Understanding SLR Viewfinders
By Mike Johnston
Finding the View
The viewfinder is the single most important user interface on any camera. Throughout the history of cameras, the method of aiming the camera accurately and communicating its view to the operator is what has determined and defined most different basic camera types.
Yet the viewfinder is perhaps the single most fudged and botched aspect of today's 35mm SLRs. With the exception of the Contax Aria of the late '90s and the more recent Minolta Maxxum 7, virtually all entry-level to mid-range cameras skimp on the viewfinder. The worst offenders are cameras that are meant to be cheap (they have mirror-box prisms) or cameras that are meant to be small (which usually have poorer coverage).
For the Sake of Clarity
To be clear, let's define a few terms about viewfinders, just in case you're not entirely up to speed. (And if you aren't, don't feel bad. Most people aren't. Why do you think the manufacturers are able to get away with such blatant skimping? An educated consumer is a dangerous consumer. Oh, did I say "dangerous"? I meant "demanding." Or maybe "discriminating." Undesirable, in any event.)
Viewfinder image: this is your view of the world through the camera, the little rectangle with black edges that shows you what part of the world the lens is looking at and whether it's in focus or not.
A typical viewfinder. (Compliments of Photography in Malaysia )
Magnification: this refers to how big the viewfinder image appears to be in an absolute sense. Like a batting average, it's usually expressed as some decimal fraction of one. 1X is the size that things appear to be when you look at them with your eye (a.k.a. "the naked eye"). Now, obviously, magnification also changes when you use different lens focal lengths — telephotos make things look bigger, wide-angles make things look smaller. So camera magnification is specified with a 50mm lens. Less often stated is that the lens must be set at infinity, because magnification also changes slightly depending on how close or far you focus the lens.
Let's say a camera's magnification is .75X. What this means is that your camera, with a 50mm lens on it, set at infinity, makes things appear to be three-quarters the size they look to be with your naked eye. .5X means half as big; .9X means nine-tenths as big. Better cameras have higher magnification. .88X is better than .67X. You're getting this.
I hope it stands to reason that magnification also determines the apparent relative size of the viewfinder image rectangle. I once tried an interesting little experiment — with identical 50mm lenses on both, I held a Pentax ME Super (high magnification) to one eye and a Pentax ZX-5n (low magnification) to the other. The ZX-5n's viewfinder image fit inside the ME Super's with lots of room to spare.
Coverage: this compares what you can see in the viewfinder with what will be recorded on the film. It's reported as a percentage. If you can see through the viewfinder half of what will be on the negative, that would be 50% coverage.
To further confound matters, coverage is sometimes reported as a linear measure and sometimes as an area measure. To simplify this, imagine a big square drawn on graph paper that has ten little squares per side. The linear measure is 10 x 10 little squares, and the area measure is 100 little squares. Now imagine that we're going to draw a slightly smaller square inside the big one that's smaller by one little square on each side. That square has eight little squares on each side. The linear coverage of the inside square is 80% of the larger one (8 instead of 10); the area coverage is 64% (8 x 8 instead of 10 x 10). You can see from this that when one camera manufacturer reports that its viewfinder has 92% coverage and another reports 95% coverage, you still can't quite be sure how they compare, because one might be reporting linear coverage and the other area coverage. (And an educated consumer is a....)
Now, if you were no expert and just taking a stab at this, you'd probably guess that you would want to see in the viewfinder all of the picture you're about to take. It stands to reason you don't want to see half of it, or a tenth of it, so why wouldn't you just want to see all of it? As with many things, however, it turns out that the uncomplicated answer is not the correct one. More about this later.
This diagram shows the light path in a typical 35mm SLR (Compliments of Photography in Malaysia )
Eyepiece: If you stick your finger out and touch the place you look into to see through the camera, you're touching the eyepiece — it's the little window on the back of the camera you put your eye up to. Add-on (as opposed to built-in) diopter correction lenses are usually incorporated into the eyepiece.
Eyepoint, also called eye relief: this refers to how far away from the eyepiece you can have your eye and still see the entire viewfinder image. Obviously, if you hold an SLR six inches from your face and look at the eyepiece, you can still see a portion of the image the camera is seeing. But you can only see a small portion of it, not the whole image out to the edges. As you move your eye in closer and closer, you see more and more of the viewfinder image, until you can see the whole thing at once. Eyepoint, usually specified in millimeters, is how close you need to get to see the whole viewfinder frame. "High eyepoint," a term coined and popularized by Nikon, means you can hold you eye fairly far away from the eyepiece and still see the viewfinder. It's useful for people who wear eyeglasses, since eyeglasses create a physical barrier between your eye and the eyepiece that keeps them a certain distance apart.
