understanding f-number

[bglick]

> Bill, losing only half a stop with a coated 18 element lens isn't bad. An uncoated triplet with only six glass-air interfaces loses roughly half a stop too. Evaluate .95^6.


Well, we all assume the ultra high transmissive value of coatings today would eliminate any throughput issue.... but I agree, with 36 surfaces elements, you start gangin losses on top of losses. This issue is rarely raised, so it can fly under the radar, till you get your film processed :-( Of course, manufacturers will NEVER provide this data, as many people buy fast lenses, because of the fast ss they need.... loosing a 1/2 stop in throughput is not a good way to sell an expensive, and often very heavy lenses.


I have never used / collected older LF lenses... I was going to ask in previous post.... I am curious about vintage LF lenses and the number of elements vs. transmissive losses. Do you have a feel for these losses through several eras of LF lenses? I understand that even the early early coatings of the 70's were much inferior vs. what we have today, but never knew to what extent. Maybe you can elaborate....

[Ole Tjugen]

The uncoated Dialytes and double-gausses are about the worst LF lenses where light loss is concerned. Eight surfaces means you lose almost a stop of image-forming light - although you "get it back" as shadow-lifting haze.

I have two 130mm f:4.5 Rodenstock Eurynars, one of which has been coated at a later date. The difference is stunning!

On the other hand I have a 150mm f:3.5 Xenar Typ D with natural bloom. The light transmission is visibly better than an uncoated Tessar of about the same age. So while the earlier (or even unintended) coatings really are inferior compared to modern coatings, you won't see any significant differences wiht LF lenses. Now with a 24-elements-in-18-groups superduperzoom lens, it's a different matter.

[Dan Fromm]

Bill, I haven't looked at Rodenstock's lens data sheets for a while, but Schneider's usually show transmission, and by wavelength too.

My experience with uncoated older lenses is limited to tessar types, triplets, dialytes and 6/4 double Gauss types. The emulsions I shoot are a little forgiving and it could be that my leaf shutters run a little slow. The upshot is that I've had no underexposure problems with any of my old lenses in or in front of shutter or, for that matter, any of my post-WWII coated ones.

I have had flare problems with one modern single coated lens, in the form of Ole's shadow-lifting haze, that could be controlled by a lens hood. 210/9 Konica Hexanon GRII. This one, not the 150/9 GRII and possibly not the others as well, is 6/6. More than sharp enough, but needs to be used with care.

Re Ole's comment on has 130/4.5 Eurynars, the VM says much the same about a pair of f/1.9 (6/4 double Gauss) Dallmeyer Super Sixes, one coated, the other not. But I don't recall flare or underexposure problems with the uncoated 50/2 Xenons (same design family) on my old Retinas. Could be that my memory is failing.

Cheers,

Dan

[bglick]

> Bill, I haven't looked at Rodenstock's lens data sheets for a while, but Schneider's usually show transmission, and by wavelength too.


I should have been more specific.... .I was referring to the super dooper zoom lenses with 36+ surface lenses for DSLR's.... LF lenses today, do not have enough surfaces for this to be a big issue. Modern coatings remove this issue for LF lenses...


Interesting about the Eurynars... a guess the saving grace is....back in the days before coatings, lens designs were not complex, limited number of elements, so while they may have ate light like a hungry rat....overall, not a deal-breaker. A stop is a lot, but I was wondering if some were even worse, but I guess not enough surfaces. Between the ultra slow film and slow lenses back in the day..... I guess we have it easy, vs. the pre 1960's era.