No. You are still shooting 400/2.8. You simply chose to crop the image size in-camera instead of during post-processing.
I never said you weren't shooting with a 400/2.8. Respectfully, please read my post more carefully. I said the 400/2.8 will
produce images equivalent to shooting with a 600/4 when in APS-C crop mode for DOF, noise, and framing purposes.
If you're not familiar with equivalency, or need to brush up on it, please read this authoritative review of it that I linked to in my first post:
http://www.josephjamesphotography.com/equivalence/
Reducing the image area does not reduce the amount of light collected by each pixel, thus the SNR is not changed.
I never said reducing the image area reduces the amount of light collected
by each pixel. Respectfully, please read my post more carefully. I said it reduces the total amount of light collected [in the image as a whole]. I'm not interested in SNR per pixel. I don't look at individual pixels. I'm interested in SNR of the image as a whole, and in problematic areas like the shadows.
If that is how you define ‘image quality’ then just use a single super-large pixel. NIIRS is a much better definition.
I didn't define image quality. Respectfully, please read my post more carefully. My describing "the ultimate image quality goal" is not the same as providing a complete definition.
Cropping discards a portion of the image. The individual pixels don’t care.
I never said cropping affects the pixels remaining in the image. Respectfully, please read my post more carefully.
Individual pixels collect photons. An image is made of individual pixels. Cropping an image is discarding pixels. Therefore cropping an image is discarding photons.
When it comes to SNR, pixel surface area matters.
No one is talking about that. Pixel surface area matters because surface area matters. If surface area matters, imaging area matters. But this entire thread is about cropping on a high resolution, high performance body and how that affects lens purchasing decisions. How do you change the pixel surface area by cropping? The thrust of your post- focusing on single pixel performance and analysis- is not relevant to this thread. The engineering underpinning each pixel, and the dimensions of each pixel, are not a variable in a discussion about cropping.
The key variables, IMHO, are total signal and total noise remaining in the image after cropping.
Have you read Dr. Emil Martinec's (professor of physics & string theorist at University of Chicago) thorough exploration of
Noise, Dynamic Range, and Bit Depth in Digital SLRs that I also linked to in my original post?
If not, page 3, section "Big Pixels vs Small Pixels" may interest you. I'll give you a preview:
"
Bottom line: Among the important measures of image quality are signal-to-noise ratio of the capture process, and resolution. It was shown that for fixed sensor format,
the light collection efficiency per unit area is essentially independent of pixel size, over a huge range of pixel sizes from 2 microns to over 8 microns, and is therefore independent of the number of megapixels. Noise performance per unit area was seen to be only weakly dependent on pixel size. The S/N ratio
per unit area is much the same over a wide range of pixel sizes. There is an advantage to big pixels in low light (high ISO) applications, where read noise is an important detractor from image quality, and big pixels currently have lower read noise than aggregations of small pixels of equal area. For low ISO applications, the situation is reversed in current implementations -- if anything, smaller pixels perform somewhat better in terms of S/N ratio (while offering more resolution). A further exploration of these issues can be found on the
supplemental page.
Rather than having strong dependence on the pixel size, the noise performance instead depends quite strongly on sensor size -- bigger sensors yield higher quality images, by capturing more signal (photons)."
Edit: you may also want to read points #7 and #8 in the "Myths and Common Misunderstandings" section of the equivalence document linked to above. I'll link to point #7 here:
http://www.josephjamesphotography.com/equivalence/#7
