It’s done with Mirrors


A while back I wrote about a new “mirrorless” digital camera being jointly developed by Hasselblad and Sony. More and more camera models are being marketed using this term, so I thought it might be useful to explain why some cameras contain mirrors and other do not.

The use of a mirror inside a camera is the heart of a reflex viewfinder system, the most common type being the single lens reflex (SLR). This system, although an old idea, evolved to its now-familiar form around 50 years ago.

The concept of the SLR is simple: Inside the body of a camera, a mirror is situated at a 45-degree angle, intercepting the incoming light from the lens and sending it upwards to a ground-glass focusing screen. Above the screen a prism is employed to redirect the image from the screen to a magnifying eyepiece. The shape of this prism also corrects the screen image orientation, making the eyepiece image an exact duplicate of what the lens will create on film.

SLR Cross Section.

SLR Cross Section. Image formed by lens (1) bounced off mirror (2) forming image on screen (5). A condenser (6) brightens the image which is routed through prism (7) to eyepiece (8) for viewing. During exposure, mirror flips up and shutter (3) opens to expose film or sensor (4).

This simple concept gets a bit complicated when a picture is actually taken. When the user presses the shutter release on an SLR, a sequence of mechanical actions takes place very rapidly:

  • The mirror, which is hinged at the top, flips up out of the way, covering the ground-glass screen and preventing light from the eyepiece from affecting the film.
  • The aperture in the camera’s lens stops down to whatever f-stop the user has chosen. Between shots, the lens aperture stays completely open in order to keep the viewfinder image as bright as possible.
  • With the mirror up and the lens at the chosen aperture, the shutter opens and closes to expose the film frame (or digital sensor in a dSLR).
  • When the shutter is once again closed, the lens aperture opens up all the way and the mirror snaps back downward into “viewing” position.

All this takes place in the blink of an eye. The photographer experiences a fraction of a second of “blackout” in the viewfinder before the image returns to the eyepiece and more photos can be taken.

All of this requires many expensive mechanical parts working with clockwork precision, but the benefits are worth it. The SLR viewfinder is far more accurate for image composition than any sort of separate optical viewfinder, especially when using longer telephoto lenses or photographing through other optical instruments such as microscopes.

Developments in digital photography created the first really viable challenge to the SLR’s dominance. The invention of live view LCD display panels on the back of today’s digital cameras means that users can now compose their photographs by simply looking at the electronic screen. In many newer point-and-shoot models, there’s no optical viewfinder at all; the user has to compose on the LCD screen. No reflex viewing system, no flipping mirror.

At first, these systems were used only on relatively inexpensive cameras with small imaging sensors aimed at the amateur market. In recent years, the market has seen an increasing number of camera models that have larger imaging sensors, interchangeable lenses, but no reflex viewing system.

The Canon EOS M is a good example. It takes the imaging sensor from the popular EOS Rebel T4i reflex camera and puts it into a mirrorless body that accepts its own set of special lenses. There are some obvious advantages to this sort of approach.

First off, it makes for a much smaller camera. The mirror box and prism assembly on a typical SLR take up a lot of space; by eliminating these components, the camera becomes more compact while retaining the imaging qualities of the larger sensor.

Canon EOS M mirrorless camera next to EOS Rebel T41. Both have the same sensor size and resolution.

Canon EOS M mirrorless camera next to EOS Rebel T41. Both have the same sensor size and resolution. The mirrorless camera fits in coat pocket.

There are also fewer mechanical parts moving around when a picture is taken. That flipping mirror is the SLR’s weakness—when HMML has camera breakdowns involving the dSLRs we use for manuscript photography, it’s almost always the mirror mechanism that has broken or worn out from countless actuations. If a compact mirrorless camera uses an electronic shutter instead of a mechanical one, there’s even fewer moving parts to break. From a reliability standpoint, the mirrorless cameras should have an advantage. Fewer moving parts should also give them an edge when it comes to eliminating vibrations in the camera.

The mirrorless cameras are also well suited to video recording. When shooting video footage with a dSLR, the mirror flips up and the user must use the LCD display anyway, so the extra reflex viewing components become useless.

The new design certainly has some advantages, but, being old-fashioned, I still prefer the SLR design despite the extra bulk and mechanical complexity. I just can’t get used to holding a camera out in front of my face to frame my shots. It’s unstable compared to way an SLR is held against the face, which creates a very steady three-point support for the camera.

I’m also not all that enamored of composing on a video screen. There’s always a bit of a lag in how fast the image refreshes as you move the camera around, and I’d rather look at a projected image on a fine viewing screen than pixels on a “tiny TV” that forces me to use bifocals to compose.

Despite this, the mirrorless design is becoming more and more popular, especially with folks who never got into using SLRs. This makes me wonder about the future of the SLR with its bulky mirror and prism assembly. Will this age-old design finally become obsolete as electronic viewfinders become better and users demand more compact cameras? Will we all be taking photos while holding cameras out in front of our faces? I’m not sure, but I do know that I’ll probably one of the last to switch.


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