With the birth of the Mirrorless Camera Forum it seems appropriate to take stock of a topic that has, and will be, debated and considered for the foreseeable future. I will try to steer clear of answering “… who is the fairest of them all” and rather focus on how the two systems differ and how that may impact underwater photography in a practical manner. I hope others will point out errors, contribute things I forgot and add more practical comments and perhaps examples beyond the more theoretical nature of my start. There is one thing mirrorless cameras still lack and that is an established simple acronym. I’ll be using Compact System Camera (CSC), which highlights compactness as one important characteristic, especially for travelling divers.
A look in the mirror
The mirror and pentaprism combination serves two distinct functions, both of which require the optical image to be redirected from the sensor to; i) the viewfinder to compose the image and judge focus, and ii) to the phase-detection autofocus sensor. A separate sensor also analyses the deflected image to determine correct exposure and white balance. During the film era this was the only option but digital image sensors can perform all these tasks, giving rise to the first CSC in 2004 (Epson R-D1) but not taking off until the first micro four thirds (mFT) format camera in 2008 (Panasonic Lumix DMC-G1). Five years later all major camera brands make CSC and technical progress has been astounding. Comparing CSC vs DSLR is a moving target but this is my current impression.
Single-shot autofocus (S-AF): Contrast-based autofocus by CSC is now at least as fast as DSLR autofocus. CSC S-AF is also at least as accurate and extremely consistent for any lens (because it determines focus right on the sensor plane). DSLR S-AF can vary by lens unless properly micro-adjusted to account for small lens-to-sensor distance variability. Even if that is done there is still a bit more variability, but probably not something you would notice by eye. In addition, careful manual focus is as good as autofocus so it you like DIY focusing it is a moot point. Roger Cigala at lens rentals has posted actual comparisons that are worth reading for technophiles. http://www.lensrenta...e-shot-accuracy
Continuous autofocus (C-AF): C-AF has been a distinct weakness for CSC because contrast-based autofocus must use trial-and-error to determine the direction of focus adjustments, which made it too slow to track moving objects. In contrast, DSLR’s phase-detection AF can predict what focus adjustment is needed and directly drive the lens accordingly. The Nikon 1 CSC introduced on-sensor phase-detection pixels and is still a leader in fast C-AF. Just today Nikon set a new bar for any camera, CSC or DSLR, with the announcement of the Nikon 1 V3, which can shoot 20fps with C-AF. Recent CSC cameras from most brands now also incorporate on-sensor phase detection. My impression is that DSLR still has a small edge, especially with less ideal lighting or subjects, but the gap is closing fast.
Video/live-view autofocus: This is an area where DSLR has struggled and CSC is well ahead. Some DSLR now also have on-sensor phase-detection pixels and I expect that over time they will become more competitive with CSC.
Autofocus summary: CSC beats DSLR on S-AF, but not by enough to select one over the other. DSLR beats CSC on C-AF but the best CSC probably now match some of their DSLR competitors. Video autofocus goes to CSC.
Continuous shooting speed: without mechanical mirror, shooting frequency is limited by read-out and image processing speed on CSC. The Olympus OMD cameras reach 9 or 10fps and the announced Nikon 1 V3 reaches 20fps with C-AF, or 60fps without C-AF. For DSLR the rate at which the mirror can be flipped up and down with adequate precision and reasonable cost becomes limiting. CSC wins and has more potential to continue improving but for underwater photography the real limitation is often strobe recycling.
Image quality (IQ): The mirror is flipped up while the image is taken so it is really up to the sensor and lens to determine IQ. Exceptions are when lens offsets are not compensated by micro-adjustment then DSLR images tend to be slightly front or back focused. The lack of a mirror also means that the lens can get closer to the sensor but I don’t know if that affects IQ.
Image sensor size: The IQ issue in discussions around CSC vs DSLR really have more to do with sensor size, which is not a mirror vs no-mirror issue and with the Sony A7 there are now mirrorless full frame cameras. However, right now if you want FF, DSLR is much more mature with many more choices in bodies and, especially lenses. A larger sensor also means larger lenses so the compactness factor of CSC bodies is less of a benefit and until the mirror/pentaprism actually becomes a liability I expect that DSLR will continue to rule FF.
The main relevance of sensor size to CSC vs DSLR is that smaller sensors require smaller lenses with shorter focal length. That benefits compactness and results in closer minimum focus distance. The latter is a distinct advantage for underwater photography and one that typically does not get mentioned. However, a proper discussion of sensor size effects would be better dealt with in a separate thread. Maybe I'll start one later.
System size: The early selling point for CSC was smaller, lighter and cheaper cameras, but you had to give up on versatility, speed and IQ compared to DSLR. In 5 years, CSC has reached a point where in a battle with DSLR it wins a few and loses a few, but prices have evened out as well. What remains as a distinct advantage, is the more compact and lighter CSC bodies and lenses, which was an important factor for me and others on wetpixel. Both for travel, lower housing cost, and more dexterity under water.
Maturity: There is no question that DSLR is a much more mature and well-supported technology with specialty lenses, flashes, and many other accessories. This advantage is most notable in the FF arena because, Nikon and Canon especially, never enthusiastically pushed lens development optimized for APS-C, and because CSC has less of a compactness advantage when dealing with FF-capable lenses. In the big picture, the presence or absence of a mirror is really not that important. If you are already invested in a DSLR brand or you need special accessories or lenses that are not (yet) available for CSC then that is all that matters. Otherwise you need to think about what things you care about most in a camera, and perhaps prognosticate which systems are in it for the long run, and make your choice. The good news is you no longer need to trade size for performance. Modern CSC can give you both but you get what you pay for.
I won’t be in the market for a new camera in the next few years (assuming I don’t flood my camera, knock on wood) but it will be interesting to see the technology develop and all the stunning images that my fellow wetpixelers will make with them.
Happy shooting! Bart