Christopher Hartman wrote:
What point are you trying to make? I'm having trouble following this.
Yes, I know. 
Christopher Hartman wrote:
If you're talking about volume, then yeah, this makes sense. But so what? How does that impact the image? Exposure is exposure and it doesn't change with the size of the sensor.
You're the only person talking about exposure, even though I've clearly stated that exposure isn't what I'm talking about.
Leggy Mountbatten wrote:
But yes, it does affect both. Depth of field is easy enough to understand. The "lost light" is that which is cropped away; it winds up on the sides of the mirror box, adding to veiling flare. The brightness of the lens is the same, but the total light gathering power of the system is reduced, due to the loss of imaging area.
Leggy Mountbatten wrote:
But it's not the same amount of light. I'm not talking about exposure settings, I'm talking about the total amount of light collected on the sensor.
The FF sensor is 2.25x as big as a Nikon DX format sensor. With the identical exposure settings, the FF sensor is going to collect 2.25x as much light.
Leggy Mountbatten wrote:
Again, I'm not talking about exposure. Why you keep bringing that up is beyond my comprehension.
Christopher Hartman wrote:
When you meter a scene, the metering device does not take into account whether you are digital, film, aspect ratio, film size, or sensor size. Shutter, plus aperture, plus sensitivity. I'm pretty sure that's it.
As far as exposure goes, you're right. But that's not what I'm talking about. You need to get that out of your head. You kinda sorta almost got on the right track for half a sentence here:
Christopher Hartman wrote:
If you're talking about volume, then yeah, this makes sense. But so what? How does that impact the image?
Now that's a very good question.
Ever wonder why a digital SLR has much higher image quality than, say, a camera phone? Is it the number of megapixels? The advanced autofocus? The manual controls?
Mostly, it's the size of the sensor. You know this, but think about it for a minute. Why does the size of the sensor matter? After all, my camera phone has an f/2.8 lens, too. And that tiny little sensor collects just as much light as my 5D Mk III, right?
But wait: the camera phone's lens is also very tiny. Sure, it's an f/2.8 lens, but the size of the opening is so small that it has diffraction even wide open. Whereas my 50mm lens at f/2.8 has an opening nearly 18mm in diameter. That aperture could swallow the entire "camera" part of my camera phone.
The answer, of course, is that the larger sensor collects considerably more light (volume, as you correctly described it) during each exposure. Even with identical exposure settings. The total amount of light collected is the single most important factor in image quality, which is why larger sensors and larger film formats have higher image quality, and why you have to use larger apertures with smaller formats to compensate for the difference.
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