Tuesday, July 2, 2013

Launch Review: First Impressions of the Canon EOS 70D

Finally! A new APS-C camera and sensor from Canon. The old 18mp sensor that first appeared in the 7D has had a long, long, ride, but we've finally found out why Canon took so long coming out with something new. It's a little bit of the expected ("70D", didn't see that name coming) and a bit of the unexpected.

What's New

  • 20.2mp CMOS Sensor
  • "Dual Pixel CMOS AF technology"
  • 19pt AF System (All Cross Type) from the EOS 7D
  • 7fps, up to 16 frames buffered in RAW or 65 shots in JPEG
  • Touchscreen rear LCD
  • Built in Wifi

  • These stats firmly bring the 70D up to snuff with Nikon, even surpassing in a few key areas. Currently, Canon seems comfortable slotting behind Nikon in outright pixel count, presumably for better per-pixel noise characteristics. Whether or not it's better noise performance per image remains to be seen, but one thing is for sure: if the 70D can best the D7100 in the so called "banding" issue, then Canon will have scored some points. Even whenyou look at some of the none-core features, like the touchscreen LCD and the built in Wifi, you are getting more features with the 70D than you are with the D7100.

    What took so long? To be honest, even though the 18mp unit is now very long in the tooth, it was leading edge to begin with, and still mostly serviceable by the majority of users. Even though Canon is a much larger entity than Nikon, their in-house approach (which did them well in the early years of the DSLR era) currently leaves them at a disadvantage. In comparison, Nikon can draw upon multiple sources for sensors: in-house, Aptina, Sony and now Toshiba, and Nikon isn't shy about sourcing different sensors for different cameras within their lineup. This effectively multiplies Nikon's research efforts, allowing the smaller company to keep pace with Canon, the market leader.


    Though the official dimensions say that the 70D is smaller than the 60D, I think I would be hard pressed to tell the difference from across the room, except for the fact that the microphone has been moved to the top of the camera (as it also did with the D7100) and a few of the buttons have been re-arranged. Still missing is the joystick controller, which was ergonomically better than the current arrangement, but deleted in the transition from the 50D to the 60D.

    On paper, the 70D 19pt AF sounds like it lags the Nikon 39-point and 51-point systems, but all is not what it seems, as this is the same (or an evolved version) unit that is used in the EOS 7D, which is competitive with both the D7000 and D7100 in the AF department, performance-wise. With the 70D, all of the sensors are cross-type, whereas in the Nikon systems, the cross type sensors are clustered in the middle of the view-finder. And surprise of surprises, the 70D finally adds an on-demand viewfinder grid display... just like the one that Nikon's had for ages. Finally, one very welcome feature has made a return, and that is AF microadjustment, which was deleted in the 60D.

    New  Imaging Sensor

    Where the fun comes in is the new "dual pixel" technology, where every pixel is split into a left and right facing component. This technology was first seen in the Fuji EXR series of compact cameras, and is the same principle used with the Nikon 1 cameras and the Fujifilm X100s. Canon claims that 80% of their sensor is capable of phase detection autofocus, which would be a paper improvement over the Fuji system, where only a few tens of thousands of PD pixel pairs are spread out across the center portion of the sensor. In other words, with 20.2 million pixels and 80%, coverage, Canon is implying that their system incorporates approximately 16 million PD sensing pixels.

    Fujifilm On-Chip PD AD. Note masking of photosites and loss of light. (Fujifilm)

    One key point about this new system: It's not the same as the Fuji EXR method, which relies on pairs of pixels to achieve PD autofocusing; the new Canon system can focus with one pixel only. This is because the Fuji system relies on pairs of green pixels that have half of the light gathering area masked. Each pair in the Fuji EXR system can detect light rays entering the photo diode from the left and right, but at the cost of reducing the sensitivity by one-half. This is why only some of the pixels are PD, otherwise the impact on sensor performance would be too great. The new Canon 20.2mp sensor doesn't mask off the light under the micro lens, but instead, uses two photo diodes in the light gathering substrate underneath. If you think about it, this is a "hybrid" sensor, but in a different way than what we are used to. It basically combines a sensor with 20.2 million microlenses and color filter arrays, but mates that with a sensor bed that has 40.4 million photodiodes.

