Wed Jun 28 09:22:07 EST 2006 graywolf wrote: > Well, it is not only that. > > Primes simply transmit more light. Each piece of glass in a lens absorbs > some light, as do each air/glass interface (although air/glass losses > are reduced by modern multi-coatings) So a Prime (f/2.8) with 4 elements > and 6 air/glass surfaces will transmit more light than the same f-stop > zoom with 10 elements and maybe 13 air/glass surfaces. The f-stops are > the same the T-stops (transmission-stops) are far different. The zoom is > likely to lose a whole stop compared to the prime.
This indeed makes sense. It is not universal, as there are primes with high number of elements, and zooms with a comparable number of elements. (BTW, you don't see only 4 elements in primes that often. I am looking at P-SMC lenses. It is more often 5-7 for "normal" lenses and higher for ultra-wide-angle and tele. In the ultrawide area, I'd say that the number of elements is even comparable.) Nevertheless, what you said it is true for many cases. What I am curious is that how much the difference is. I don't know what is the per surface lost for the MC (SMC) lenses, and I don't know how many of the elements are coated (only the front ones or the inner ones as well?). I am doubting that the difference is indeed as much as an equivalence of a full f-stop (t-stop). Igor PS. By the way, I am still puzzled how your second posting (above) is different from the previous one (below)? In my view, both are talking about the same. So, I am confused by "It is not only that". :-) Also, I might be missing something, but I don't see how "aspherical elements, and extra-high-dispersion glass" play any role in increasing transmission. If anything, extra-high-dispersion glass would increase the reflection from the surface/interface, as the reflection from the interface dependence on how different the optical densities (refraction index) of the two media are (glass-air in this case). In my view, high-dispersion glass should have higher refraction index. However, high-dispersion glass can decrease the total thickness of the glass, hence possibly reducing the overall absorption, but I am not sure how significant that is, especially taking into account that high-dispersion glass might also have higher absorption. Sorry, if I somehow missed your point here. Wed Jun 28 09:32:59 EST 2006 graywolf wrote: > Tell you what, Bill, take out that zoom and a prime of the same f-stop. > Use an external meter to set the exposure for both of them. Examine the > unprocessed images closely, then get back to us with the results. > Multi-coatings help, but I do not think they completely eliminate the > problem. It is the fact that metering is now done mostly BTL (behind the > lens) that has made this pretty much unnoticeable. Current zooms with > their multi-coatings, aspherical elements, and extra-high-dispersion > glass minimize the effect, but once again I do not thing they completely > eliminate it. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
