That was a long post!
I read it all and I think you are right.
In a simplistic view - then the light gathering capacity of the camera, if you use the sam F-number is proportional to the sensor area.
The light gathering capacity of the lens has nothing to do with sensor area. The intensity of the light at the sensor is the same for any respective sensor for a constant light source and the same f stop. If you mean that the physical size of the opening the light passes through at a given f stop is proportional to the sensor, you are correct because the actual FL of the lens for a given crop equivalent FOV will be proportionally different.
But - then we make two assumptions.
1. Both sensors use the same technology level.
Light gathering capacity has nothing to do with technology level. The the same lens gathers the same light regardless of the sensor. The only variables are focal length and aperture (which in this case means the size of the front element of the lens and should not be confused with the aperture control diaphragm inside the lens). The sensitivity of the respective sensors at a given ISO would also be the same. ISO 100 is ISO 100 regardless of sensor format.
2. Both systems are made for the same quality output.
This also has nothing to do with lens speed or light gathering. # of pixels, perceived IQ, or processing by the camera's ADC or processing engine have anything to do with light gathering which ends at the back end of the lens.
But - in practice this is not true. Therefore - the actual difference is probably somewhat smaller.
I think we're talking past each other. You're talking about two things Light Gathering capacity of the lens and IQ (or what the sensor and processing in the camera can do with a given level of light.
Some of the confusion is caused by the mental image of all the light that's "wasted" by falling outside the sensor area. This has to be disregarded as it's irrelevant to anything in a photographic sense.
You are correct that a smaller sensor will yield noisier images because the number of photons per photosite are fewer. As light levels decrease, there's a greater variation of output between adjacent photosites because they are working with less photons per photosite, and these differences are amplified by the processing in the camera to create higher ISO. . . but we all already knew that, and that's a different subject -- IQ.
As regards to resolution. I think that you are incorrect in implying that the smaller sensored cameras have lenses that are sharper because it's easier to design a sharper lens that's smaller. If lens mfgs were capable of making lenses with current materials with 5.5 times more resolution, I think they'd have done it or at least something close for 35mm. They'd do it, if nothing else, for bragging rights, regardless of price or practicality (look at the Sigma 250-500 f2.8 -- certainly a bragging rights lens).
Consider that with the same pixel density as the Q's sensor, an APS-C sized sensor would have 156MP. Consider that using the same lens (I picked the Canon EF 100mm f2.8 USM Macro for no particular reason) in Photozone's testing scored 2030.5 with an 8MP APS-C and 2416 with a 15MP APS-C in Line Widths per Picture Height at f5.6. From this it's clear that pixel density can have a positive effect on resolution. My experience with my 6MP DS and my 16MP K-5 with the same lenses has confirmed this, and the added sensor resolution also increases the ability to crop with acceptable resolution. I don't think that anyone would dispute either of these points.
On the other hand, there is a point where sensor resolution will equal lens resolution, and adding more sensor resolution will not improve image quality further, but it won't make it worse. The smaller sensor will have lower IQ, but considerably greater pixel density. These will play against each other, and I don't think anyone can accurately predict how the balance will tip.This really is highly dependent on the lens quality, the point at which lens resolution is topped out, and the user's perception of IQ. The mind boggles. . .
The ultimate variable is "acceptable". Each photographer has a bar that he/she's not willing to cross in this, and each individual's bar is placed at a different height. And the bar could be placed at different levels for the same photographer for different purposes and in different circumstances. As a birder, I know that my standards are higher than some, and considerably lower than others in my genre.
Will the IQ of the Q be acceptable for me? I don't know, but I'm willing to wait and see. Will the IQ of the Q with a K adapter and whatever lens I choose to slap on it be good enough? I also don't know, and this answer will partly be contingent on the answer to the first question.
I'll also mention "godfrog"'s point that the Q doesn't have a mechanical shutter in the camera, so any use of a K to Q adapter would have to rely on the electronic shutter in the Q. I really have no idea how this would effect IQ, but since they include a diaphragm shutter in the higher quality lenses, the assumption is that electronic shutters are inferior. How much and in what way, I don't know, but I do know that an electronic shutter would cause zero camera shake, and with an extreme ultra tele lens, camera shake matters, so there might be some tradeoff even here.
I've never shot at 1600+mm EQ, and don't really know what my standards might be for this. They will be considerably lower than what I expect to get from my K-5 and FA* 300/2.8 or FA* 300/4.5. All I do know is that theoretically, this can work, and if I get a Q, I'll be willing to try it.
When one considers what a 1100mm (1650mm EQ) f2.8 (we're straight with the f stop thing, right?) lens would cost for my K-5, the price of the Q and some time spent experimenting doesn't quite seem so steep.
Scott
Lets call it my fault.
