Each time I read about a new mirrorless camera, I wish I had the money to buy that camera. Having purchased a DSLR (Canon 700D) about one year and four months ago, I cannot afford to invest in a new camera system. To add to the woes, most mirrorless cameras available are pretty expensive. Additionally, the number of lenses available is limited.
Considering the limited number of lenses available, I have wondered why mirrorless camera systems are designed with new lenses. Why not use the ide range of DSLR lenses available?
After much thought, I believe I have an answer. In a DSLR, the lens aperture is kept at the widest while composing the picture. Only when the shutter button is clicked does the aperture size change to the one specified by the photographer or the one chosen by the camera. In a mirrorless camera, the EVF (Electric View Finder / display) shows the true picture. By true picture, I mean that the image captured by the camera is exactly like the one being shown on the screen. (This not the case in a DSLR). To show the true picture, the camera has to set the aperture to the value selected.
The mechanism of managing the aperture is the main difference between a DSLR lens and a mirrorless camera lens. I believe that this feature of the DSLR lenses is what prevents their AS-IS usage on a mirrorless camera system.