Prism: This is big funny-shaped (pentagonal) chunk of glass that forms the big "lump" at the top of an SLR and that "rectifies" the lens image — that makes it look right-side-up and right-way-round when you look through the eyepiece. This is usually a beautifully crafted piece of fine optical glass with silvered surfaces. Sometimes, to save weight, SLR makers use a hollow box made of inward-facing mirrored surfaces, called, appropriately enough, a mirror box (or a "roof mirror pentaprism" or various smokescreen proprietary names that not-so-helpfully obscure from the consumer what's really going on).
Viewing screen, or just "screen": This is the piece of plastic between an SLR's mirror-box and prism, on which the lens's aerial image is cast. Also sometimes referred to as ground glass, since that's what screens used to be made of (and still are, on some view cameras). Think of a slide projector. When the pprojected image is in the air, you can't see it; but when it hits the projection screen and reflects back at you, then you can see it. If the slide projection screen were semi-transparent, then you could get behind it instead of in front of it and still see the image through it. This is what a camera's viewing screen does. The viewing screen is where the camera maker puts things like focusing aids (split-image circles and microprism collars, if you're old enough to remember such things), grid lines, and the little marks that show you what the camera is autofocusing on. Better cameras may have "interchangeable screens," meaning that you can take out the one with the split-image circle on it and put in one that has grid-lines on it, or whatever.
A cutaway view of the familiar 35mm SLR viewing system. You can clearly see the lens, the mirror, and the glass pentaprism. The screen is the flat piece just above the mirror; the eyepiece is the bit furthest to the right, where your eye peers into the whole thing.
Brightness: this should be self-evident. It refers to how bright or dim the viewing screen is. This isn't measured, at least in camera tests, but rather reported subjectively: "the view screen was admirably bright," "The screen was as dark as Injun Joe's cave," etc.
Focusing "snap", a.k.a. ground-glass coarseness: this not only can't be measured objectively, it's also usually not remarked upon in camera tests. Rather, it's assumed that the brighter the screen is, the easier it is to focus. This is sometimes, but not always, true. More about this later.
Diopter correction: This changes the diopter of the viewing system, same way eyeglasses change the correction of the lens in your eye. The virtual image of the information displays in the viewfinder is about a yard away from your eye, so the diopter correction your eye needs to see an object clearly at about three feet is the same correction you need to see the camera viewfinder image clearly. Diopters can be in the eyepiece and screw or snap or slide on, or, in the better cameras, they can be built-in. Some manufacturers provide a combination of the two for greater effective range. Most viewfinders have a standard correction of -1, and should have correction available from at least -2 to +1. If you're in doubt about what diopter you need, go to an expert — your optometrist — and ask!
Life is a Cabaret, Old Chum
Now, if you are a Good Amurricun, you are saying: give me everything. I want it all. I want an optical glass viewfinder with 1X magnification and 100% coverage that is as bright as life and easy to focus manually, that I can see with my eye an inch away from the eyepiece.
Oh, but you cannot have it all, friend. For two reasons. First of all, it turns out that some of these design parameters fight each other from an engineering standpoint. Secondly, if you are an average Amurricun, you want it all without having to pay for it. No such luck. Good viewfinders are e-x-p-e-n-s-i-v-e.
Roughly speaking, there is a trade-off between magnification and coverage. The higher the coverage, the lower the magnification. Ironically, magnification for 100% finders has to be made lower so that viewfinder information displays can fit in the finder and be within the user's field of view. That's why all the top pro SLRs have 100% coverage but only so-so magnification.
If you've been following along here, you've probably already realized that lower-magnification viewfinders, since they're smaller, are going to easier to see with your eye further away from the eyepiece — i.e., higher eyepoint. So magnification and eye relief also oppose each other to some extent.
