your eye will focus all of the light from one point onto a single point on you retina so you see it as a single point. If your vision is not so good, the light from a single point may be spread over a small are rather than focused on a single point so you see a "blur" rather than a sharp image.

You can, of course, has lenses in glasses made so that they correct that problem- light passing through both the lenses in the glasses and the lenses in your eyes will be focused on a single point on your retina.

However, the lenses in you eye, because they are not made of rigid glass, can alter slightly- making it possible to focus on objects that are close up, then change to focus on objects that are farther away. Your glasses can't change like that so if you have difficulty seeing objects both close and far (more common in older people), you may need glasses that have two separate parts that focus differently, on part that lets you focus on objects close to you, another that lets you focus on objects farther away- those are "bifocal" lenses.

There are, today, "trifocal" lenses that allow you to focus in between as well.
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Old Jan 15th 2018, 01:53 PM   #5
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If you have good vision, the lens in your eye will focus all of the light from one point onto a single point on you retina so you see it as a single point. If your vision is not so good, the light from a single point may be spread over a small are rather than focused on a single point so you see a "blur" rather than a sharp image.
Please note that this does not account for either short or long sight.
This effect is called aberation.

The lens in the eye should bring the image to a focus at the retina.

If the lens in the eys brings the image to a focus in front of the retina this is called short sight.

If the lens in the eys brings the image to a focus behind of the retina this is called long sight.

Either way the image you see is out of focus at the retina, without a correcting lens.
If the lens rotates the image this is called astigmatism.
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Old Jan 15th 2018, 07:15 PM   #6
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Originally Posted by s22345 View Post
no. i mean focal lenses like bi-focal
some glasses are called Bi- focal this means each lens is in fact 2 lenses ,each with a different focal length ,examine the lens you will see it has another lens ground in one side of the large lense ,

this allows the wearer to use the same glasses for reading , and looking far , he just looks through a different part of the lens .
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