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Man looking into optical viewfinder on DSLR camera

By Rudy Winston

Canon Autofocus Series: Still a Big Benefit - The Optical SLR Viewfinder

By: Rudy Winston

May 29, 2018

There’s no question that the completely electronic viewfinder in some recent interchangeable-lens cameras — think of “mirrorless” cameras, like Canon’s EOS M-series models — brings some cool features to their users. Some of these include the ability to see the effect of changes in camera settings, like exposure or white balance, and to see additional information like histograms and so on, before a picture is taken.

But there’s a lot of benefit to the traditional “optical” viewfinder, used in EOS digital SLRs like the EOS Rebel T7i and EOS 77D. We’ll look at those benefits in this article.

The Optical Viewfinder

Just to be clear, the term optical viewfinder refers to the type of eye-level finder where you look at an image as it’s projected into the camera by the lens. It’s not an electronic screen; instead, it’s the full image delivered optically onto a very precise, flat surface called a focusing screen. When the camera’s lens is not sharply focused upon a subject, that subject appears blurred…and it becomes sharp in the viewfinder when the lens is focused sharply. You are looking through the lens at the actual subject, again not at an electronic screen.

Autofocus: Power, Precision and Speed

With the optical viewfinder, EOS DSLRs use a totally different type of AF than so-called “mirrorless” cameras do — and this continues to bring significant performance benefits to the DSLR user.

Instead of relying on focusing off the imaging sensor itself — which of course is primarily designed to capture image information — the phase-detection AF system in an EOS DSLR uses a separate and totally dedicated AF sensor for all focus detection and calculation. We discussed its benefits in-depth in another article in this series. But simply stated, the type of AF used in cameras like the EOS 77D and Rebel T7i (in their optical, eye-level viewfinder) has advantages such as these, over most competitive systems that read focus off the imaging sensor:

Cross-type AF coverage, at up to all focus points
This is covered in a separate article in this series, and can be found here. But the short story is that (with most lenses), each AF point gets distinct horizontal and vertical coverage, from separate sensor elements, to read your subject or scene. AF is far less likely to be thrown off by things like thin, repeating lines, very fine detail and texture, and so on. Cross-type AF is responsive AF, whether you use the center AF point, or AF points away from the center.
Totally phase-detect AF technology — no “hybrid” system that also uses contrast-based detection
This is a big difference. Phase detection AF is able to not only immediately tell if a subject is in-focus or not, but can instantly assess two vital things: what direction the lens needs to be driven, and when the subject is actually in sharp, final focus.
Many competitive “mirrorless” systems have to add what’s called contrast detection for final focus adjustment. This is a type of sharpness detection where simple sharpness is read off parts of an image sensor — and this often requires the system to move focus toward and then past the point of sharpest possible focus, and then drive the lens in reverse to go back to the detected sharpest plane of focus. You can often see this as the lens “hunting” back and forth slightly, to settle on the sharpest focus.
With a totally phase detect AF system, this usually isn’t the case, as long as the active AF point(s) in question are seeing some detail or texture at the actual subject. Phase detection technology lets the AF system know immediately which direction to drive the lens in, and when to precisely stop to achieve sharpest focus. The now-traditional AF through an optical viewfinder in a DSLR doesn't have to rely on starting with phase-detection, and then handing off to a contrast-detection system to finish the job of sharply focusing the camera’s lens.

This is a picture of the 61-point AF sensor, used in an EOS-1D X Mark II camera. It’s similar in appearance to the AF sensors in all current EOS DSLRs, even those with fewer total AF points. As discussed above, this type of AF sensor is exclusively used for focus detection, and has the inherent benefits of immediately knowing not only when a subject is out of focus, but approximately how far out of focus it is, and in what direction to drive the lens to bring that part of the scene into sharpest focus — without having to hunt back and forth. The technology used here is called “phase detection” (not to be confused with “face detection” — a totally different AF feature!)

