Why Are Mirrorless Bodies Smaller?

No mirror, no mirror box, so a narrower body back to front

No mirror, no mirror box, so a narrower body back to front

Hello everyone. As mirrorless bodies continue to gain market share, there remain a number of misunderstandings and misconceptions about them. This time I want to answer the question of how mirrorless bodies can (could) be smaller than a DSLR.

DSLRs evolved from SLRs where the R stands for reflex. The Reflex in this case refers to the presence of a mirror and the mirror box where it lives.

In a DSLR, space must be allocated to hold the mirror box and its mechanism. Despite having been around for a long time, mirror boxes are pretty technical things. It must have no reflective surfaces and it must contain not only the mirror, but also the mechanism to move the mirror up and out of the way for an exposure and then to return it to its normal position, consistently, shot after shot after shot.

The mirror must sit in the box at an angle, so light entering the camera from the lens can be reflected up into the pentaprism to be reflected and righted to look correct in the viewfinder. One of the perceived advantages of the DSLR is that the viewfinder is optical, you see exactly what the lens sees directly, with no middle man.

To accomplish all this work, the mirror box must be quite large. You could consider it a cube for our example, being slightly wider than the width of the sensor and having equivalent depth and height. Now before someone writes in to say I am being misleading, this is obviously an approximation for the purpose of visualization.

This makes the body reasonably thick from front to back and accounts for the grip depth on modern DSLRs. A body with a crop sensor needs less space for the mirror box and that is why some DSLRs are narrower than others. Compare Canon’s very successful SL2 with the equally successful 5D Mark IV. They do serve different use cases, but there is also a tangible size difference front to back.

The size of the mirror box also has an impact on the lens design. A lens, to be successful in implementation must produce an image circle slightly larger in diameter than the diagonal measurement of the sensor. Thus we include the depth of the lens mount in our body assessment. This does allow for very compact lenses because the rear element can be a bit smaller than on a similar focal length mirrorless lens. Take Canon’s 40mm f/2.8 “pancake” lens. Whether one likes it optically is immaterial if size and weight are primary considerations. It delivers an image circle appropriate to proper coverage of Canon’s APS-C sensor. Compare this to something equivalent in Canon’s M lineup of mirrorless cameras that use the same APS-C sensor. The lenses are not as wide across the front in general, but are as deep if not deeper than the DSLR mount. This has everything to do with the projection requirements of the image circle to properly cover the sensor plane.

When Sigma made their excellent ART lenses available for Sony FE, one of the initial criticisms was that they were much longer than the same ART lenses for Canon and Nikon full frame. This is expected because the lenses were designed for a mirror box, and needed to be redesigned to mount properly and provide a proper image circle to the sensor in a camera body that was quite a bit narrower given that there was no mirror box.

Working very briefly with Canon’s EOS R and now with Nikon’s Z7, I can say that while the cameras are indeed thinner, the lenses are at least the same size as a comparable mirror mount lens. You will note that most of the mount adapters are really just spacers to allow the lenses not designed for mirrorless to project the image circle properly on the sensor in a mirrorless body. You will also note that the native lenses will have a much larger flange size and use a larger rear element, mounted closer to the back of the lens in order to maintain as small a size as possible without compromising image circle coverage or introducing optical vignetting that would need to be corrected for by the CPU in the camera.

You will by now have gathered that these facts are representative regardless of camera sensor size. The greatest impact on body depth is the required image circle to cover the sensor, so full frame mirrorless bodies will be thicker than a micro four-thirds mirrorless body. This is why some folks feel cheated when they discover that their new full frame mirrorless is not a lot smaller or a lot lighter than the DSRL version. It’s because the only significant architectural difference is the presence or absence of a mirror box which is, mostly open space. A larger lens flange may actually increase weight a bit. We are also not likely to find that mirrorless lenses are any lighter than DSLR lenses when we compare apples to apples.

I hope that this short article helps clear up some misunderstandings.

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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.