An Interchangeable Lens System refers to a camera that accepts interchangeable lenses (such as prime, wide-angle, telephoto, macro, zoom) as opposed to one that has a fixed lens. Digital Single-Lens Reflex (DSLR) cameras fall into this category. So do mirrorless cameras.
The category “Mirrorless” has become the accepted term to refer to cameras that:
1- are system SLR level in capabilities
2- accept interchangeable lenses
3- have done away with the redundant mirror and optical mirror prism/viewfinder
4- use a high resolution electronic viewfinder (EVF)
The mirrorless camera sees not only what goes through the lens, but also what exactly exposes onto the image sensor. It allows photographers to see what exactly they are capturing instead of seeing one thing through an optical viewfinder but then obtaining a different result when the image is finally captured onto the image sensor.
Seeing how “The future is mirroless™,” some camera manufacturers have started branding their cameras as mirrorless. They use a different, dictionary-based, definition: anything that does not have a mirror is therefore “mirrorless.”
That’s wrong and they are only inviting ridicule for themselves and their products (and often, very good ones at that).
The term “mirrorless” applies only to an interchangeable lens system camera.
Perhaps a short history of how that term “mirrorless” came to be accepted as the de-facto name might shed some light. When the first mirrorless cameras came about, photographers (and camera manufacturers) struggled to come up with a proper name for those new type of cameras:
- One term that gained early traction was “EVIL”, short for Electronic Viewfinder Interchangeable Lens camera. Right off the bat, you can see that those cameras accepted interchangeable lenses.
- Next, since no camera manufacturer was going to brand its camera as “EVIL,” a flurry of other terms quickly cropped up: DIL (Digital Interchangeable Lens), DSLM (Digital Single Lens Mirrorless, still in use by Panasonic), MILC (Mirrorless Interchangeable Lens Camera), DSC (Digital System Camera), ILC (Interchangeable Lens Camera), etc. Notice that a necessary component was that these cameras accepted interchangeable lenses.
- Finally, the name “mirrorless” slowly started gaining traction among both photographers, camera reviewers and editors (shorter to write than “mirrorless interchangeable lens), and eventually stuck. Though the term “interchangeable lens” is not included in the name, you can bet it is always there, and has never gone away.
So an Interchangeable Lens System includes all cameras that accept interchangeable lenses, so both DSLRs (and film SLRs) and mirroless cameras fall into that category. However, since DSLRs are increasingly being pushed aside to make way for the newer mirrorless technology, in the future the the term “Interchangeable Lens System” will mostly refer to mirrorless cameras.
To repeat, a mirrorless camera is an interchangeable lens system camera.
