Last updated: 2nd May, 2014
Question, What is the fuss about the f2.0 lens of Canon G1X mark ii.
Last updated: 2nd May, 2014
Answer,
F2.0 vs F3.5
When you look at the spec, G1Xii comes with a lens that has max. aperture of F2.0, which is a great deal.
Generally digital compact(DC) cameras do not have lens that come close to this. You may find some DC specify F2.0 or even F1.7, unfortunately due to their smaller sized sensors their effective max. aperture is reduced to F3.5 to F5.6
What it actually mean is, G1Xii can use this advantage to throw many things out of focus, thus, producing perception of depth in a photo and also to isolate the background from your main object.
Let us leave the technical stuff and show you some photos.
The first photo you see is exactly what it meant. Everything is blur except the subject. (You can [Right-click] the photo and choose Open image in a New Tab, to view the full-size photo.) 
The above photo was taken at f2.0 with G1Xii in full AUTO mode. You may have noticed, the background is blur, and that makes the flowers, the main subject of the photo, stands out.
I can assure you that not many DC camera nor phone can come close to this performance.
Also notice how the background was rendered. Those out of focus effect is called bokeh. A good bokeh is usually smooth, as you can see G1Xii produces very pleasant Bokeh.
As I have mentioned earlier, not many DC camera comes with a f2.0 lens. A decent one may have a effectively f3.5 lens, but most come with something worse.
Let us have a look at how f3.5 defer from f2.0, the difference in numbers may be small, but the results are significantly different.
The above photo was taken with the same camera but at f3.5 in AV mode.
Which one of them is a better photo, is down to personal preference. However, it is good to have both to choose from.
G1xii with a f2.0 lens allow you to have both options, you can decide to have the first photo or the second. Many people may not know what they have missed with a Digital Compact camera that come with a max aperture of f3.5.
Sensor size, APS-C vs mirco four-third
Do remember that the actual depth of field or the degree of out-of-focus effect of DC also depended on it’s sensor size, given the same equivalent focal length. Generally the effective f number is a reference to a Full size camera with changeable lens. (SLR/ DSLR).
A full size DSLR is called Full frame DSLR, camera like Canon 5D, or Nikon D800 is a Full Frame DSLR. Their sensor size is referred to as 35mm or Fullframe.
For economic reasons and portability, manufacturers also produce SLR with smaller sensors. It is the APS size sensor, which is about 60% of the size of a Fullframe sensor. You can find this sensor in camera like Canon 7D, or Nikon D5100
Other camera manufacturer introduce other sensor size too. Like Olympus who once a successful camera maker, joint venture with Panasonic to introduce an even more portable format, in order to stay relevant to the camera market. This format is called Micro Four Third, or Micro 4/3. Example of camera using this sensor size is Panasonic GF5 and GF6
The following chart shows the sizes of sensors stacked together for comparison.
The chart also include the sensor size for a typical high-end Digital Compact camera, like Olympus XZ-2. Also a typical point-and-shoot Digit Compact camera like Panasonic Lumix ZS30, which constitute the largest number of digital camera in the market. And finally, the sensor size of an iPhone 4s is also included.
The same chart when shown in approximate size of the real thing.
As shown in the chart, the G1Xii comes with a sensor larger than micro 4/3 but smaller than an APS.
Without going into the technical explanation, it is generally accepted that, the larger sensor produces better image quality, due to the extra amount of light gathered by the sensor.
Other than that, sensor size also affect the effective aperture given the same f-number of the lens.
For example, the effect of a f2.0 lens on a micro 4/3 sensor is not the same as a f2.0 lens on a APS size sensor, given the same effective focal length.
in fact a f2.0 lens on a mirco 4/3 size sensor is equivalent to a f3.5 lens on a APS size sensor. That mean a f1.4 lens is needed on a mirco 4/3 size sensor to achieve the same result of a f2.0 lens on a APS size sensor.
Apparently, iPhone 4s has a f2.4 lens, but it is equivalent to f16 on a Full frame camera.
Confused?
Simply put, f2.0 lens on a G1Xii produce shallower depth of field than and f2.0 lens on a GF6. because of the larger sensor size. And it is definitely better than the f3.5 kit lens that come with the camera.
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