Compact vs. full size zoom binoculars – Basically, binoculars with an objective lens smaller than 30 mm are considered “Compacts”, while ones with lenses larger than this are described as “Full Size”. In most cases, I recommend people to go with a full size binocular for watching birds and nature. One reason is that they are just a lot easier to look through. A reason for this is compact binoculars have correspondingly smaller exit pupil lenses (the smaller lenses you actually look through) that sometimes make it hard to see the whole field of view.