What follows is my understanding, as a photographer and not a scientist. I've shot documentary, fashion, beauty, and portraits for 25 years, on everything from 8x10 down to a Minox. My experience with these formats reveals a different look associated with film/sensor size, at a given distance and working within "normal" apatures for teh format. My experience is that 8x10 has a particular look because at a normal portrait distance you are always closer to macro type magnification/depth, which manifests itself in fall off, while the in focus area is very, very sharp. So take a normal lens for each format (300 for 8x10, 150mm for 4x5, 80mm for 645, 50mm for 35mm), frame for a portrait, practical F stop for typical light, and the larger formats will always have faster fall off contrasted with very sharp in focus areas. I don't know if this is a concern for landscape guys because it seems landscapers desire everything to be sharp, but for people, the larger formats and the fast falloff of larger formats can create a striking photo.
I think the Canon 85 1.2 and the 135 2 can get close, but it is different than say 4x5 of 8x10, or even 645 with an 80 1.9. It is more than just a wide aperture. If anyone can chime in who (a) can see a difference in look between formats and (b) has some physics to explain, please be my guest!
I think both of us discussed this before in this forum and agreed in this point. I definitely see the difference, so (a) is confirmed.
I try (b): I see two main reasons for this:
1. The transition from sharp to unsharp depends on the maginification. In case of a portrait the head is biggest on 8x10 inch film. The smaller the format the smaller the head on the film/sensor. You are completely right that 8x10 is already in the macro world if you take a portrait, even with a full length shot you are at about 1:7 (25 cm on film, 175 cm the real person). With 35 mm you are at 1:50 (3.6 cm on film, 175 cm the real person). Consequently the dof in case of 8x10 is almost symmetrical around your focal plane. In case of 35 mm you have the 1/3 to 2/3 rule: dof is 1/3 in front of the focal plane 2/3 behind. The further the distance of the subject the less difference in magnification. Everything further away than 15 meter should look pretty identical on 8x10 and 35 mm if you have the same resolution. Everything between 1 and 10 meter should look pretty different. Even if you use corresponding f-stops you should see the effect - and I can see it. Corresponding f-stops are the f-stops which leed to "similar" dof, but the distribution of the dof in front and behind the focal plane is different (diagonal of 8x10 / diagonal of 35 mm is approximately f-stop 8x10 / f-stop 35 mm, numbers 300:43,3 = 5.6/0.8, that means if you use an f-stop of 5.6 with an 8x10 inch camera you would need 0.8 with 35 mm —> Noctilux).
2. The lenses for smaller formats are heavily corrected to get higher resolution. This leads to a different form of sharpness fall of. The rule of the thumb is: the higher the correction, the less pleasing the fall of. You can see this even in one format: Macro lenses usually have a slice of super-sharpness and they run out of sharpness quickly. Compare a 50 mm macro with a 50 mm standard lens on 35 mm. I compared the Rodenstock 360 Apo Ronar, the 360 Sinaron-N and the 360 Fujinon on 8x10 and I see the difference in a portrait - not a resolution chart.
Things like that where discussed over and over by the developers of the various lens designs in the 1920ies. You need old books about this stuff to find what you are looking for. Google for Paul Rudolf, he designed/co-designed the Protar, Planar, Tesar and Plasmat lenses, he wrote a lot about it – not about sharpness and resolution but how the render space and people. I'm not shure whether you will find english translations. It is always a question which lens errors you leave uncorrected and which you correct. The smaller the format the less options you have.
It is not easy for me to write this in english, because it is not my first language. But at least I tried.
If I want to take a photograph, I first think of the lens I want to use to translate this set in two dimensions, then the format and camera is dependent of the lens. Lenses I like for portraiture and environmental portraits are the 360ies I mentioned, the Sinaron-N 300 obviously 8x10 film, the Canons 50/1.2 and 85/1.2. I like the Planar 80/2.8 and the Sonnar 150/4 for Hasselblad V but not what they show in combination with a digital sensor.
I learned all this when I studied the images of my heros: Nadar, Atget, Sander, Sudek and Lartigue. Look at Avedon, Penn and Roversi - in my opinion they learned also from these masters. Now I try to forget about all the technology to be able to make at least one or two meaningful pictures in my life.
What was this tread about? Sorry, it just came over me.
Hope that helps,