Most (every?) eyepiece has a field stop, similar to a baffle. If I design for 1.25", and the field stop is 1/2", I will be cropping/blocking/losing over 50% of the "illumination".
I'll have to build a telescope, buy some eyepieces, and start experimenting.
Yes and no.
Yes you will be blocking out the illumination for field you are are not interested in.
No you won't be blocking any light from the target of interest in the 1/2 inch field.
The telescope primary makes an image of the sky at the focal plane. The size of the total image is unbounded really and the size of individual targets is dependent on the focal length of the objective. A longer objective makes a larger image of any target. For example, the size of the image of the moon is about one half degree and an image of this is 0.009 X focal length. A 1000mm scope creates an image of the moon (or the sun), that is 9mm diameter. A 500mm scope makes an image of the moon 1/2 this size or 4.5mm. A 2000mm scope makes an image of the moon that is 18mm diameter.
All targets have an angular size in the sky and the image created by the objective can be calculated as focal length of the objective multiplied by the tangent of the angular size of the target. The tangent of 1/2 degree is about 0.009. You can easily lookup the tangents of any angle using the Windows calculator.
Deciding on the field size is really determining how much you will see of the primary image made by the objective. So determine the size of your targets and calculate how big the images will be to determine how big an illuminated field is required. Then choose what eyepieces you need to view this image For a wide field RFT scope, you may want to illuminate 4 degree field. For a refractor to split double stars or for planetary viewing, a much narrower field can be chosen. Often the moon, 1/2 degree is recommended as the minimum field for a general purpose scope.
It should be added that you can't have a long focal length scope and a very wide field of view. A 4 degree field in a 1000mm scope has an image about 70mm diameter . An image that size doesn't fit in any eyepiece and would require a monster sized focuser tube.
You need to make choices based on what you desire to observe.
hope this helps,
Edited by dan_h, 05 August 2019 - 12:08 PM.