Hi Moose and all,
From: Moose <olymoose@xxxxxxxxx>
>From a strictly mathematical standpoint, I slightly disagree.
>
>f1.4 + 1/4 stop = 1.26
>f1.4 + 1/3 stop = 1.22
>f1.4 + 1/2 stop = 1.15
This is interesting. So, f/1.15 should be the half-stop between f/1 and
f/1.4. Since this is a geometric progression, the full-stop increment should
be the _square_ of the half-stop -- just like two-stops (2x the F-number, as
I learned at about 7 y.o.) is one stop _squared_, 1.4 x 1.4 = 2 (approx.)
But then, 1.15 x 1.15 = 1.32, too far from 1.4. Something must be wrong
there.
>Because I wasn't sure how to do partial stops using direct math, my long
>ago created spreadsheet calculates the reduction of lens area,
I don't know what formula is using your spreadsheet, but there's no need to
bother -- the math(s) is/are already done... the old ASA steps are actual
third-stop increments. Of course, these are directly related to shutter
speed values; in order to obtain aperture ratios (which must be squared to
be fully comparable, since as we all know exposure depends on the _area_)
just compute the square _root_ of the values (or their multiples)
The ASA values (divided by 100 for convenience, the FULL stops in brackets):
[1] - 1.25 - 1.6 - [2] - 2.5 - 3.2 - [4] - 5 - 6.4 - [8] - 10 - 12.5 - [16]
- 20 - 25 - [32] ...
And their square roots, which should represent the actual F numbers in
third-stops:
[1] - 1.118 - 1.265 - [1.414] - 1.58 - 1.79 - [2] - 2.24 - 2.53 - [2.82] -
3.16 - 3.54 - [4] - 4.47 - 5 - [5.66] - 6.32 - 7.07 - [8] ...
Which may (and must) be safely rounded to this sequence of usual values:
[1] - 1.1 - 1.25 - [1.4] - 1.6 - 1.8 - [2] - 2.2 - 2.5 - [2.8] - 3.2 - 3.5 -
[4] -4.5 - 5 - [5.6] - 6.3 - 7 - [8] ...
As stated before, the half-stop increments (1.1892...) are easily computed
from the square root of the full-stop increment, about the usual 1.4
(already rounded):
[1] - 1.2 - [1.4] - 1.7 - [2] - 2.4 - [2.8] - 3.4 - [4] - 4.8 - [5.6] - 6.7
- [8] ...
>None of this precision matters, considering normal sample variations in
>real lenses.
Absolutely right. Marked focal lengths and f-numbers are ALWAYS rounded to
nominal values. But I don't think it's variations between samples of the
same lens, just a design feature of each model -- Zeiss publish the actual
focal lengths of its lenses, besides the nominal values.
>I'm not sure what this might mean in context. I was only talking about
>the Zuiko 50/1.2 vs. 50/1.4.
Well, you're right in the first place, optical performance alone won't
justify the purchase of the 1.2 versus the 1.4. Wide open, the "advantage"
of the 1.2 turns into handicapped performance, sharing a similar technology
-- although neither of them is stellar at such apertures. And yes, the speed
difference is in any case very small in practical use.
>I am not convinced that what's best for astro is best for terrestrial.
Absolutely right. I've had many surprises in one way (Zeiss 85/1.4, great
terrestrial and so-so astronomical) and the other (Tamron 135/2.5, just OK
on earth but great on space)
>Yes, I understand that. What I was saying is that the latest 50/1.4
>design and the 50/1.2 design were made at about the same time
I may be wrong, but was the 50/1,2 available at the same time of the
(latest) 1.4s? I remember (early 90's) checking Oly price lists for 50s...
only 1.8 and 1.2 were available, not the 1.4. Probably there was some time
with no F1.2 lens available in the Oly range...
>So one would expect them to be more like each other than like the much
>earlier 55/1.2 and single and early multi coated 50/1.4s.
Could be. I had a 55/1.2 briefly, but I don't recall it to be similar in
"taste" to the early 50/1.4 -- which I still own and has some nice
character. And the 50/1.2 was probably designed from the scratch, whereas
the latest 1.4 could be the result of a series of improvements on a much
older formulation. I could be wrong, though.
Cheers,
--
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|