Optical finder or EVF?

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As the year comes to a close, I thought I’d address one of the recurring topics amongst photographers: optical finder, or EVF? Both have their advantages and disadvantages, and oddly they seem to be converging towards the middle of late. That’s to say: optical finders on the whole are getting worse, and electronic finders are getting better. Where does this game end? Is there an ideal solution for you? Let’s take a look…

Not that long ago, the choice was clear cut: optical finders were superior in every way to staring at a small, dim, blurry TV screen at the end of a drinking straw. Making a good micro LCD panel was expensive, and even then they had limited resolution and gamut – not to mention limited dynamic range. Optical finders were generally large, bright, and of course showed a real time view of the world in both framing, and to some degree, focus. Technology has progressed and reversed: it’s now more expensive to build a good optical finder than a good LCD panel, and that’s before you consider the additional complexity that has to go into the system if the optical finder is to actually be useful.

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Of the three kinds of optical finder – the uncoupled framing device as seen in accessory hotshoe finders, like the one on my previous Leica M8 – is the simplest. It’s also the least useful, because it does not show the view through the lens and is therefore subject to parallax error that increases as focus distance reduces. On top of that, it does not show a representation of depth of field, nor does it show the focal plane, or even where the focus points lie. At worst, you get a hole. At best, there are some frame lines to indicate an approximate field of view; these are imprecise and usually much tighter than the actual field of view of the lens it is meant to represent in order to provide some margin of error. All that said: a good accessory finder is really a little lens in itself, and can be quite complex to design and manufacture; the advantage of this type of finder is size, interchangeability, and a very bright view (if designed well, like the modern Zeiss Ikon finders). They’re also often referred to as ‘brightlines’ for this reason. The only situation in which I’d prefer this kind of finder is if the only other option is no finder at all. Even a back panel LCD is more useful as you can always slip a magnifying hood over the top and use it as a much bigger EVF.

A good TTL optical finder isn’t easy to build, because not only do the optics have to be high resolution and distortion free (and preferably also chromatic aberration free, making it easier to determine critical focus) but the distances involved must be precisely calibrated. A TTL optical finder requires that the distance between the lens flange and the recording plane and the projection plane of the finder be identical across the entire field of view; if not, then both magnification and apparent focus will change. This negates one of the biggest advantages of optical finders: the ability to determine what is in focus and what isn’t. With modern digital cameras and ever smaller circles of confusion, the tolerances have to be even tighter. I’m sure we’ve all experienced plenty of occasions where something appears to be in focus in the finder but isn’t on the final image; this is usually the result of a mirror that is either at the wrong angle (only top or bottom out of focus), or a focusing screen that is at the wrong distance (whole image consistently out of focus by the same amount). On top of that, the focusing screen and mirror play a significant role, too: the mirror must be transparent enough to let sufficient light through for the AF system to work effectively*, and the focusing screen determines both brightness and ‘snap’.

*Which is one of the reasons modern DSLR finders are dimmer than old manual focus cameras: they had a fully reflective mirror instead of a partially transmissive one.

The coarseness and grind of the screen affects both: too fine and you won’t have much differentiation between in and out of focus, but with a brighter image; too coarse and the image will be very dim with slow lenses. And this is before we’ve begun to consider focusing aids such as split and micro prisms. Originally, focusing screens were made of ground glass – this has since changed in modern times to be either bundles of fibre optics cut transversely, or very fine fresnel lenses to increase brightness. Both were changes in a quest to compensate for ever slower consumer lenses: try mounting an f5.6 kit zoom on a 1960s SLR and you’ll find the view incredibly dim. Yet, switch to an f1.4 prime and the the vintage camera becomes pretty amazing; bright and very easy to focus; the modern DSLR won’t improve by anything as much as you’d imagine. In reality, most modern DSLR finder systems do not differentiate much below f2.8-4, which is also one of the reasons why fast lenses appear to have a greater degree of blur in the final image than the viewfinder. Note: the focusing screen does not affect autofocus performance, since light for this system is split off at the mirror beforehand.

