Correct me if I'm wrong, but after reading here at CR and elsewhere for quite a few months I sense that it comes down to this. High resolution via smaller pixels is a killer of high ISO capability. APS-C will indeed give one a reach advantage when it's not possible to get closer to fill the fame with the subject but there will always be the requirement of adequate lighting (thinking tele and birds here). One has only to imagine a sensor size that keeps shrinking far beyond APS-C and thus improving the reach to realize that there is no free lunch and IQ has to suffer. Higher gain in electronics always brings amplification of noise, that much I know for sure.
Still the lingering question in my mind is, is it possible that with very good lighting a 7D2 could give an IQ similar to say my 6D and provide me with the added reach I long for (and the action advantages)?? In that case I'd buy the imaginary 7D2 as the second camera I would pack around. Any thoughts on this?
IQ is more than just noise, gotta remember that. It is currently possible for some compact and mirrorless cameras that have SMALLER sensors than APS-C, and smaller pixels, to produce better IQ than your 6D under just decently good lighting. Those sensors tend to use much more advanced manufacturing processes, are usually BSI designs these days (so while pixel size is smaller, it isn't as much smaller as you might think), and the quantum efficiency of many of those sensors is over 60%. They also usually have relatively weak AA filters, which while you run the risk of encountering color moire, the sharper results help offset the increase in noise.
So yes, it is most certainly possible for the 7D II to produce excellent IQ, in decent light or better, that would rival your 6D. The 6D would always have less noise, but in the event that reach and detail is the more important factor, it doesn't really matter how much noise the 7D II has, so long as you capture that extra detail. (Sharp detail has the effect of greatly mitigating the perceived
impact of noise.) It is only when speaking purly in terms of noise, and especially in the context of identical framing, where FF sensors will always be better than crop sensors. Make sure you understand that: Identical Framing. You would either need a longer lens on FF, or you would need to be closer. Why does that allow the FF to be better? Two significant reasons:
1. You are putting more pixels on the subject, as FF sensors usually have more pixels than APS-C sensors.
2. You are putting larger pixels on the subject, as FF sensors usually have larger pixels than APS-C sensors.
Between those two things in concert, you can get SIGNIFICANTLY better IQ...you have less noisy pixels, and probably more of them (at least in the case of the current 7D...a 7D II might have more than the 6D.) Scaling a larger more detailed image down to the same size as a smaller image is going to reduce noise even more, and result in even sharper results. That is why a 1200mm f/8 lens on a 5D III will still produce better results than a 600mm f/4 lens on a 7D. You have 22.3 megapixels of larger, less noisy pixels covering the same relative area of your subject. The 5D III also has a slightly weaker AA filter, so the detail it resolves is sharper to boot, which has that perceptual impact to noise. Scale those 22.3mp down to 18.1mp, and the results just get that much better (as scaling averages noise, reducing it further.)
The 7D II is just as likely to come in at 24mp as at 18-20mp, so it is possible that it would have more pixels. That would be where your REAL reach advantage comes from. Depending on how strong the AA filter is, those 24mp might be crisp and sharp, somewhat susceptible to Moire, but maybe detailed enough to achieve parity with the 6D/5DIII. That's just in the interim, though. At some point, Canon is going to release higher resolution FF sensors. We might see a 46mp FF sensor in the near future not long after the 7D II. When that happens, FF will once again have both a total pixel count and a pixel size advantage. Even tough the pixels will be smaller than the 5D III or 6D, they will still be larger than the pixels of a 7D II. So while there may be periodic leapfrogging that temporarily brings APS-C bodies to parity with older FF bodies, that benefit will itself always be leapfrogged.
That's the nature of the game, though.
You just gotta evaluate your needs, weigh your choices, and make a decision. A camera body is generally only going to last a few years before your going to find a good enough reason to upgrade. My 7D will barely be three years old by the time I purchase my next body. My 450D didn't even last much more than two years before it was the most significant thing holding me back. This is why you will regularly hear photographers most ubiquitous quote: A body is ephemeral, glass lasts forever.
If you are truly serious about photography, buy the right glass for the things you photograph. I love birds and wildlife. I need reach, but I also want quality. The 7D is a good camera, but it's showing it's age. I live in an area where birds are extremely wary of people, so it's difficult to get close, even with skill. I intend to move to full frame at some point, so I'll need even longer lenses (400mm was the sweet spot on APS-C, and 600mm is the sweet spot on FF.) I am no pro (not yet, at least, I have aspirations!
), and my work is ok, but by no means great. I spent over ten grand this year on a 600mm f/4 L II lens. I used up a huge chunk of savings that, in hindsight, I could have used to help me through the time I'm going through now. But I don't regret the purchase. It was the best purchase I made. The investment account I had my money in has languished, not really moved anywhere, and it's not really expected to move. But I managed to purchase the lens for about $3000 off list price, and the current sale prices are still about $2500 over what I paid. If I amortize the cost of the lens over my lifetime, another 35-40 years, it actually costs me less than $1 per day!
The best value for your money is glass. It lasts for decades, and there is nothing that surpasses really good glass. You could work your way slowly towards the ultimate prize, something like the 500mm or 600mm lens, but your really just increasing your total long term cost. If you have the ability, save, save like you've never saved before, invest in big dividend payers or monetize your current photography or whatever you can to scrounge up the money you need to buy the lens that will really do your photography justice. A 500mm f/4 or 600mm f/4 lens will last you forever, and in that same eternity, you'll buy a dozen camera bodies, and each one will always be better than the last (even if they are mid or lower end, like the 6D). And the quality and reach of either of those lenses will be the biggest boon for your bird photography, hands down.