Its glass quality that counts not lens and tube diameter especially in difficult conditions like last light against a dark background.
Some years ago a freind and I tested a whole range of scopes against a dark background in the evening. These from memory were from Luepold VX3 ( 40mm), Swaro PVS (50mm), Ziess Dia Vari C (36mm), Simmons ( 20mm), Nightforce NSX ( 50mm) and a couple of large 56mm US scopes built for very long range target work.
The winner by at least 5 minutes was my 1994 built German made Ziess, DiaVari C 3 x 9 x 36, 36mm lens and 25mm tube with Plex ret, a remarkable scope, if you can find one it will handle all you want out to 250 yards with ease. One of mine is on a 17 Ackley Hornet and has knocked crows and rabbits over slightly in excess of 200 yards with relative ease. When I spotted another for sale I had to have it, but its an older model and the turrets work the other way round.
The Diavari was the cheaper of the two Ziess scope ranges, yet it outperformed every other scope on test including Swaro PVS which at the time was the top of the line Swaro scope.
My worry with 56mm lens is that the tube is usually 30mm, 34mm or now I see 38mm on the new Ziess, thats not only a hell of a lot of excess weight to be carrying ( you will note that once you get over 55!!) a lens that high may well cause your rifle to be inconsistently mounted on the shoulder usually resulting in the bullet going high. Mate of mine has a 56mm Ziess Victory, at night other than the larger field of view there is nothing to choose between that and my 36mm Ziess. although my old eyes are now not as good as they once were.
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