Hi Robert,
At 2000 August 7 - Monday 8:14, RobBurn@xxxxxxx <olympus@xxxxxxxxxxxxxxx>
spoke about *Re: [OM] What lens to carry (again)...* saying
> In a message dated 8/3/00 12:49:12 PM US Eastern Standard Time,
> jlind@xxxxxxxxxxx writes:
>
> << Considering lens speeds, there is often a misconception about f/3.5 versus
> f/4. They are only 1/3 stop apart, not the half stop many think it is.
> That's right, f/3.5 is only 1/3 stop faster. For me, f/3.5 versus f/4
> wouldn't be a consideration.
> >>
>
> I was always under the impression that the difference between aperture
> settings on the lens was one stop difference in light transmission. That is,
> the difference is measured by light transmission, not by numerical value. If
> so, how could there be a 1/2 or 1/3 difference in light transmission? At the
> same shutter speed, there should be twice as much light transmitted by the
> f3.5 aperture setting than by the f4. The consistency of the one stop rule is
> what makes possible the exposure correlation between aperture and shutter
> speeds. For example, f3.5 at 1/125 second would allow the same light
> transmission as f4 at 1/60; f8 at 1/250 second would allow the same light
> transmission as f11 at 1/125. In other words, to maintain exposure
> consistency, if light transmission is doubled then shutter speed is halved;
> if light transmission is halved then shutter speed is doubled.
>
> Have I just been confused for the past 50 years?
In a word, yes. But only at the biggest aperture. Lens makers stretch
beyond the normal sequence for just a little more light. That's why
you'll see f/6.3 or f/1.7, or f/3.5.. But the normal sequence is the
square root of the series 2 4 8 16 32 64 etc, i.e. 1.4 2 2.8 4 5.6 8
etc. Sometimes, they skip the proper number nearest the widest
aperture, so the 50/1.8 doesn't have an f/2. On the other hand, many
lenses transmit slightly less light than nominal at their widest
f/stop, so the 1.8 may transmit just double the light of 2.8.
Going further, these numbers are "nominal" and are more accurate for
depth of field than exposure. For example, mirror lenses usually
transmit 1 stop less light than their nominal rating because of the
obstruction in the middle of the lens. Uncoated and zoom lenses,
because of light lost reflected, transmit less light.
The movie business needs very accurate light measurement to minimise
light changes between shots and different lenses, and they use "T-
stops", which are measured for each lens, and are always higher than
the f/stops.
Here's the proper sequence with 1/3 stops, calculated by excel:
f/stop +1/3 +2/3
0.71 0.82 0.91
1.00 1.15 1.29
1.41 1.63 1.83
2.00 2.31 2.58
2.83 3.27 3.65
4.0 4.6 5.2
5.7 6.5 7.3
8.0 9.2 10.3
11.3 13.1 14.6
16.0 18.5 20.7
22.6 26.1 29.2
32.0 37.0 41.3
45.3 52.3 58.4
64.0 73.9 82.6
90.5 104.5 116.8
128.0 147.8 165.2
Tom T
------------
Tom Trottier <TomATrottier@xxxxxxxx> ICQ: 57647974
Abacurial Information Technology Consulting
400 Slater St. Suite 415, Ottawa ON Canada K1R 7S7
__o +1 613 291-1168 fax:594-5412 (877)247-8796
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