The Life & Death Of The
After Leica's rangefinder popularized 35mm film in the 1920's, SLRs ability to see through the lens took it one step further in the 1960's. The 1990's saw the rise of the DSLR, but why did professional digital cameras embraced the SLR design and why is that no longer the case?...
First DSLRs - Nikon D1 (1999) & Canon D30 (2000)
Before the DSLR and even before film SLRs were retrofitted with digital components - Digital sensors started out in "compact" (mirrorless) form. These cameras were initially consumer toys, playful with deign and ergonomics, but crippled by a reliance on emerging technology. Small / low resolution sensors, slow / tiny memory, poor processing, slow autofocus and terrible screens. All this made the experience less than ideal and certainly nothing a professional photographer would want to use.
The DSLR could mask this by forcing the photographer to interact with mechanical components (like the shutter, mirror and prism), making these cameras a considerably more elegant tool and one that could continue using existing lenses. Apart from already being familiar to photographers it also made a much nicer shooting experience than digital technology alone could provide (at the time).
The optical viewfinder, mirror and shutter mechanism of the DSLR are a fascinating piece of engineering history, but it has also held them back. As technology has evolved its once greatest strength has become its greatest weakness. This design now restricts lens design, limits their focus systems, makes previewing depth of field difficult or even impossible. It limits their usefulness in dark conditions, with ND filters or simply doesn't work at all in UV or IR wavelengths (usually after conversion). Information overlaid on an optical viewfinders is extremely limited and using them to look towards the sun can be dangerous.
The exponential improvement of digital components over the last few decades has enabled mirrorless cameras to leapfrog over the venerable DSLR's functionality. Although mirrorless was initially marketed on simply being smaller & lighter, ultimately they're becoming more capable (reliable, versatile, faster an silent). To illustrate what I mean let's compare features from Nikon's latest flagship DSLR (D6 - 2020) to Sony's first flagship mirrorless camera (A9 - 2017):
Feature | Nikon D6 | Sony A9
Burst (max fps)
Buffer (# of RAW)
Shutter Lag* (ms)
Shutter Speed (max)
Startup Time (sec)
AF Coverage (%)
DxO score: DR
DxO score: ISO
NOTES: Better / Worse / Similar
* Shutter Lag: A prefocus shutter lag time to state how responsive the camera is in its natural mode.
** Silent Penalty: Whether the camera slows down or loses functionality during silent shooting.
*** Lens Calibration: Whether lenses suffer from front/back focus issues, requiring calibration.
Mirrorless technology and electronic viewfinders (EVFs) have allowed cameras to keep all the functionality from DSLRs while being smaller, but they also bring a host of additional features. Many of these things don't show up on a specification sheet, but can make a tangible benefit to your photography:
Live Exposure - No more guessing exposure. Makes shooting full manual much easier, faster and better
Silent functionality - the viewfinder & primary autofocus functions equally when shooting silent or video
No Chimping - reviewing images is now unnecessary - correct DoF and Exposure are shown in real-time
Augmented Info - the viewfinder can show: histogram, levels, eye-AF, focus peaking, clipping etc.
True Focus DoF - permanent DoF preview that shows the correct depth & unaffected by brightness
Focus Magnification - makes manual lenses or checking focus, very fast and accurate
No light leaking - viewfinder doesn't need to be covered during long exposures
Focus & Speed Unhindered in Live-View - due to no mirror moving to switch systems
Shooting Into The Sun - is no longer dangerous for your eyes
Image Review In Sunlight - when you struggle to see the rear screen
Crop Lenses - show just like full frame (zoomed in and 100% coverage)
Viewfinder Colour - showing the world with white balance adjusted colours (full spectrum game-changer)
See With Opaque Filters - strong ND, IR or UV pass can still show a normal exposure in the viewfinder
IBIS - DSLR viewfinders would bypass the sensor stabilisation effect, if any DSLRs had the feature
DSLRs fastest burst speeds are seemingly at their limit due to the mirror and shutter mechanisms. Speeds, like 14-16fps (for the latest Nikon and Canon DSLRs respectively) mean the mirror is only in position (for the AF to function) for a tiny fraction of a second. In many circumstances this is not enough time for focusing system to track subjects reliably, forcing these speeds to drop.
The focal-plane autofocus sensors of mirrorless cameras are more efficient and accurate than an SLR system (never needing focus calibration). They can be more responsive with no mirror to move out of the way and if you remove the shutter as well they can constantly see and track the subjects while being silent, giving them a huge speed and reliability advantage.
DSLRs try to incorporate both systems to desperately compete, but having two entirely different focus systems is expensive, increasingly unnecessary and downright confusing if you switch modes often. Even if you don't mind the extra bulk, complexity and cost of these two systems, DSLRs still lose their viewfinder when using the more advanced features. Their battery life can drop below that of a mirrorless camera despite being much bigger and usually don't have in body stabilization or articulating rear LCD screens.
