D-SLR Lenses on an MILC

Those of you who are actively following the technological and commercial developments in the field of photography would have noticed that little or no development is taking place in the realm of D-SLR systems. All the major manufacturers (OEMs) seem to be currently concentrating on mirrorless cameras (MILCs) and associated lenses.

- Ashok Kandimalla

One important point you should bear in mind is that all OEMs have opted to use new lens mounts for their MILCs. While this results in incompatibility with the older D-SLR lenses (even from the same manufacturer), there are compelling technical reasons to justify this move.

Let us see in detail the reason behind this issue and how we can overcome it. Picture 1 (a schematic, not to scale) shows how the MLCs have relatively thinner bodies as they lack the reflex mirror that is present in the D-SLRs. Hence, the distance between the flange (Picture 2) which is where you mount the lens, and the sensor, called the Flange Back Distance (also known as Flange Focal Distance) is shorter.

In Picture 1, this is indicated by a red arrow for the MLC and a blue arrow for the D-SLR. In Picture 1 the flanges of both are aligned (the green dotted line) and you can see how the sensor of the D-SLR is positioned more to the back compared to that of an MILC.

A direct consequence is that a lens designed to work with a D-SLR will not focus correctly on an MILC even if you were able to mount it though this is technically not possible since mounts will be different. The lens will focus on a plane beyond (or to the back of the) sensor (Picture 3). This is a major barrier to those photographers who want to shift to MILCs as the investment in the D-SLRs lenses, which can be considerable, will go as a waste. Manufacturers have recognized this problem and have developed a solution, which is a device called an adapter. Let us look at how an adapter works.