My ignorant Lens Question

Started Nov 6, 2012 | Discussions thread
DUSTY LENS Veteran Member • Posts: 4,308
Re: My ignorant Lens Question

Signalz1123 wrote:

So I have a stupid lens question I've always wondered about.

Why is it that when you hold a lens up to your naked eye and look through it, objects appear out of focus, even when your eye is exactly where the sensor would be. Specifically, I'm looking through a 300mm prime and while objects 1ft away appear sharp, the distant objects it's supposed to be focusing on are all blur. For the distant objects to become clear, i have to hold the lens over a foot away from my eye.Why will the lens focus distant images for a sensor but not for an eye the exact same distance away?

Your eye needs light which is mildly divergent . As it is while it travels out from any reflecting object .

Light coming from very distant objects , ( moon , stars , houses across the street ) is almost traveling with no divergence as it enters the lens of your eyes .

Light from your hand still has a very small amount of divergence when it enters your eyes .

Light from the rear of a lens is strongly converging from the lens which your eye can not deal with for the purpose of focusing it on your retina .

As this light from the lens passes through it's focus point , ( where the sensor would normally be , ) it becomes divergent again , but the amount of divergence is far too great for your eye to deal with for the purpose of focusing it on your retina .

You need a second lens having just the right amount of lens power and in the right direction ( convergent or divergent ) to make this light almost parallel ( not too divergent and not convergent ) as it enters the lens of your eyes , for your eye lens to be able to make it converge to a point exactly at the distance of your retina .

The second lens also must be located at the exact distance from the first lens , to be able to focus the theoretical image projected from the first lens . This will add the focus distance of the first lens to the focus distance or the second lens for the correct distance between the two lenses . This is a crude telescope .

I have no explanation for what you are seeing from the rear of your lens . It is too complicated for me to think about it or to understand it .


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