Leaded K9 vs Unleaded BAK4 prisms

[FredinBoise]

I understand that K9 is a Chinese designation for BAK7 glass with a 9% lead content.

Lead has always been considered good in prism glass.

With the current BAK4 stuff I understand they are lead free.



Question is which is better NOW; K9 leaded or BAK4 unleaded?

 

[chainsaw23]

To my knowledge,

BK9 is also so used for lenses, chandeliers, jewelry etc. Less dense, high clarity, but as a result less capable of withstanding wear. So not as well suited for a front lens or use in a dishwasher. Common name: Crystal. 9% lead.

BAK4 ( Barium Crown) with slightly higher refractive index than BK7 has better capabilities for total internal reflection of rays at steeper incident angles in short focal systems like binoculars. So it is better in avoiding a bit of light loss.

One can see if binoculars has BK7 or BAK4 prisms by looking at the exit-pupil. BAK4 pupil is round, BK7 has a bright square.

 

[FredinBoise]

In my research on spotting scopes ran into this little puzzle.
With lead free glass, The BaK4 (little a) is a Chinese designation for glass that is just slightly better at refracting than the BAK7. Both are behind the BAK4 (big A).
Then along came the K9. Chinese for the BAK7 but with 9% lead in the glass. I recall when lead free or ecco glass came into use. ALL the binos and spotting scopes took a hit on brightness. Don't know about resolution.
Several Spotting Scopes of Chinese origin are sporting K9 prisms.

My question is simply that with 9% lead in the PRISM Glass, is the Chinese copy of the BAK7 the equal or superior to the current non-leaded BAK4 (with the Big A)?