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finder scope

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Finder scopes are a short focal length so that means that the depth of field is generally large, basically infinity starts not that far from the scope. If you work the values for an object 100 times the focal length away then the image falls very near to the focal plane and so it appears to be at "infinity" as would the moon.

It is something that was "learnt" in the days when cameras were manual and had film in them, it is amazing what bits were learnt from having a camera with a hand winder, no battery and the camera had never heard if the prefix "auto". :happy8:

Also the aperture "changes" the depth of focus as it gets smaller then the depth of field gets larger.

If you say 50 ft so 15 meters and lets say the finder is 150mm focal length then the difference in focal plane between the moon at infinity and the object at 50 ft is just 1.5mm, the moon being at 150mm the 50ft thing at 151.5mm. There will be a depth of focus involved and very likely your eye just adjusts to maintain a good image. It is easy, or a pretty good approximation, to say that any object at 1000 times the focal length away is in effect at infinity


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