My understanding is it depends on the size of your pupil when viewing vs the exit pupil of the eye piece. If the exit pupil of the eye piece is greater than your pupils diameter, your eye won't receive all the transmitted light and your scope will become "inefficient" as a percentage of the transmitted light is not reaching your pupil.
An analogy would be like trying to poor water from a bucket into a bottle. Most of the water wouldn't even hit the opening of the bottle, it would just splash around and be wasted.
So with that in mind, the exit pupil of an eyepiece is the focal length of the EP divided by the focal ratio of the scope. In your case, 40mm/4.7 = 8.51mm meaning for your pupil to receive all the transmitted light, it would need to dilate to 8.51mm or greater, which is up there in the higher octaves of standard pupil diameter.
Going by Tony's advice, with an approximate pupil diameter of 7mm at a dark site (35mm/5 = 7mm), the largest focal length EP you should use is 32.9mm
My question to the pros is how can I increase my field of view once I've hit this EP focal length barrier?
Hope that helps some Liz
