Taking Pictures Through the Stereo Window: Matching Lens and Eyepiece Magnification for Immersive Viewfinding

30 Dec 2014 06:17
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"Keep both eyes open" is common advice to new photographers. It keeps you more aware of your surroundings and more attentive to your subject-including compositional elements and interesting situations developing outside the frame! Maybe even more personable, with at least one normal peeper alongside the camera's giant Cyclops-eye and, over time, fewer creepy squint-wrinkles.

But looking through a viewfinder at one magnification-typically small, i.e., a reduction-and into the real world at its own size is distracting. It's hard to have more than the faintest impression of one while paying attention to the other.

(This assumes the now typically fancier kind of camera with a eye-level viewfinder, such as an SLR, typically associated with photography students and enthusiasts. Those finders' optics provide a fairly wide, bright view that appears off in the distance like the real scene. It's pretty much impossible to pay attention to a little picture of the outside world on a digicam's or phone's plain screen at arm's length, and to the outside world itself, at once.)

The problem pretty much goes away if the viewfinder and the outside world have the same magnification: none at all, or 1X.

A simple direct viewfinder, such as that in some rangefinder cameras, can pretty easily have this magnification and provide sets of framelines for various lenses' fields of view. (At least normal and tele: wide-field eyepieces require special complicated bulky optics.) But these, like most separate viewfinders and sights, suffer from parallax and, often, overall imprecision.

Through-the-lens viewing as in an SLR is very precise, but the overall magnification of the viewfinder depends on the combined magnification of the finder system itself (typically quoted with a 50mm lens, whether or not that is the "normal" moderate-angle focal length for the format) and the lens.

To match the magnification on an optical finder, you'll need some sort of optical adjustment to the eyepiece. Ideally this might come from zooming elements built into the eyepiece or a purpose-made attachment. A modern computerized camera could even auto-adjust to match the lens or zoom selected, and the assembly might shift to put the image "straight ahead" with the camera just as the photographer wants to hold it.

But there's an easy solution right now: eyepiece "extenders" to reduce magnification for tele lenses, and magnifiers to increase it for moderate wide-angles. (Ultrawides might require special wide-field optics and, as the name implies, require a wider than usual gaze to take in at full magnification; in any event, they're for getting right into the middle of things-not a quick glance over.)

A typical extender is the Canon EP-EX15. There's also a version II, with slightly different compatibility. And it has Chinese clones such as the EC-2 from the often-good "JJC", but, for this accessory at least, the Canon's reliable high quality is not grossly overpriced. This extender decreases magnification by 30% (that is, multiplies it by about 0.7), increases eye relief, and adds a little "barrel" (center bulges out) distortion. It goes between the camera's removable eyecup and the viewfinder, and can be stacked for increased effect. A stack of three is about right for 135mm focal length on a crop-frame DSLR or 200mm on a "full-frame" (35mm film or expensive digital) camera. It does stick out from the camera but can come off for storage and seems likely to itself break rather than damage the camera's harder-to-replace eyepiece flanges.

Typical magnifiers, of several Chinese makes, similarly clip on to the camera's eyepiece mount but come with their own eyecups and do not provide for stacking. Some zoom, however. They reduce eye relief and can require eye movement to see corners or simply not show them well at all. It may come as no surprise that Canon seems not to make one itself: a big, bright view is impressive, pleasant and generally practical, so they've already designed in as much magnification as practical without being likely to trouble many customers. These should work well for moderately wide angle lenses such as 35-40mm on full frame: past that, they'll still do their thing, but will not bring the picture all the way back to life size.

To use the life-size adapted viewfinder, simply select a number of extenders or a magnifier and, if applicable, its zoom setting to approximately offset the magnification or reduction of the view through the camera and lens. If the viewfinder adapter or the lens itself is a zoom, tweak one or the other to merge the images through the viewfinder and the other eye perfectly. (From this point, zooming the lens will mismatch the images.) Similar but not identical magnifications may work fine too. You should see a dark frame with a brighter, 3D image within your field of view. That's your viewfinder! Simply turn to what to photograph - you'll naturally keep everything aligned. Awesome for fast-moving sports and birds!

Keeping the two images merged might be more difficult with very narrow viewing angles against plain backgrounds, such as sky. A self-compensating red-dot (not laser projecting) sight, which can also project frames of adjustable sizes, might be better for those, at the cost of less maximum precision. It also doesn't have to go behind the camera, allowing it to be held underarm or on the shoulder—just beware of potential resemblance to a weapon.

The smaller view for telephoto might seem like a drawback at first, but it's not as bad as one might think. The fine details aren't as important when you don't have to be sure they're sharply focused: the overall composition will be very visible and the AF-confirm lights will sparkle over what's in focus.

An (eye-level) electronic viewfinder could simplify automation. Choose a fancy camera with one, or add an adapter eyepiece to an LCD monitor. Although the colors won't match as well, the electronic image size could easily be "zoomed" to maintain perfect life size magnification at the eyepiece and framelines and any other desired data could be added. A great programming project for Magic Lantern or a high-end cameraphone!

For the ultimate in oneness with your camera, try this with eye-controlled focus as on a Canon EOS 3. Spooky! Canon please bring that back…or someone - it's probably soon out of patent and even that exact same autofocus array would be super-intuitive sprawled all the way across a crop-size digital sensor area. No more think-focus-reframe or loss of expensive image quality to cropping or field curvature! Just look-oooh-click! click! Click!

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