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Why mirrorless cameras are taking over the world

Renee Johnson
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Nikon’s introduction of its Z6 and Z7 cameras might make it the most recent company to hop on board the professional mirrorless-camera bandwagon, but the venerable Japanese firm is hardly the first to take a crack at cutting the flipping mirror of single-lens-reflex cameras out of the optical path.


Sony’s A7 III, a popular mirrorless camera of late

Olympus and Panasonic were some of the pioneers of the digital mirrorless camera with the Four Thirds and Micro Four Thirds systems. Fujifilm’s X System and Sony’s Alpha family of cameras are some of the most popular mirror-free picture-takers available today. Why is the industry making the mirror persona non grata, though? Let’s take a little trip into the history books.


Nikon’s original F. Note the mirror inside the lens mount. Source: KEH.com

The vast majority of interchangeable-lens cameras made since 1959 have been single-lens reflex designs, or SLRs. SLRs include a mirror between the rear of the lens and the film or sensor to redirect light through a focusing screen and erecting pentaprism. That system meant that you could, well, focus, compose, and even preview the depth of field in your image at the aperture you chose, since you were looking directly through the lens.

That direct, one-to-one view was the major advance of 1959’s Nikon F and other, earlier SLRs over the Leica M and other rangefinder cameras without reflex mirrors: the image in an SLR’s viewfinder was always representative of what you would see in the final shot with practically any lens, from the widest possible wide-angles to the longest long lenses.


Leica’s M3, the seminal rangefinder camera. Note rangefinder window on the upper right of the body. Source: Rama on Wikimedia Commons, CC-BY-SA 2.0 fr

Rangefinders have endured among photo enthusiasts, however, in part because of the optical quality that’s possible when you don’t have to design lenses that account for a flipping mirror between the rear of the lens and the film or sensor plane. Rangefinders can be smaller, lighter, and quieter thanks to their less-complex designs, as well, but image quality is the primary reason they still have their adherents.

That advantage is especially true for rangefinders’ wide-angle lenses, the rear elements of which can be placed as close to the film or sensor as is practically possible. SLR lens designers have to artificially increase the distance between the rear of the lens and the film or sensor to make room for the mirror and its surrounding infrastructure.

As the article by Bruce Sirovich above notes, that design principle requires complex retrofocus lenses with many glass elements, and complex lens designs tend to be harder to keep sharp, all else being equal. Leica cameras and lenses—the most prominent rangefinder system still in use—have a deserved reputation for being among the sharpest in the world in part because of the inherent advantages the rangefinder camera design offers optical engineers.


A view through the Leica M3’s finder. Note the lens intruding in the lower right. Source: kiemchacsu on Flickr

Rangefinders have all sorts of inconveniences for practical photography, though. As I alluded to in the first paragraph, one inconvenience is parallax at close distances. By design, rangefinders make the photographer peer through a separate viewfinder with its own perspective, not directly through the lens, so the mismatch between your eye’s perspective and the lens’ perspective grows as you focus on closer subjects.

This mismatch makes macro (or extreme-close-up) photography practically impossible with rangefinders, barring the use of specialized focusing devices (e.g. Leica’s Visoflex). Rangefinders can also show more of the image than you’ll actually get in their viewfinders, so it can be difficult to achieve precise compositions at the edges of your frame. Rangefinder lenses can even intrude into the view offered through the finder, marking yet another way it can be more difficult to achieve the precise composition you want.


Leica’s Universal Wide Finder. Note hotshoe mount at lower left.

On top of those issues, rangefinders have a limited set of frame lines in their viewfinders that they can show to define the edges of your composition, so the handy zoom lenses common from SLR makers are practically unheard of in rangefinder systems. Rangefinders also can’t show the full view from wide-angle lenses, so you have to compose your wide-angle shots through another, auxiliary finder that’s different for every wide-angle lens you have in your bag before focusing using the limited view through the rangefinder window. That separate finder is in yet another position versus the lens itself, so precise composition remains difficult.

Finally, with long lenses (above 85 mm or so), rangefinder photographers have to cope with calibration issues between the focusing system and the lens itself (usually controlled by a mechanical coupling between lens and rangefinder). Rangefinders and lenses that are poorly matched make it hard to get sharp shots, and fixing that mismatch usually requires the purchase of a different lens or sending the camera and lens back to the manufacturer to get them in closer agreement. That’s one reason why you don’t generally see lenses longer than 90 mm in common use with today’s Leicas.

In short, even though rangefinder cameras don’t need flippy mirrors or pentaprisms to assist with composing or focusing, the fact that rangefinders can’t see through the lens attached to the camera makes them specialized tools, not the Swiss Army knives of photography. That mantle has, for many years, been claimed by the SLR.

