Barlows and field of view

If a given Barlow doubles the mangification of an EP, does it also reduce the true field of vision by 50% or is it unchanged?


Thanks.........Mike

No free lunch. Easiest to describe by example. Suppose that an eyepiece with a 50 degree apparent field of view when used in your telescope produces a 1 degree true field of view. When you use a 2x Barlow the apparent field of view will (hopefully) still be 50 degrees, but the true field of view drops to 0.5 degrees.

I said "hopefully" because some eyepieces cannot handle what the Barlow does at the edge of field and then the apparent field of view goes down a little bit also. People call this "vignetting", but that's a detail that only affects some eyepieces, i.e. one 25mm 50 degree eyepiece may vignette with a given 2x Barlow while another 25mm 50 degree eyepiece may not. The general principle, however, is if the magnification goes up by 2, the true field goes down by a factor of 1/2.

Yes, its reduced to 50%.
Anything that increases the magnification for a given eyepiece will reduce the true field of view by that same amount.
Michael