FWB 50 MM or 400 mm scope?

There are some practical differences between 40mm and 50mm objective lenses. * 50mm are heavier. * Bigger lens means the scope must be mounted higher above the barrel to clear. Some shooters prefer one or the other. For shorter distance shooting the lower mounted scope is better. Very long distances a higher mounted scope is an advantage. * 50mm have shorter depth of focus / depth of field / focal length (Think camera lenses) which might help with AO parallax setting and range finding using the focus wheel. A 40mm scope you don't have to adjust for focus as much with varying distances. * In very low light the 50mm will increase light availability but in normal daylight it has no advantage.

For myself, I prefer the 40mm over the 50mm and all my Hawke scopes have the smaller objective lenses.
 
On that rifle I'd go with a 40mm. A bigger lens will get you a little more light gathering ability up above 8x. The way you calculate it is to divide the objective lens diameter by the magnification. I believe that's called the exit pupil and the maximum a human can make use of is about 7mm, but that's for someone young and with perfect eyes and for the rest of us anything above 5 is likely wasted.

The larger lens should also give you a slightly larger field of view.

The downsides are more weight, higher cost and having to use higher mounts to clear the objective.
 
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Bigger objective means shallower depth of field. That means it will snap into focus and "range better" for field target competitors. The smaller lens with greater depth will be better for hunting as long as it still allows 100% light transmision at your prefered power. 40mm is big enough for up to 12x, so 4-12 40mm is by design.
 
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