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Field of View and Add On Sights.

Have you ever wondered why when you take a typical 1:1 scope and place it behind a typical dayscope, you get a very narrow image? Here's an illustration I created some time ago to explain this.

The cause is the 25mm ( sometimes 26 or 27mm depending on scope ) objective lens of a 1:1 monocular. ( The focal length of the scope is an equivalent focal distance if the lens was a pinhole, so is useful for working out image sizes. )

With the typical dayscope having about an 18 degree apparent FOV, it simply can't illuminate the photocathode. That shouldn't be surprising given that the monocular has a FOV of about 40 degrees.

So what can you do about it? Well, a 2:1 zoom attachment would work fine and fill most of the image. Likewise, you can fit a 50mm lens ( usually referred to as a 2x zoom lens ) also if other lenses are available.

These are both functional solutions that will help you fill the photocathode.

There are more than aesthetic reasons to do this. An insufficiently filled photocathode will concentrate the light in the middle of the image, increasing wear, lose resolution and may interfere with the function of automatic brightness control and bright source protection.

However, it's not critical, so you should be able to keep using your monocular in the mean time under most circumstances that you normally would be able to.

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