The old Pentax Asahi's are good.....there are plenty better older binos though. The yellowish coatings which you refer to as anti-glare coatings actually have the opposite effect. These coatings actually reflect light back at the eye while viewing, especially when bringing the binocular to the eyes to acquire a target. This is has the effect of closing down the pupil, which may enhance perceived sharpness or depth of field, but it can be uncomfortable, blinding even. For nightime viewing this is not a problem as there is little light to reflect. The rest of the build and design is good though.
Bk 7 prisms are virtually a non factor when using such as 7x50 or 8x40 binos in broad daylight. The dimmed area is not entering the pupil, but the stray light may reduce contrast some. Ive got an excellent pair of old porro 7x50's with Bk 7 prisms.
Are there examples on the market of evenly illuminated exit pupils resulting from the use of low cost crown glass (i.e., with porros)?
No. Bk 7 prisms will always produce a diamond pattern with light falloff. This cant be defeated with oversized prisms of Bk 7 glass.
The majority of binoculars more than 20 years old will have BAK 7 prisms. BAK 4 has only become cheap enough and plentiful enough in the last decade or so to filter down to the non-premium brand models.
This is incorrect also. Its the same now as it was then. Cheap binos used the cheaper Bk 7 and more expensive ones used the more expensive Bak4. Ive got B&L porros from the 50s that have Bak 4 prisms. And I know that Zeiss and B&L were using it before WW2.
Pyrex is also a borosilicate glass. It has properties similar to the Bk7 glass, it is very chemical and heat resistant. Bk7's low expansion under heat and resistance to chemicals is useful as the outer element in binocular and telescope achromat objectives. Indeed it is nearly universally used as the outer lens in achromats, though others are used as well as various ED and Flourite. BTW, Bk 7 glass is a low dispersion glass.....EDish. Pyrex is also used as the glass of choice for telescope mirrors, because of these properties, especially the low thermal expansion.
I agree with most here that coatings over the last 30 years have shown the most significant improvements, others being the use of ED and Calcium Flourite and Aspheric lenses. Come to think of it, I dont know of a binocular that has Flourite lenses, scopes and camera lenses do though.
BAK4 is a better glass type than Bk7 if, and only if, the product was designed around it. Optical engineering is not mix and match.
This is almost correct. Bak 4 glass is neither better or worse than Bk 7 only different with differing properties that can be used to good effect in a lens system. But Bak 4 is a much better choice for prisms, invariably. A bino with Bk7 prisms can be better overall than a bino with Bak 4 prisms, some older binos with Bk 7 prisms are better overall than newer binoculars with Bak 4 prisms and modern coatings, even on just an optical performance level.
I don't really consider changing chasis material from aluminum to magnesium much of an advancement, but more of a simple refinement.
Magnesium has been used as chasis material for binos since the WW2, although not universally. Aluminum was more predominant and some steel and brass. Mostly brass and steel were used pre WW2 with some aluminum.
