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[OM] Re: (OM) Solstice web-site news

Subject: [OM] Re: (OM) Solstice web-site news
From: Moose <olymoose@xxxxxxxxxxxxxxx>
Date: Wed, 12 Jan 2005 22:47:53 -0800
Fernando Gonzalez Gentile wrote:

> .............
>
>. All this, and my worries about the Zuiko f-stops too :^)
>[Hey list, did you all read my post of Jan 12 'till the end? - why so few
>answers? - it's a true ON TOPIC post !!]
>
Oh dear, I've tried to stay out of this one. I can't add much in the way 
of satisfactory answers, but perhaps I can add some clarity in terms - 
and confusion in details.

In optics, F-stop is, by definition, a purely mechanical number. it is 
the ratio of the opening of the front element of the lens divided by the 
focal length. so, for example, I just measured the diameter of the front 
element of a 50/1.8 that is not obscured by the mounting at somewhere 
just under 29 mm. It's a little hard to measure perfectly without 
banging on the lens coatings. Anyway, 50/29 = 1.74 and 50/28 = 1.78, so 
it is indeed an f1.8 lens. Now, if it had a special element that was 
semi-opaque and blocked half the light through it, it would still be an 
f1.8 lens. that's because the "F" in F-stop is short for focal ratio.

There is another lens characteristic called a T-stop, where the "T" 
stands for transmission. Because there is some loss of light in any 
lens, the T-stop is always less than the F-stop. In pre-coating days, 
there could be quite a large difference between f and t stops in a lens 
with more than 3 or 4 air to glass surfaces. With coatings, the 
difference went down and with MC technology, it is essentially zero for 
practical purposes in prime lenses and possibly even most zooms.

Now comes the tricky part. Manufacturers don't tell you what is really 
indicated by the "f-stop" markings on their lenses. The most obvious 
case is with mirror tele lenses where the secondary mirror/mount uses up 
the central part of the front element. So if they were marked in f-stops 
and used on manual cameras, underexposure would be common. But it isn't. 
So I took a little survey of some mirror lenses I happened to have lying 
around. What I found was that an f ratio calculated from the diameter of 
the FE was always lower (faster) than they were marked. When I 
calculated the area taken by the central spot, subtracted it from the 
total area and calculated the diameter of a circle with the smaller 
resulting area, and f-stop calculated from that virtual diameter and the 
focal length was close to the marked f-stop.

Now whaat about zooms? In many zoom designs, the full clear diameter of 
the front element isn't used at some of the focal lengths and a smaller 
one could be used at that focal length, but not at others. Also, even 
with MC coatings, some zooms have so many surfaces, that the difference 
between f and t stop may be significant. Also, many early WA lenses had 
disproportianatly large front elements. My old Vivitar 28/2.5 that I 
used for many years has a front element diameter of about 46.5 mm. Based 
on that, its f-stop should be 0.6, super fast. The Zuiko 28/2 is better, 
with a clear FE of about 33 mm*. So it is faster than the Viv and has a 
smaller FE, but the calculated FE for 28.2 is 14 mm.

So, blah, blah, blah, what is he talking about?? I'm just trying to 
point out that the whole business is pretty complicated when you get 
into it and trying to make sense of an explanation by a non-native 
English speaker, who was probably not an optical engineer, strikes me as 
not being worth the brain cells it would use up, so I just didn't try to 
read all that stuff.

Now, if you followed me this far, don't you feel the same way? There's 
another piece of your life you could have used for soemthing more fun, 
better for you, almost anything else, right?

So the bottom line is, many Zuikos exhibit a T-stop change, as measured 
by the light meter, smaller than indicated by the aperture ring 
markings. Useful to know, but not much so for me, as say 98% of my shots 
are taken on Auto and another percentage somewhere in the 90s are not 
taken at full aperture.

But there are fun alternatives! We could discuss whether Oly cheated. 
And once somebody with calibrated equipment determines whether the wide 
open of stopped down markings are more accurate, we can wonder whether 
everybody else cheated, or whatever it turns out to be, or if Oly 
managed the whole camera/lens downsizing that changed the industry based 
on smoke and mirrors.

Moose (Now looking on the floor for any stray, recoverable brain cells. :-)

* The 28/2 is famous for easily damaged front coatings and there is a 
smaller opening behind the FE than in front, so careful measurement 
isn't in the cards.



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