In 35mm terms, its 6.1-30.5/1.8-2.8 lens is the equivalent of a 28-140/8.3-12.9 lens. Nothing too special.
Don't want to go off topic, but in 35mm terms it is still an f/1.8 - 2.8 lens. It might have an equivalent field of view as a 28-140mm lens. But the shutter speed is based on the f/1.8 to 2.8 aperture, which is pretty good. The equivalent background (or lack of) is what you'd expect from a 6.1 - 30.5mm lens (which is largely independent of sensor size). That is, you'd have similar background blur at 30.5mm at f/2.8 on G15 as you would with a 16-35mm zoomed to 30.5mm at f/2.8 on a 5Diii. And in dark shooting conditions, shooting macro, or when capturing action with the fastest possible shutter speed, the ability to have more in focus for a given aperture is sometimes just as important. (And besides, in post production it is easier to blur a background than add more detail in).
In a round about way, just trying to say that the aperture range of a lens should be viewed independently of sensor size. From there, most sensible photographers can use their own judgement as to whether a particular system/sensor size is suitable for their purpose. Need more background in focus, shoot m43 or the G15. Need more background blur, shoot medium format (or adjust in post production). Need it just right, shoot full frame.
Imagine this scenario - Canon releases an EF-S 400mm f/4 with image quality, build and price equivalent to the EF 400mm f/5.6. Even though some might say it is still a "f/5.6 equivalent", most people would instantly see the benefit of the faster aperture.
People have been misled by this insane nonsense for years.
The "35mm equivalent" is what is really important and nothing else.
From a physics perspective the "35mm equivalent" is capturing identical information. What really matters is the geometry of the light hitting the sensor:
Generally the technical difficulty of achieving a particular geometry is INDEPENDENT of sensor size, meaning it's equally difficult to create a 17-55mm f/2.8 IS lens as it is to create a 24-105mm f/4.0 IS.
The front element of a 1/1.7" sensor lens that is 8-30mm f/0.3 lens would be 11.4 inches, and so would the front element of a 35mm sensor that has a 28-140mm f/1.8 lens.
Going back to the 35mm equivalent discussion, consider this:
On 7D compared the the 5D Mark III
The sensor is 1.6 x 1.6 times smaller.
35mm equivalent aperture - Multiply by (1.6 x 1.6 / 2 ) (an f stop is a base 2 log, so we divide by 2 to multiply between base 10 and base 2 if you were wondering, this just converts the number systems, nothing else)
35mm equivalent focal length - Multiply by 1.6
35mm equivalent ISO or light sensitivity - Multiply by (1.6 x 1.6) (bet you haven't heard of that, but if you do the math the 7D's sensor amplifies the signal 1.6x1.6 times more at a given ISO than the 5D3)
The point is that people are often misled by manufacturers changing the geometry of a camera system, particularly putting in small sensors and then claiming otherwise impressive performance numbers which are incredibly misleading because you are measuring them on a different scale.
It's like saying:
I have a million dollars, and then failing to mention these are Zimbabwe dollars worth $20 not, American dollars.
Yes aperture ISO and focal length are fixed numbers, but so are monetary figures, and the most important thing even the most basic dealing of currency has is WHAT currency you're dealing with, and 99% of people require an "equivalent" frame of refference to understand foreign currency or need to do a conversion. Likewise with cameras, geometry (type of currency) is the most important thing when dealing with the performance of a camera system, and the first thing anyone needs to do is bring up a conversion to the local frame of reference, APS-C 35mm, whatever.
To respond to your post though, there is NO benefit to a 1 stop faster aperture on APS-C sensor vs full frame because they (more than) cancel each other out. You don't stop action any quicker at all, whatsoever, because remember the ISO is skewed too, so all you're doing is just turning up the ISO sensitivity in a roundabout way. You have been misled into thinking there is a benefit.
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