Similarly, "focusing snap" and viewfinder brightness don't exactly go hand in hand. The super-bright screens are essentially bundles of very small fiberoptic cables, sliced crosswise, or miniature fresnel (flattened) simple lenses. While they transmit a ton of light, they can be very difficult to focus on. Everything looks pretty sharp; it's not very obvious what's in focus and what's not. (The effect is worse with wide-angle lenses, which have more depth-of-field.) Old-fashioned ground-glass screens had better focusing snap the coarser the grind (surface texture) was. But, the coarser the surface, the dimmer the finder.
One reason bright screens are so widely used today is that many SLRs are now autofocus, and focus isn't dependent on your eye, so focusing snap no longer matters. If you want to see what a really bright viewfinder looks like, take a gander through Minolta's Maxxum 7 next time you're in a camera store. It's an example of a finder with very good brightness, but it doesn't have very good focusing snap. The Maxxum 7's magnification is excellent as well, at least by AF-camera standards. Although manual focusing with this finder may be tough, it's an incredibly bright, contrasty, crisp and clean AF viewfinder. Frankly, compared to most cameras these days, the Maxxum 7 is a pleasure to shoot with because it's such a pleasure to look at the world through. But I digress.
Here are some of the specs of some of the viewfinders being offered today in top-of-the-line cameras, and a few others thrown in for the sake of illustration. All of these except the N55 are glass-prism viewfinders. Specs are listed in this order: magnification, coverage, built-in diopter correction range, and eyepoint.
Minolta Maxxum 9:
.73X, 100%, -3 to +1, high-eyepoint (22mm)
.74X, 100%, -3 to +1, high-eyepoint (not specified)
.82X, 95%, none built-in, high-eyepoint (25mm)
.75X, 96% vertical, 97% horizontal, high-eyepoint (not specified)
.72X, 100%, -3 to +1, high-eyepoint (20mm)
.75X, 100%, built-in but not specified on website, high-eyepoint (20.5mm)
.75X, 92%, -2.5 to +1.5, not high-eyepoint
Here's an example of typical entry-level SLR viewfinder specs:
Nikon N55 (mirror-box prism, not optical glass):
.6X to .68X*, 89%, built-in but not specified on website, eyepoint 17mm
*Note: I'm afraid I have no idea what this could mean. I've never seen magnification given as a range before.
Finally, here's an example of a great viewfinder from days gone by:
0.84X, 97%, -3 to +1 (built-in), eyepoint not known.
Now take a look for a moment at the comparative specs for the two Contaxes, the RTSIII and Aria. Contax gives its users the best of both worlds: those who want 100% coverage can choose the RTSIII. With the Aria, which, like most Contaxes, has a particularly excellent viewfinder, the coverage goes down slightly to 95% (still very good), but the magnification goes up, to .82X. This, plus the Aria's excellent "focusing snap," makes manual focusing easy. Combined with the Aria's outstanding eye relief (more than one inch, better than both the Nikon F5 and Canon EOS-1V!), it makes for a very good viewfinder indeed.
The Nikon N55, on the other hand, is an entry-level camera that exemplifies the poor viewfinders typical of today's entry-level breed. (Of course it's better than what it competes with: point-and-shoots, most of which have even worse finders.) The N55 has what one photographer referred to as a "Lincoln Tunnel" finder — meaning, it looks small and far away. You can't see to focus manually. It doesn't show you everything that will be on your negatives. It's a pointing device. It sucks.
Dirty Little Secrets
Viewfinder coverage (see definition above) is perhaps the dirty little secret of today's SLRs. The Nikon N55 is typical — it shows you, in the viewfinder, less than 90% of what you're going to get on your film. Of course, this generally works out fine, because most people who use entry-level SLRs also use drugstore and supermarket film processing — which actually use less than 90% of the negative to make enlargements with!
This highly approximate system is self-correcting. It hides slop in many different places. First of all, since consumers never see the edges of the pictures recorded on the negative, they'll never know that their consumer-grade lenses aren't very good at the edges and corners. Since the camera's viewfinder doesn't begin to cover the whole area seen by the negative, it doesn't matter if maybe the viewfinder image isn't centered properly. Since the photofinisher is cutting off the edges of the pictures in enlarging, it doesn't matter if his equipment is registered well, either. Finally, it covers slop in the actual picture-taking — because perhaps the most common failure of tyro shooters is that they don't get close enough or frame tight enough. So, your camera and your processor "zoom in" a bit on your behalf. Everybody goes away happy.