However, there are still very good optical finder available: the one in my Hasselblad H5D and H6D, for instance. It’s huge, bright, and has both sufficient snap for determining critical focus manually (there’s a little shimmer visible in the finder caused by the frequency of the fresnel lens grid at the focal plane) and calibration is spot on: if it appears in focus in the finder, it is also so on the sensor plane. Eye relief is also excellent: it’s one of the few finders that’s both large and still completely visible, including info LCD, even when wearing spectacles and with the deep eyecup in place. Unfortunately, the replacement part by itself also costs more than most cameras. For an experience that won’t break the bank, but clue you in to just how a good finder should look – take a peek through any of the older SLR finders, or better yet, a 6×6 medium format camera with a magnifying hood: this is live view!

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But if there’s one thing that a TTL optical finder can’t do, it’s accurately simulate actual exposure. You see what the scene sees, you get some indication of what the camera’s meter thinks the exposure should be based on a set of predetermined parameters, and then it’s up to your individual experience to determine if you need to apply compensation or not. It’s nigh on impossible to determine precise exposure to the right simply by looking, even if you have some significant experience. On the other hand, the EVF can not only show you exactly what the sensor is seeing, but also any preview effects, for example to help visualize in monochrome. There’s also magnification and peaking for more precise focusing.

Most of the problems with early EVFs revolved around resolution, dynamic range, and refresh rate: insufficient resolution to determine critical focus or examine small elements in a composition, inability to determine overexposure or even see some highlight or shadow elements, and slow refresh leading to lag and timing errors. Arguably, the former was not a huge problem if the recorded resolution was also low, but has become a signifiant one today when you’re at best seeing 1.2MP (RGBx3, 3.6MP, Leica SL) of a recorded 24MP). Dynamic range and simulations (filters applied to the raw sensor video stream) have improved significantly, to the point that the camera may well see more than our naked eyes under some situations – very low light, for instance. Refresh rate has also improved to the point that we see less lag than mechancial-optical systems. Theoretically, it stands as low as about 6-7ms for the 120fps refresh models. Add in 1-2ms on the physical shutter button, and none for an electronic front curtain camera (no opening and closing required to start exposure) and you’ve got a camera that beats out the best of the mechanicals (on the order of 15-20ms for leaf shutters, mirror up SLRs and Leica Ms; a fast DSLR can be as low as 30ms including mirror response time and first curtain opening). For video, of course, EVFs rule supreme: almost none these things are a problem because output much more closely matches what is actually able to be displayed by the EVF.

The latest generation of EVF panels have only one problem left to overcome, and I’m not sure how this can be done: one of night vision. Under very low light situations, the current EVFs are like staring at little light bulbs – your shooting eye might be acclimatised for the finder, which can result in a difference between both eyes (and physical discomfort) or loss of night vision. I can still focus for astrophotography through a good finder; all I see in an EVF is a snowstorm of dots that I’m not sure are stars or just amp noise. You could dial down the brightness, but the panels only deliver full color and dynamic range gamut at a certain output level – usually close to maximum. Even if the signal is ‘turned down’ to reflect the actual scene to reduce apparent amp noise, it doesn’t change this property of the panels themselves. Perhaps OLED technology might be the way forward, where black pixels are truly not radiating any light and full gamut is available at lower luminance levels. Whilst they’re at it, we could use physically bigger panels, and higher magnification optics (you still need relay lenses for your eye to focus on a panel that’s physically often a centimetre or two at most from your eyeball). To some degree, this would drive cost up again: higher magnification means more resolution required to provide the illusion of continuity (and invisible pixels); you’d also need a larger panel and better optics for magnification and evenness.

I think you can see for now why my choice still remains a good optical finder; but it isn’t clear cut under every situation. On the balance, most of the time, I’ll still take the size and cost and lack of exposure simulation and focus difficulties of an optical finder. A good optical finder can overcome the last, and experience the former. On smaller formats, I prefer an EVF – both because getting exposure right is far more critical to capturing an optimal image, and because the physical sensor size is so small that an optical finder tends to be either very small, very dim, or both (brightness is somewhat proportional to the total amount of light captured, too: bigger formats simply capture more light, which if condensed into an output projected image of a fixed size will be much brighter than a smaller format ‘expanded’ to cover). Some time ago (probably around the original Olympus E-M5), I said that EVFs were ‘good enough’: that still holds, but it isn’t the same as good (or excellent). Large format, anybody? MT


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