    One advantage of this system immediately shows up on the published spec sheet: the AF works down to apertures of f/11. Most conventionally PD AF arrays have been limited to f/5.6, with the central point being extended to f/8 with Nikon and Canon's recent updates. This is because the aperture restriction is not about brightness, but about the geometry of the light entering the autofocus array; if the effective aperture is too narrow, then a conventional array no longer reliably receives the "information" that it needs to calculate distance. 

    Conventional image sensor cross section. (Sony)

    However, there is a disadvantage to this system as well. Just because the light is not masked off at above the substrate does not mean that the sensor is not wasting light. With double the number of photodiodes, the amount of wiring interconnects in the sensor bed increases as well.  Looking at the diagram above, you can see that in a conventional image sensor, the wiring interconnects run above the light gathering substrate. This means that light is collected as a charge in the lowest part of the sensor, but is read off by the interconnects in the middle of the sensor. Doubling the number of photodiodes would also reduce the amount of the effective light gathering area, meaning that such a sensor would not have the same high ISO efficiency as a similar conventional 20.2mp sensor... unless this design is also backside illuminated (BSI) as well, in which case it would be a very interesting sensor indeed. Another way to mitigate the light loss is to move from the older aluminum fabs to the newer copper fabs. Because copper has a higher conductivity than aluminum, the thickness of the metal wiring within the chip can be shrunk, thereby opening up the path of the light into the photodiode. However, the published specs make no mention of the fine details of the new sensor, so as always, we're anxiously waiting for the objective testers to get on with their work.

    Also, its worth remembering that more is not always better. (Remember, with conventional PDAF, the 7D matches well against the D7100 despite having less AF points). Just because the number of discrete phase detecting elements numbers into the millions, does not mean that the 70D is necessarily better at focusing in live view than a Nikon 1 or Fujifilm X-E1. This would also depend on the development of the image processing that accompanies new AF hardware. Again, we are anxiously awaiting the objective tests. The proof, as they say will be in the pudding. The hybrid CMOS chip used on the EOS-M and the the EOS T5i (EOS 750D) was underwhelming, producing live view focus speeds no better than conventional contrast detect systems. Canon has since announced a firmware update, but it remains to be seen how much of an improvement it turns out to be.  

    Canon 70D vs Nikon D7100

    This will be the one that everybody will be asking; I know I'm asking this already. Since the Nikon D7100 has been out for a few months now, it's a known quantity: excellent for stills, files aren't as abusable as with the D7000, reliable autofocus but not as quick as the D300s. The 70D is a bit more of an unknown.... at least the body and handling will be familiar.

    Advantage Canon 70D

    • Touchscreen LCD that now swivels
    • Build in Wifi (Optional on the Nikon)
    • Traditional AF system from the 7D
    • Theoretically better live view focusing, but pretty much anything is better than Nikon's live view focusing
    • Probably a better movie mode, maybe even dramatically better if the live view autofocusing lives up to its promise.  Combine this with the flip-out LCD screen and you have a serious DSLR video device.
    • 7 fps burst rate compared to 6fps for the D7100 (Actually, not a real world difference)
    • Larger buffer (The D7100 can only hold 7-9 shots NEF depending on compression/bits)

    Advantage Nikon D7100

    • Tougher body with partial metal frame
    • Dual card slots
    • Has hard more time on the market and has already taken some bite out of 60D/78D users waiting for upgrades
    • Greater number of AF points means that you can be more precise with your AF point placement
    • 100% viewfinder coverage vs 98%. Hardly worth mentioning, actually

    Of course, there's one area where we can't say for certain who is better at the moment, and that is with image quality. For the most part, there likely won't be a difference, but the old 18mp sensor, though competent, was starting to show its age. But one thing is for certain, Canon versus Nikon, the age old (pointless?) argument just got interesting again.

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