Sell the R5 and get an R7 and R? Crazy?

Oh yes, I apologize if some of this got lost in my write up. The depth of field and bokeh from the 300mm f/2.8 mk ii is extraordinary. I love it. But it doesn't have enough reach for birding in my opinion on the R5, even with the 1.4x TC mk III. I don't like the IQ with the 2x so I'm less keen to go that route. So...getting a 1.6x crop on the 300mm f/2.8 and keeping the DOF and out of focus backgrounds is the goal. That's why I hypothesize that the 300mm f/2.8 + R7 is a killer combo for birding, giving nice DoF/bokeh, subject differentiation, and reach (480mm equiv naked; 672mm with 1.4x TC)

Hmm, I still don't think you've figured this out right.

The R7 has 0.0032 mm pixels; the R5 has 0.0044 mm pixels--about 1.38 times larger. With a 1.4x TC on the R5, you will have VERY similar results as on the R7. You will have almost exactly the same number of pixels per duck either way. The R7 is not a step up in that respect.

I think you are being mislead by the 1.6 crop. It just means that you will have a slightly narrower field of view with the R7, compared to the R5 with 1.4x TC.

Now we don't know exactly what optical effect the 1.4x TC will have on bokeh, etc., but I'll bet it's pretty good. Someone did a resolution test recently, and there was very little image degradation with the lens he tested.

OK, two things here: first, pixel size and number of pixels are both relevant to image quality, noise, etc but they are also two separate things. You have referred to both here in your response. So let's just slow it down for a second. 1) yes, R7 and R5 have different pixel sizes. 2) the R7 will have more pixels on subject than the R5 (about double), albeit smaller pixels. Now, pixel size matters and we can discuss that in the context of image quality, noise, etc. 3) what you've laid out here in the 1.38 times is that the pixel area will be roughly the same with the R7 naked compared to the R5 with a teleconverter. That's interesting to make explicit with the calculation. However, what we've shown is that the R7 with no TC can offer similar pixel area to the R5 with a 1.4x teleconverter. But that isn't the whole picture is it? The R7 can take the 1.4x teleconverter as well. I would recommend compare.

I totally get the desire to be explicit with this stuff and walk through examples but I don't think the question was ever: does the R7 compare to the R5 with at 1.4x TC...this is interesting as you demonstrated, but I wonder if you might consider how the comparison looks when you add the 1.4x to the R7 as well. That is the apples to apples comparison as far as I'm concerned.

Finally, I think the easiest way to just demonstrate the difference between the R5 and R7 image is to use some of the controlled examples that folks have provided. For example, here are two examples of RAW files shot at 12800 on the R5 and R7 with no teleconverter provided by bird photographer Glenn Bartley. I've just opened these in Capture One and taken at screen shot with them both at 100%. The difference is clear in pixels on subject at 100%.

R5 vs R7; 100% crop. Glenn Bartley raw files provided on his website.

Another way to compare is to just bring the magnification up on the R5 file so it matches the field of view comparable with the R7 so I did that as well. To my eye, the R7 offers a better image despite its limitations and this is at ISO 12800. Consistent with your observation (I think), the R5 required 140% (not 160%) to match the field of view on the R7.. That said, they are remarkably similar images and I concede this. Certainly the R7 image doesn't blow the R5 one out of the water even when cropping more on the R5.

R5 vs R7; 100% crop for R7; 140% crop for R5. Glenn Bartley raw files provided on his website.

I was a little concerned about how well the R7 files would clean up. So I ran the ISO 25600 image from the R7 through DxO PureRAW2 and was satisfied enough with the result to consider it viable. Again, 100% crop and screenshot.

R7 100% crop ISO 25600 run through DxO PureRAW2. Glenn Bartley raw files provided on his website.