Of course, all this has some disadvantages, too. It makes your lenses longer than they really are. Paid extra for a zoom that goes to 28mm on the wide end, did you? Thought you were shooting at 28mm? Not really — your photofinisher, like your viewfinder, has cropped it down to the angle of view of a 35mm lens. Thought you were using all the negative area available to you for the best quality enlargements? Think again — you're using a patch out of the middle of your 35mm neg that's barely bigger than APS size. And of course, precise framing is pretty much out of the question. Plunk your subjects in the middle of the frame like most rank snapshooters, and you probably won't mind. Otherwise, you'll mind.
One might reasonably ask, what's the point of using an SLR at all if you still can't see what the lens sees?
Ignorance Ain't Bliss!
The main reason coverage is a dirty secret is that this is one camera specification that ordinary consumers simply aren't aware of. One industry insider let it slip to me that most camera purchasers aren't aware that their cheap SLRs have poor viewfinders simply because most camera purchasers have never seen a good one! Yeow.
One of the delightful little lies the industry just loves to tell is that "amateur cameras have 93% finders to correspond to the image area of a mounted slide." I love this one. It makes perfect sense — until you stop to realize two inconvenient little facts. Fact one, less than 3% of film sold in North America is slide film. Fact two, if this is such an advantage, then why do all the top pro cameras have 100% finders — especially since pros are the ones who shoot the most slide film?
The whole situation, when you think about it, is ironic. As more and more people buy cameras by the spec sheet, features and specifications have become more and more important, and manufacturers have loaded down their cameras with features to try to make them sell. Yet when it comes to the most important interface on the camera, most consumers are still ignorant of what the specifications mean — with the predictable result that most cameras have considerably poorer viewfinders now than average SLRs had twenty-five years ago.
So let's say you want to be an educated consumer when it comes to viewfinders, and try to pick through the consumer nightmare? (Oops, did I say "nightmare"? I meant "minefield." Oh, no, I meant "wealth of choices." That's it.) What should you look for?
— Try to buy a camera with at least a 95% viewfinder. Even this modest standard is tough to meet these days, but they're out there. 97% or 98% is better, but good luck.
— Don't accept less than .8X magnification unless the coverage is higher than 95%. Never accept less than .7X in any case — and accept less than .75X only if you're getting 100% coverage in return.
— See if you can find out what the eye relief is, in millimeters. If it's less than about 15mm, pass.
— If you want to manually focus, make sure you can easily see what's in focus and what's not. Crank the focus back and forth — is it clear when an object is in focus and when it isn't? This isn't important if it's an AF camera and you expect to use AF all the time.
— Look at the brightness and the contrast of the viewfinder image. Do you like it? Is it comfortable to look at? Is it uncluttered, so you can concentrate on what you're looking at? Is the information provided likely to be helpful, or just distracting? (How anybody takes a decent picture with an EOS 3 is beyond me. I'd just sit there staring at the gee-whiz light show display in the finder all the time. As moronic features go, viewfinder features that directly obscure what you're photographing are right up there.)
— Do some quick comparisons. Ask to see a top model of camera, switch the lens you've been trying out to the better camera, and compare. Can you see more from edge to edge? Is the image a lot brighter, and yet also more contrasty? Use the Contax Aria as a standard for focusing snap, the Minolta Maxxum 7 as a standard for AF finder brightness and contrast, and older manual cameras such as the Pentax ME-Super or MX as comparison standards for high magnification. Remember to compare the cameras with lenses of the same focal length on them. And don't be dissuaded if the counterman or -woman doesn't know what you're talking about. Like consumers, some of them have never paid attention to viewfinders before. Persevere!
— Finally, don't be afraid to make a personal, subjective appraisal. Remember, the viewfinder is the single most important user interface on the camera. The user is you; the viewfinder is what you must use to connect your subject with your photograph. If you enjoy looking at the world through the camera, you'll very likely take more, and better, pictures. The viewfinder is important. You're justified in treating it as such.
The manufacturers would just as soon keep you in the dark about the differences between good and bad viewfinders. But you can't shoot well if you can't see what you're shooting, or if your camera is only giving you an approximate idea what you'll get on film. Know your viewfinder!
— Mike Johnston
Mike Johnston writes and publishes an independent quarterly ink-on-paper magazine called The 37th Frame for people who are really "into" photography. His book, The Empirical Photographer, is scheduled to be published in 2003.