E náo, Bak-7 não é inferior a Bak-4. O problema de bak-7 pra binóculos é o amplo campo de visão que cria um halo escuro nas bordas da imagem. Pra telescópios o Bak-7 é tão bom ou melhor que o bak-4.
"BaK-4 is preferable to BK-7 as a prism glass only when the focal ratio of the telescope objective lens falls below about f/5. Binoculars are mostly around f/4 so BaK-4 is needed for their prisms to achieve total internal reflection at the edges of the fast f/4 light cone.
Virtually all spotting scopes have focal ratios above f/5 so there's no disadvantage in using BK-7. It achieves total internal reflection just as well as BaK-4. In some telescopes it's actually a slightly better choice because a prism made with BK-7 has a little less spherical overcorrection and chromatic aberration at blue/violet wavelengths than BaK-4."
"BaK4 (Barium crown) has no advantage over BK7 (Borosilicate) when used in in a situation where the incoming light beam angle is very narrow. However in wide field applications such as low power binoculars there are advantages - increasing the brightness of the edge of field while having no effect on the on-axis image. There would be no advantage to BaK4 over BK7 to any part of the image for instance in a Binoviewer because of the narrow beam angle wherein generally they are slow systems without wide fields.
The disadvantage of BaK4 is that it introduces more spherical aberration into the optical path than BK7. BaK4 also introduces more chromatic aberration than BK7."
Comments
Sim, é!
E náo, Bak-7 não é inferior a Bak-4. O problema de bak-7 pra binóculos é o amplo campo de visão que cria um halo escuro nas bordas da imagem. Pra telescópios o Bak-7 é tão bom ou melhor que o bak-4.
http://www.cosmobrain.com.br/cosmoforum/viewtopic....
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Aqui umas dicussões em foruns sobre Bak4 e Bak7
"BaK-4 is preferable to BK-7 as a prism glass only when the focal ratio of the telescope objective lens falls below about f/5. Binoculars are mostly around f/4 so BaK-4 is needed for their prisms to achieve total internal reflection at the edges of the fast f/4 light cone.
Virtually all spotting scopes have focal ratios above f/5 so there's no disadvantage in using BK-7. It achieves total internal reflection just as well as BaK-4. In some telescopes it's actually a slightly better choice because a prism made with BK-7 has a little less spherical overcorrection and chromatic aberration at blue/violet wavelengths than BaK-4."
http://www.birdforum.net/showthread.php?t=156753
"BaK4 (Barium crown) has no advantage over BK7 (Borosilicate) when used in in a situation where the incoming light beam angle is very narrow. However in wide field applications such as low power binoculars there are advantages - increasing the brightness of the edge of field while having no effect on the on-axis image. There would be no advantage to BaK4 over BK7 to any part of the image for instance in a Binoviewer because of the narrow beam angle wherein generally they are slow systems without wide fields.
The disadvantage of BaK4 is that it introduces more spherical aberration into the optical path than BK7. BaK4 also introduces more chromatic aberration than BK7."
http://www.cloudynights.com/ubbarchive/showflat.ph...
É o mesmo sim. O BAK-7 é pior e mais barato do que o BAK-4 que é melhor e mais caro (lógico).
Esta é uma especificação da composição dos blocos de vidro óptico de que são fabricados os prismas ou lentes.
Existem os BaK-4 e os BaK-7 . Existem diferenças de transparência e de índice de refração.
O vidro BaK-4 é melhor e mais caro que o BaK-7.
Dicas:
Ba = Bário
K = Potássio