|
I have also thought about this issue. Even though some of the light is "wasted" once the magnification gets too low, the consideration may be more a field of view question, as in your case. Unless the viewing site is very dark, even a younger eye may only dilate to 5 or 6 mm due to ambient light (street lights, the neighbours, local ovals etc). However, if you just want to fit the maximum in the field you might be able to push the magnification a bit lower (especially if it is a refractor), but if gets too low you may also start to get problems with vignetting of the field. So even with a refractor there will be some absolute minimum magnification where all you would see is smaller circle with the same image in it.
My 30mm gives a mag of about 33x and although extended objects should appear "brighter" the sky background also gets brighter so you don't actually see as much as you might think. The stars, being point sources, will appear much the same brightness but the contrast with the background will reduce.
At this mag I get a FOV of around 2 degrees (4 Moons).
|