Objective lens size differences

[endure]

Looking at 8x4? binoculars Porro prisms are usually 8x40 whereas roof prisms are 8x42. Is there any reason for the extra 2mm?

[ronin]

For whatever reason 42's seem to be the "normal" these days.

It is very likely a best compromise between light gather, weight, cost and aberations.

Also they are from the nature watching side of life, which has slightly different criteria.

Have 2 sets and very happy with them for general work, nature and looking at the skies.