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Old Feb 27th 2017, 07:54 AM   #1
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Lightbulb How far from focal length of a lens does the nearest infinity begin

Q. How can we make out whether the the object is beyond C or infinity?
Q. At what nearest distance does infinity start from,from the focus(focus Of any lens or mirror)?
Means is there any relation between the point from where infinity starts and the focal length or curvature of any lens or mirror?
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Old Feb 27th 2017, 09:26 AM   #2
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"Infinity" is not any specific distance. The effect of distance from focal point to such things as "magnification" decreases as the distance increases. "Infinity" is the distance at which that effect is "small enough to ignore". That depends on how much error you are willing to ignore.
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Old Feb 27th 2017, 12:56 PM   #3
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Thank You Sir.
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Old Feb 27th 2017, 01:25 PM   #4
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When I was teaching optics I would frequently use the distance from the front of the room to the back and call that infinity. A point at "infinity" typically does not even need to be all that large a distance away in practice.

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Old Feb 27th 2017, 10:17 PM   #5
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So helpful!! Thanks
Am I free to consider any distance beyond 2F as infinity?
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Old Feb 28th 2017, 03:59 AM   #6
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For an object to be considered as at "Infinity" the distance has to be "Big" compared to the Focal Length.
Your question is how can one define "BIG"
There may be some standard convention, but I'm afraid I don't know it.

However I think some consideration of trigonometry will give an indication.
You basically want the object to be far enough away that any straight lines taken from the object to the lens are so close to parallel that the difference is negligible.

There might also be a consideration of the diameter of the lens as well as the focal length.
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Old Mar 3rd 2017, 11:38 AM   #7
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Originally Posted by Naazima View Post
Am I free to consider any distance beyond 2F as infinity?
Please answer in YES or NO.
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Old Mar 3rd 2017, 03:25 PM   #8
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Originally Posted by Naazima View Post
Please answer in YES or NO.
It can be, yes. But to repeat Woody's comment, it can depend on the experiment you are doing. The best thing to do is give it a try and see if it works with your equipment. If 2F does the job then fine. If it doesn't then try larger distances.

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Old Mar 5th 2017, 06:14 AM   #9
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Consider a digital camera
Now select a point in the scene being photographed.

If every "light ray" from that point that passes through the lens is focused onto the same pixel, then that point can be said to be at "infinity".
If if light rays (from that point) that pass through the bottom part of the lens are focused onto different pixels to the light rays (from that same point) that pass through the top part of the lens, then the image will be fuzzy and the point is too close to be considered at infinity.
This is one reason that phone camera lenses are so small, "infinity" is closer for a small diameter lens than for a large diameter lens (of the same focal length).

A big lens is needed to get lots of light in, but if you have a very sensitive detector and/or a very bright scene, then a small lens will (generally) be able to focus on closer objects.
In old school photography, one would close up the aperture, effectively only using the central part of the lens, when the scene is bright and thereby get a much crisper image with a much deeper field of focus.

It is instructive to think of the pin hole camera.
Here you don't have any lens at all, but you can still produce a surprisingly good image (of a very bright scene) because the aperture is so small.
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Old Mar 20th 2017, 08:31 AM   #10
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Thank you very much sirs. I got the catch of it.
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