I was watching Generation Kill...

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I was watching Generation Kill...

Postby celot » Fri Sep 25, 2009 6:14 am UTC

and I noticed that when the marines have their M4s with a scope, the objective lens is covered by a piece of cloth, except for a small hole, about as wide as a pencil, centered over the lens. I assume this is to minimize the sun's reflection off the glass.

Why do their scopes still work?

If they do work just as well, then why aren't scopes manufactured with tiny lenses?

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Re: I was watching Generation Kill...

Postby phlip » Fri Sep 25, 2009 8:14 am UTC

The light from an object passes through all of a lens, before being focused on the other side. So if you cover most of the lens, you'll still be able to see about as much as you could before... it's not like the image you'll see will be covered over except for this one hole.

However, it does have downsides - for one, it means less light comes through, which would make everything darker. So you wouldn't want to use this at night, say, or indoors. Though, in both of those situations you don't have to worry about getting glint off the sun, so you wouldn't need the cover anyway.

Have a look at the Wikipedia pages on apertures and diaphragm... they talk mostly about photography, but the principle is the same. Apparently, having a small aperture also causes a larger range of objects to be in focus, which could be useful. But yeah, it's not permanent 'cause it'd make the scope less useful in low light. I guess the low-tech cloth-with-a-hole-in-it is used instead of a mechanical iris so that there's less moving parts to break or something?

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Re: I was watching Generation Kill...

Postby Technical Ben » Fri Sep 25, 2009 9:53 am UTC

I do believe (if movies are anything to go by) that some scopes do come with apertures that can be swapped over.
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Re: I was watching Generation Kill...

Postby frezik » Fri Sep 25, 2009 6:49 pm UTC

If I'm understanding correctly, it's the same reason why, when you focus a newtonain reflector telescope correctly, you don't see the spider vanes and center mirror blocking the image.
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Re: I was watching Generation Kill...

Postby Spen » Fri Sep 25, 2009 9:35 pm UTC

It'll also make the user's aim better as it will ensure that the rearsights and the foresights are lined up (about to be made to look like a bell and told that they don't have foresights I think, oh well). It would definitely be used to prevent glare and reflection, in at least one of the episodes they come under attack at night and you see he's removed the cover.
That's a link to the sight used on most brit rifles most of the time, rather than having an attachable cover they have a small rubber hood type thing, other solution to glare is iron sights but they can't be used effectively over 600 metres unless you have INCREDIBLY good eyesight.

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