400mm * 2x (tc) * 1.55 = 1240mm @ 10MP or 1736mm @ 5MP
400mm * 2x (tc) * 2.47 = 1976mm @ 10MP or 2766mm @ 5MP
I don't understand the point of this.
Why would you care about comparing images that have the same pixel count, instead of images that were shot with the same effective focal length?
Here, you are comparing an image with an effective focal length of 1240mm to one shot with an effective focal length of 1976mm. What is the point?
I'm here. My subject is (say) 100 feet away. I'm going to shoot with the same lens focal length, regardless of whether it's an a9 or a7rIV. Suppose that the result leaves me wanting to crop the image by 1.5X, to get the composition I want in the final product. I'm going to want to do that 1.5X crop, whether the original image is 24MP or 61MP. Are you saying that, if, for example, I shoot the image with an aperture of f/11, the result (from the 61MP source) is going to have worse image quality if the source is 61MP vs 24MP (given a 1.5X crop from either source)?
Are you saying that, if you were shooting an a9, you would get closer to the subject, in order to compensate for not having as large of a max effective focal length? I.e. you can't crop it as much, so you get closer to make up for that. If you can get closer, why wouldn't you also get closer while using an a7rIV?
I am pretty much of a beginner hobbyist photographer, so I apologize if any of this comes across as sounding really ignorant. It just does not make sense to me that you'd compare two different FF cameras by comparing the image quality after cropping to the same number of MP. It seems like you'd want to compare after cropping to the same effective focal length. I.e. so the final images are the SAME - not an image of a whole bird in one (i.e. 1.5X crop) and an image of just the bird's body in the other (i.e. 2.5X crop).
A friend referred me to this thread because I am also in the process of deciding between an a7r IV and an a9 II.