This type of AF works well with a huge variety of lenses
Phase-detection AF, like you see in DSLRs with optical viewfinders, is a long-proven technology, and it works well with lenses ranging from ultra wide-angle thru long super-telephoto. And, it even works well with fast-moving subjects — often a challenge for AF systems that rely on focusing off the imaging sensor to get their focus data. This is something prospective users often won’t see in a brief hands-on demo at a dealer. But it may well be something they experience out in the field, with subjects like birds in flight, or sports with long telephoto lenses.

When you use the optical viewfinder, a separate, dedicated AF sensor does all autofocusing. One of the many benefits is that this system is compatible with every Canon EF or EF-S lens that offers AF… whether you’re using an ultra wide-angle lens, or a super telephoto. And, this applies to every Canon EOS SLR, from the first EOS 650 film camera of 1987 to today’s latest digital SLRs.

Other Benefits of Optical, Eye-level Viewfinders

Again, our purpose isn’t to bash the concept of the electronic viewfinder. But understand that in some cases, their advantages (like being able to see changes in exposure or white balance in the finder, as you change settings on the camera) come with some compromises as well. Some things that the traditional optical viewfinder still brings to the photographer:

Very little battery consumption
An optical, eye-level viewfinder (such as that found in the Canon EOS Rebel T7i, or EOS 77D) actually does have a thin, LCD overlay in its optical viewfinder, which displays the AF points, grid lines (if you’ve activated them), and so on. And there’s an illumination system as well, to light up the focus points and other displays. So they do consume a small amount of battery power.
But this is far less than the amount of power typically consumed by modern electronic viewfinders in many mirrorless-type cameras today. In fact, according to some camera makers, their eye-level, electronic viewfinders use more battery power than using the larger LCD monitor for Live View shooting. (Of course, power consumption specs vary, and many other factors can contribute to how many shots are possible per battery charge.)

With transparent LCD overlays, information can be placed over the focus screen, on top of what you’re viewing. Here, you can see the focus points and camera shooting settings, in a simulated viewfinder from a high-end EOS 5DS camera.

Optical viewfinders stay active during continuous shooting
When you fire multiple, continuous shots (a sequence of a moving subject, for instance), there is momentary black-out in an SLR’s optical viewfinder, as the mirror goes up and returns. But anytime the mirror is down, even if it’s only for an instant, you see right through the lens. With some electronic viewfinders, when you fire-off a sequence of shots, the camera isn’t able to continually display what it’s seeing, and you instead get a quick sequence of each shot you took — making it harder to see and keep a moving subject well-composed in the viewfinder. You may sometimes even find a subject leaves the frame entirely, and you’re not aware of it as you’re shooting, because the electronic viewfinder can only show a delayed display of finished shots as you fire your sequence of pictures.
See the subject or scene in the actual lighting it’s being illuminated by
Electronic viewfinders can simulate what different camera settings will do to an image before it’s taken, and sometimes this can truly be beneficial. But there’s also value in constantly seeing the actual subject, in the actual lighting it’s being bathed in as you compose and shoot it. Optical viewfinders give you a true view of your subject, without imposing artificial simulated looks before you shoot. For some users, this can be a more useful way to work, even in dim or unusual artificial lighting.
(Sometimes) easier to see what’s in sharp focus, and what’s not
This will vary, depending upon a host of factors, when you compare an optical viewfinder to an eye-level electronic viewfinder. But there are situations where an electronic finder makes lots of things simply appear sharp to your eye, when in fact if the lens is wide-open (or near wide-open), some elements in a scene may in fact be out of focus. And, in some cases, an optical viewfinder’s focus screen can make it easier to determine not just if something in a foreground or background is out-of-focus, but how out-of-focus it is.

Summary

It’s sometimes difficult, in the face of impressive new technologies, to avoid taking for granted the value of traditional designs. The optical-type viewfinder is a perfect example. It’s evolved over time, adding features like LCD overlays to display grid patterns and up-to-the-moment status of AF, but at its heart, optical viewfinders remain a powerful way for the critical photographer to work with his or her lenses, and his or her subjects. It uses virtually no battery power, nor does it compromise on a user’s ability to shoot continuously and view their subject.

We’re hopeful that this reminder of the value of traditional SLR camera construction will be useful insight, to both current and future camera owners.