Dark Lord of Optics: the advantage of small objective scopes

I can be a bit evangelical about the excellence of 2.5x20 scopes.  This video explains why they are so good.


  • My understand of of obj lens size, is that it only needs to be big enough to provide an exit pupil at the given magnification you'd like to use, no smaller than your eye's pupil dilation at whatever light level environment you'll be shooting.

    If you want to be able to shoot into the dark shade of a wooded area, at a longer sniping distance, a larger objective can be useful.  If you don't shoot at longer distances in lower light, a larger objective lens is entirely wasted.

    So if your eye's pupil dilates betwee 2mm in bright light to as much as 7 or 8mm in the dark, and you're magnification is as high as 4x, you'll never need an objective larger than 32mm for shooting at night in minimal light. (Ignoring the dangers of shooting at a poorly identified target).

    If you shoot at 3-9x, and want to be able to see into dusk/dawn shadows at 9x, a 40 to 50mm objective is worth the trade-offs this guy's video describes.  If you try to look into a shadow at 9x, and its too dark to see well, you can turn your magnification down to about 7x (for a 40mm obj.) or about 8x (for a 50mm obj.) and your (perhaps larger than average) low-light pupil size of about 6mm might let you resolve that previously too dark image at 9x.

    On bright days, with your ~3mm pupil and 4x magnification, your little 20mm objective lens is ~8mm bigger than it needs to be.  If you're not going above 4x, and it gets darker, and your pupil grows to 5mm, you're making full use of your 20mm obj. and wouldn't need any larger obj lens scope, unless your eye's pupil gets bigger than 5mm, or your magnification gets higher than 4x.

    I don't know the degree to which all of the above is BS, because of the higher density of rods and cones near the center of your eye's vision.  I've never verified the above math/theory experimentally, by looking through two scopes that were otherwise similar, except for the obj lens sizes, knowing that there wasn't some other confounding factor.
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