Canon EOS DSLRs and the virtues and drawbacks of Mirrorless
The DSLR lets you preview your image with real photons from your scene!
I'm primarily a Canon man, perhaps largely for historical reasons. I always think Canon DSLR bodies feel 'right'. However, I recently looked through the viewfinders of two Nikons, a D3 and a Z6. They're DSLR and mirrorless cameras respectively, the D3 having long since progressed so we are on to the new 2020 D6. I switched on, looked at a subject, and half-pressed the shutter button for autofocus. They are both really nice. For this photographer though, one camera shooting experience conveys more precision and class, and lends more sense of occasion. That would be the D3. Admittedly, this was a top-notch optical system and not all DSLRs are this good. With top-end Nikon optical viewfinders, I would expect only minor (if any) improvement since the D3's launch in 2007.
I'm an enthusiastic user of Canon EOS DSLRs. The 'SLR' part of the mnemonic is hardly new, and stands for 'Single Lens Reflex'. The concept has some real advantages. It signifies that one lens is used both to generate an image for the photographic recording medium, and also to allow the eye to register what the sensor will 'see'. Generally the lens itself is interchangeable. SLR is therefore a method of allowing composition of shots. The recording medium itself was once a chemically-treated film. Now it is usually an electronic sensor array. The later gives rise to the 'D-for-digital' appendage. The first commercial DSLRs appeared in the 1990's but resolution did not compete with 35mm film until the early 2000's. Back in the 35mm film era I used Olympus and Canon film SLRs. I now use full-frame EOS 5D and cropped 7D series cameras, with the smaller frame 7D used mostly for macro and telephoto. Canon are also making pronounced moves toward the R Series mirrorless cameras and associated RF lenses. The 5D- and 1DX- series remain extremely popular with professional photographers, most of whom still seem to prefer their traditional optical finders. Wildlife, news and sport photographers in particular like to have their photons presented first-hand when deciding the correct composition and moment. Canon remain part-committed to the DSLR and EF/EF-S mount at the moment. Their newly-released (in 2020) EOS 1DX III flagship retains the format while incorporating many mirrorless-type features in its live view mode.
The DSLR, like the film SLR before it, includes a movable mirror to divert the light from the lens into an optical prism assembly pending shutter release. The mirror moves out of the way while the exposure is taken and returns again. This is the arrangement which allows the eye to view the framed scene utilising direct light transmission via the main lens. Mirrorless cameras instead convey the light from the lens directly to the sensor during composition. The electronic signal from the main sensor is fed to a miniature electronic screen display providing the Electronic Viewfinder (EVF) image. To me these never convincingly emulate the optical displays of DSLRs. Clarity, contrast and resolution are always inferior, generally combined with perceptible lag. Other unnatural effects can often be seen when panning. Some of the immediacy of DSLR photography is removed.
Subject Tracking with EVF cameras- a potentially significant concern for mirrorless
There are two delay issues with precise stills action photography.
The first is viewfinder lag. This is essentially absent with the optical viewfinder of a DSLR. With a mirrorless camera finder, it is significant. With Electronic Viewfinders the delay in relaying reality, while only of the order of a few 10's of milliseconds with the latest cameras like the R5, does represent an appreciable error in tracking a rapidly moving subject. When the shutter is released, it is being exposed to reality as it has become some milliseconds later, rather than to exactly what the viewfinder gave you in the DSLR scenario. The optical viewfinder works at the speed of light, and the delay will be insignificant, measured in nanoseconds or microseconds. You will still be tracking reality essentially in real time when the shutter goes off, and while there will be a delay with the shutter before the exposure happens, you need not stop your accurate targeting while the shot is taken. Admittedly, there will be shutter blackout in the viewfinder, but provided you 'follow through' the target, you're likely to get good framing. You won't be doing any better than that for tracking. With mirrorless, you aren't tracking in real time, by a small but sometimes very significant amount.
The second issue with fast, capture-the-instant photography, is shutter lag. This is the time between pressing the shutter and the sensor being exposed. With a DSLR, pressing the button moves the mirror out of the way followed by the first shutter curtain. This takes around 50mS, somewhat less with a 1DX series. With mirrorless, the main sensor is feeding the EVF, and when you press the shutter button, the shutter has to be positioned to blank the sensor before it can be released to expose the shot. This seems to take slightly longer, with a figure of about 80ms quoted for the R5. In both cases I've assumed that focus and exposure are already mostly correct, as is likely the case when tracking a subject. Again, a pro film SLR is the fastest of the lot, since there is no requirement to prime the electronic sensor. Total shutter lag in the single-figure millisecond range is entirely possible.
Viewfinder Lag vs. Shutter Lag
Shutter lag is not dissimilar between the two systems, with optical viewfinder systems probably still holding a small advantage. However, in terms of framing, shutter lag is not the main issue. This is because a slight delay in the instant of exposure may not matter provided the subject is being tracked accurately throughout. With a DSLR, that is perfectly possible. But with mirrorless? When the shutter releases with a mirrorless camera, it will accord with where you thought the subject was, but actually it isn't. Your eye is basing your framing on the lagged EVF view. This is most significant with subjects moving fast tangentially to the line of sight, and compounded by long lenses and the small angular field of view they bring. A 500 or 600mm lens gives a field of view of about 3 degrees. If your line of sight to the subject is turning 15 degrees per second, as is entirely possible with a bird in flight or a close aircraft, for example, your entire frame is traversed every 200 milliseconds. A 50 millisecond lag EVF is presently a fast EVF, but it will give you a framing error of a quarter of a frame. If the subject shifts 30 degrees a second, it may move out of the field entirely because of EVF lag. Use APS-C or smaller sensors and things will get even worse, lag-for-lag, focal length for focal length. If you intend to photograph these sorts of targets-in-motion, get yourself a DSLR with a good viewfinder-mode autofocus system. This situation may not improve substantially very soon, for mirrorless. Why? Faster sensor-display systems for EVFs are going to have even higher power consumption and mirrorless battery life is another problem area. This will be further compounded as attempts are made to improve resolution. Power consumption of digital electronic systems is proportional to clocking speeds and higher EV resolution means faster digital processing and yet more power, all other things being equal. For some applications at least, DSLRs may be with us for a while.
