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Message: <blockquote data-quote="Tantien" data-source="post: 3150601" data-attributes="member: 116813">Some Optics EducationHi Wachi: The combination of magnification [the '8' in the 8x25] and objective/aperture  [the '25" in the 8x25] is the "rating" (often misnamed power). When the objective gets larger, the binoculars generally get bigger and heavier. But this "rule" can vary if the bins are built with a polycarbonate body or a magnesium body or have different amounts of glass in them. But brightness generally decreases when the objective gets smaller or the magnification increases. At the same aperture size (25mm, 32mm, etc.), as you increase magnification the image becomes less bright, and this is more obvious at dawn and dusk. Thus, a 10x32 binocular, technically having more “power” than an 8x32, will be less bright under low light conditions. More power isn’t always a good thing! So, (all other things being equal) a 25 is smaller, gathers less light than a 30 which gathers somewhat less light than a 32 mm.The higher the magnification (10 vs. an 8) will introduce more shake to an image (unless you have an IS to compensate). Some people can handle a 10X pretty well without IS but some may prefer the relative steadiness of an 8X (as well as a brighter image). When you are looking at the combinations above, there is a shaft of light called the "Exit Pupil" (EP) which is the actual amount of light hitting your eye. This diameter of the EP (expressed in millimeters) exits a binocular’s eyepiece and enters through your pupil. The EP is calculated by dividing a binocular’s aperture by its magnification, e.g. 42mm / 8x (for an 8x42 binocular) = 5.25mm.This is one of the primary reasons most birders choose an 8x42; it’s the best combination of magnification and aperture, keeps the binocular’s physical size from being unwieldy, and still delivers a bright image at moderate power.Here are EPs for some binocular ratings:8x25: 3 EP8x32: 4mm EP8x42: 5.25mm EP10x21: 2mm EP10x32: 3mm EP10x40: 4mm EP12x36: 3mm EPJust remember that your age will determine the Max EP that you can use.A very young person have an maximum eye pupil of 8mm.After 30 years old it will be smaller than 7mm.After that about:40 years old: 6mm50 years old: 5mm65 years old: 4mmAnd finally to the very end of life about 3mm.So just keep these "few" things in mind when buying a pair. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite2" alt=";)" title="Wink    ;)" loading="lazy" data-shortname=";)" />For me, I have recently gotten a pair of 8x25s, not to replace my full size binos, but to provide a more packable, decent quality pair that will fit easily in my backpack and will not weigh me down. I have a pair of HD 8x32s which for me are the best all around combination of size, weight, and performance since I do little birding at dawn and no birding at dusk.I'll let the others on this forum argue the merits of quality and small size (IMHO, you can get very decent small bins for not that much money but you also can give up durability and build quality for less money), but I thought I'd give you some basics. Hope this helps!Glenn</blockquote>

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