Sigma 15mm F/2.8 EX Diagonal Fisheye vs Sigma 15mm F/2.8 EX DG Diagonal Fisheye — Which Is Better?

Current status of lens manufacturing (e.g., in production, discontinued).

Specific branding codes used for professional-tier lenses.

The lens mount will tell you what type of lens range you can fit onto the camera body. Often the same camera company will have different lens ranges for different cameras.

Indicates which camera system the lens is designed to attach to.

Largest sensor size compatible with the lens.

Width of the lens at its widest point

How long the lens is from front to back.

Some lenses are compatible with front filters, some are compatible with back filters and some are compatible with both. Front filters are usually more convenient to add and remove.

Shortest to longest focal length of the lens (zoom range start - end )

Largest aperture at the lens's shortest focal length to longest focal length (aperture range start - end)

Distance from lens mount to sensor.

Structure and design of the aperture

Number of diaphragm blades in aperture

Zoom allows you to vary the focal length, change between long shots and close-ups

Minimum distance for a subject to be in focus.

Largest size a subject can be reproduced

Weather sealing capabilities will give you more confidence when shooting in unfavourable conditions.

Coating to repel water and oil from lens elements.

A teleconverter (extender) is an extra piece of equipment that fits between the lens and the camera body to enhance magnification

Lens elements refer to individual pieces of glass within the lens.

The number of aspherical lens elements which can help correct aberrations such as spherical aberration.

Elements with two non-spherical surfaces

Elements that reduce chromatic aberration

Advanced elements further reducing chromatic aberration

Built-in features to reduce chromatic aberration

Internal Focusing means that the lens does not change in size during focusing, and the front element does not rotate.