I'll admit I've only had limited experience with mirrorless cameras, but even the latest EVFs also seem artificial and lacking in impact compared with a DSLR viewfinder. Additionally, I like the surprise element of later evaluating how the camera sensor managed to reproduce what my eyes saw in the optical viewfinder. It's a bit like waiting to get your film developed. With mirrorless, what you see is pretty much what you get, as far as exposure is concerned. That can be seen as a good thing.
Mirrorless cameras have other compensating advantages. They are lighter, for one thing. The large mirror box and viewfinder prism are not needed. Sometimes this trimming-down is not a great advantage, because mirrorless camera lenses are necessarily much the same size and weight as the DSLR equivalents, and the same flashguns are used. Most present mirrorless cameras allow use of legacy lenses via an adaptor, further reducing the weight and size advantage.
Autofocus System Differences
Mirrorless cameras add the EVF finder but delete the autofocus sensor, the main and autofocus mirrors, the pentaprism and some other optical viewfinder components, and also the exposure metering sensor. Mirrorless achieves not just viewfinder feed, but also metering and autofocus, with the main image sensor. The DSLR, like the automatic film SLR immediately before it, has separate systems for all four aspects of operation.
The Canon full-frame mirrorless cameras use dual pixels at each main pixel site to achieve focus. Newer DSLRs like the 5D4 and 90D use this method in live view (rear screen framing) mode. They also allow focusing to be achieved via sophisticated algorithmic analysis of the main sensor image. More complex methods can be used in an attempt to discern the intended focus point or points. Reliable and precise automatic focus on the eyes of a subject is possible, for example. And because what is seen in the finder is essentially the electronic image being recorded, one can see any problems like dynamic range overload straight away. Any camera has limited ability to handle different light levels within a particular scene, and if this is causing a problem, the electronic viewfinder allows you to see it before pressing the shutter.
Autofocus errors are a risk with DSLRs unless everything is set up perfectly, because the light path for the autofocus sensor is different from the image sensor light path. The autofocus sensor is in the base of the camera and light is directed there using a secondary mirror behind the main mirror. Mirrorless cameras autofocus directly using the image sensor, and are therefore considerably simpler optically-speaking. There are no alignment issues between the main imaging path and the autofocus and exposure metering paths. Top tier DSLRs allow micro-adjustment of the focussing system to optimise individual attached lenses. It may be necessary to perform this adjustment to get the very best results. The newer EOS 1,5 and 7 D -series cameras and some others allow your lenses to be individually registered by model and even serial number to camera-stored autofocus correction factors.
Perhaps I'll change my mind, but at the moment I prefer the overall DSLR experience. For fast motion photography, the issue is less subjective and is rationally in favour of the DSLR for accurate framing. And the very latest, albeit very expensive, Canon DSLR, the 1DX III, looks likely to overcome most of the autofocus intelligence limitations of the format. (I haven't tried this camera at this point in time). To describe why in detail would take another article and some detailed input from Canon design engineers. Suffice to say for starters is that another complication with DSLRs mentioned above is the inclusion of a separate exposure metering sensor, normally located near the viewfinder prism at the top of the camera. This is another DSLR sub-system which is integrated into the camera in a way which continually prepares for the shot. If this sensor has adequate resolution, it can also be used to help automatically select the autofocus point in the same general way as is done with mirrorless systems, by firmware analysis of the image. In some of the newest DSLRs the metering sensor includes infrared sensitivity to help find people and animals in the scene by body temperature. The 1DX III uses two small full sensor arrays (of smaller size and lower pixel count than the main imaging sensor), one each for exposure metering and autofocus. Since each of these sensors is receiving a version of the main lens image before the shutter is pressed, they can be analysed continually for autofocus purposes. This can be performed in a hybrid manner. A subset of the autofocus sensor pixels is used to perform the traditional, very fast, SLR phase-detect focus method. At the same time, image analysis firmware similar to that used in a mirrorless system is performed, potentially using information from both sensors. The metering sensor obviously also performs its primary task concurrently. The EOS 1DX III uses a separate, powerful processor to rapidly co-ordinate these tasks iteratively and optimise autofocus and exposure. The algorithms can be tuned for various scenarios, using options set by the user. Settings are available for erratic subject movement, subject likely to leave and return to the frame, etc. Of course, there's a full auto mode which will get things right most of the time. The most capable DSLRs, epitomised by the 1 DX III, are very complex beasts, and correspondingly rather expensive. The aim is to leverage all the advantages of a large sensor, interchangeable lens system with rapid and simple operation. Seize the moment and get a good quality image. The 1D series cameras have a trustworthy reputation and track record with this.