Electronic shutters have a host of benefits (higher shutter speeds, reduce mechanical wear, silent shooting), but unfortunately rolling shutter has been an annoying limitation. This is due the time it takes the sensor to read and store all the pixel data (from top to bottom). This can be a confusing issue to understand, but without the physical shutter there is no easy way to stop traditional CMOS sensor photosites from gathering light before they get written to memory. Simply writing the data faster has not been an option due memory speed and heat issues, so they simply scan slowly, like a flatbed scanner.
For static subjects this is not an issues, but panning with moving subjects can distort objects that can ruin an image. Thus, with the electronic shutter, "rolling shutter" (or "scan speed") can be very important. Although it's something that's gradually getting better over time, many cameras are still too slow to shoot fast moving subjects without distortion. Here are some sensor scan speeds from a few popular cameras that I could find around the internet (for stills photography, NOT video). Colour coded to help illustrate severity.
NOTE: Take this data with a pinch of salt as most come from single independent sources (usually not from the manufacturer).
Rolling Shutter | Speed | ms
Fuji GFX 50s
Fuji GFX 100
Sony A7R IV
Sony A7R II
Canon EOS R
Sony A7 III
Canon EOS R6
Canon EOS R5
Sony A7S III
Several professional cameras (Canon 1Dx III / Nikon D6) boast high "electronic" burst speeds without mentioning rolling shutter performance, despite being built for action photography. If this speed were high enough not to cause issues you can be sure they would shout about it, thus if it's something you think could affect you I would advise avoiding these models for silent shooting.
Some people are more sensitive to this issue than others, but this will mostly depend on the subject you're shooting and what it's doing. Larger birds in flight will often not be an issue, smaller birds - more likely. Panning shots with geometric detail in the background is a common issue, but it will depend how fast you're moving, how long your focal length is and how much the subject fills the frame. High speed spinning things like propellers, fans etc. are the worst case so consider carefully how likely you are to photograph these things in silent or burst mode.
Sensor scan speeds can be faster in various video resolutions, due to reading a smaller area of the sensor or skipping lines. The opposite is true of the Sony A9 however, it is slower when shooting 4k video (not 1080p). Scan speeds can also be affected by the bit-depth; Cameras tend to drop to 12bit (rather than 14bit) in silent shooting modes, potentially affecting their dynamic range and noise of your images. This is likely due to silent mode being required for fast burst speeds. Sony cameras can still shoot 14bit in silent shooting (and/or compressed RAW), but only if set to single shot.
It's clear why high resolution sensors tend to be slower, but this is also true of larger sensors. The worst cases above are all medium format sensors. Likely related to thermal performance (in the same way that mobile SoC's are more efficient ). This makes the three best cases above interesting considering they're full frame, beating many smaller sensors (for a price). What's strange about these high-end Sony cameras is that they actually get faster the higher their pixel counts are, which is counter-intuitive.
Silent shooting reduces the strain of mechanical fatigue on a camera. The Sony A9 & A1 are almost at the point where the physical shutter isn't needed any more. The ultimate goal of this is a "global shutter", but we're not quite there yet. This will entirely remove the need for a physical shutter mechanism and any moving parts during shooting. Ultimately this will drastically reduce key points of failure in future mirrorless cameras.
UPDATE: The current rumours of a Canon R3 and Sony A9 mkIII are discussing the removal of the physical shutter.
Battery life is one of very few objective advantages for the DSLR. Although it's worth noting that this is mostly true when using the viewfinder. In live-view mode (using the rear screen), this advantage often can flip back to mirrorless cameras, despite their smaller batteries.
Like film cameras in the early 2000's, DSLRs will still function, be enjoyable to use and available on the second hand market for some time to come. However, it's increasingly clear that their commercial viability will soon be over. Mirrorless cameras simply offer more features in a more reliable package for less money as time goes on. Sure some people be disappointed that they can't go out and buy a brand new DSLR for a while, but eventually they will realise that they gained a lot more in the transition than they lost.
Some might assume that camera phones (and computational photography) will someday replace mirrorless cameras, the way they did with compact cameras. However, this is unlikely to happen given how large lenses have always been intrinsically linked to high image quality, light gathering and depth of field. Shorter flange distances from mirrorless cameras have allowed wider lenses to be a little shorter, but many modern optics have also gotten larger due to an increased demand in optical perfection.
Lens size is most likely a fundamental issue of physics, however there is hint that curved image sensors will soon be developed commercially. This will remove the need for so many corrective elements in the lens groups. Although this will shrink lenses further it won't bring them down to phone sizes and they will have some really difficult caveats to overcome before they are viable for different focal lengths. The short answer is don't hold your breath for pocket sized wildlife cameras, or shallow depth of field.