If you select "The Microcosmos Microscope" option you will receive the microscope with four achromatic objectives. If you choose to upgrade to the plan objective option, you will receive the microscope and a set of four plan achromatic objectives.

Achromatic - These objectives work great for people who want to get into amateur microscopy. The image will look great and will be clearly focused in the middle, but due to the nature of the lenses, they display a curved image which results in the far edges of the field of view being slightly distorted and out of focus. If you’re just getting into microscopy and aren’t planning on taking a lot of pictures and videos of your samples, these objectives will work great for you.

Plan Achromatic - These objectives produce a flatter image across the entire field of view. This is ideal if you're interested in capturing high quality pictures and videos of the things under your microscope since the center of the image and the edges of the image will all be in focus.