 

Tolerances test the SLR’s limits

SLRs aren’t a perfect solution to the problems of the rangefinder, though. Since you’re relying on an agreement of mechanical alignments between light, a partially silvered mirror, the optical focusing system above the mirror, and an autofocus system underneath the mirror, you can end up with subtle focus problems that weren’t visible on film but are becoming more and more painfully evident with wide-aperture lenses and high-resolution, full-frame sensors.


Nikon’s D850, one of the most recent DSLRs

That’s because wide-aperture lenses offer so little depth-of-field, or the front-to-back “slice” of the image that’s in focus, at their maximum aperture. The full-35-mm-frame, high-resolution sensors popular with today’s professional cameras reduce the effective depth of field for a given focal length compared to smaller-sensor cameras (like Micro Four Thirds and APS-C), making any focusing errors with those lenses all the more obvious.

You can get around that problem by calibrating your SLR’s autofocus system to compensate with some lenses, but it’s not a cure-all by any stretch of the imagination. The focus errors between lenses and autofocus systems aren’t necessarily consistent, either, meaning that a compensation made at one end of the focusing range might not work well for another focus distance.


The AF Fine-Tune controls of my Nikon D810, showing a particularly egregious mismatch between lens and AF system

It’s probably fair to say that we are reaching a limit of what’s possible with the SLR design today, just because of the practical limits of mechanical tolerances of a moving mirror system versus sensor size and resolution.

Mirrorless digital cameras, then, are opening the next frontier of demanding photography by blending some of the virtues of rangefinders and SLRs with their own distinct set of advantages. Since mirrorless cameras acquire focus directly from the sensor plane, photographers are (in theory) assured of precise focus even with long, wide-aperture lenses on large, high-resolution sensors. The agreement between lens and focus system is theoretically no longer an issue, since the camera’s focus system is a closed loop and can compensate for any imprecision in the lens itself.


Nikon’s Multi-CAM 20K AF sensor module, which sits under the mirror in its DSLRs. Source: Nikon

Since mirrorless cameras’ phase-detection (read: fast) autofocus (AF) sensors are ideally just another functional element on a silicon substrate—a principle that Canon pioneered and is now used in practically every high-end smartphone nowadays—engineers can spread out those AF “pixels” over the whole sensor, rather than being limited by the practical size of a separate autofocus module that can be embedded under an SLR mirror. That limitation tends to result in a small cluster of AF sensors in the middle of the frame.

Since mirrorless cameras can spread out their AF sensors this way, subject tracking and autofocus over the entire frame is a real possibility now, as demonstrated by Sony’s A9 camera, among others.


Canon’s dual-pixel autofocus system, in principle. Source: Canon, via YouTube

Mirrorless digital cameras also take another page from rangefinders by allowing lens designers to put the rear element of the lens as close to the sensor as is practically possible, so it’s easier to make wide, fast, and sharp lenses that aren’t huge. You still get the direct view through the lens with mirrorless cameras that made SLRs so popular, thanks to the relatively high-resolution and high-refresh-rate electronic viewfinders that draw their video feed directly from the image sensor.

With all those advantages, though, why didn’t interchangeable-lens mirrorless cameras take off much sooner, given that we’ve had practical digital SLRs since 1999’s Nikon D1? The answer likely comes down to lenses (as well as massive leaps and bounds in technology since, but I’m going to blame lenses). Professionals and amateurs alike can invest many thousands of dollars in glass, and those lenses can remain useful for decades, if not longer, even as camera technology advances. Camera makers who want to build enduring systems and loyalty among camera buyers avoid lens-mount changes like the plague.


Nikon Z7, showing the new Z mount

For example, the last major mount change in the Canon-Nikon duopoly was Canon’s introduction of the EF mount in 1987. All EF lenses still work perfectly with today’s Canon DSLRs, and even manual-focus Nikon F-mount lenses from 1959 will still work with the company’s latest semi-professional and professional cameras with some mechanical modifications for aperture readout. That reticence to change mounts means Nikon’s new Z mount is a rare event in the world of cameras, and it’s no surprise that the company is offering a bridge to photographers already invested in F-mount glass with its FTZ adapter.

All told, mirrorless cameras give engineers more flexibility to cope with the challenges of high-resolution sensors and optics now that film isn’t the primary medium on which we’re recording images, and it’s no shock that the industry is embracing them as the future of camera design. Nikon might have been the next domino to fall on the way to mirrorless cameras’ dominance of the high-end photography market, but it’s unlikely to be the last.