Jan Steinman wrote:
>> From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>>
>> Granted,
>> the 7-14 covers a wider angle at 7mm than the Canyon at 16mm but the
>> difference is only 5% (114 vs 108 degrees on the diagonal).
>>
I didn't really pay much attention to Chuck's original post on this
subject, unlikely as I am to buy either lens. Somehow, this post led me
to plug the numbers into the AOV spreadsheet I worked up some time ago.
My numbers differ a little bit. I don't know who is right. Mine are
simple trig. calculations in Excel.
On the diagonal, I get 114.5° vs. 107.0°, for a 7.0% difference. But
the diagonal measure seems meaningless to me, especially with different
aspect ratio sensors. Horizontally, it's 102.4° vs. 96.7°, for a 5.8%
difference.
HOWEVER, that's all theory, and practice is another story. I don't know
how contemporary testers deal with actual focal lengths. I suspect many
don't even check. In the old days of Modern Photo tests, their standard
was that ± 5% was within normal manufacturing tolerance.
When I was looking for a super wide for film, I did some comparisons of
lenses from tests:
Variance
Spec. AOV Meas. AOV Nominal mm AOV
Zuiko 18/3.5 100.5 18.76 98.1 18 4.2% -2.4%
Zuiko 21/2 91.7 20.15 94.1 21 -4.0% 2.6%
Tamron 17/3.5 103.7 17.75 101.3 17 4.4% -2.3%
Samyang 18-28/4-4.5 100.5 17.81 101.1 18 -1.1% 0.6%
Vivitar S1 19-35/3.5-4.5 97.4 19.1 97.1 19 0.5% -0.3%
As you can see, the particular samples of the Zuiko 18/3.5 and 21/2
lenses were actually only about 1.4 mm different in focal length. So the
nominal 9.6% difference in AOV was actually 4.3%. Had the variance been
in the opposite direction....
I am not proposing that these individual lenses are representative of
anything specific. To conclude, for example, that zooms are closer to
nominal fl, is not meaningful from single samples.
What may be concluded from the stated tolerance for testing by a
serious, scientific tester and the results is that there is considerable
variance in camera lens focal lengths between nominal and actual. Here
are the results for various Zuiko lenses in their tests. Notice that
none exceed Modern's idea of acceptable tolerance.
18/3.5 18 18.76 4.2%
21/2 21 20.15 -4.0%
40/2 40 41.33 3.3%
50/1.2 50 52.48 5.0%
50.1.4 50 50.94 1.9%
50/1.8 50 51.87 3.7%
90/2 90 90.44 0.5%
100/2 100 100.65 0.6%
180/2 180 180.57 0.3%
Once again, the temptation to conclude that longer fls are more
accurate, is statistically invalid here. Even the obvious supposition
that variance tends to be on the long side, is questionable. There just
isn't enough data. It is interesting to note that this 50/1.2 was really
a 50/1.3.
>
> Heh heh. Real Wide Freaks won't give up no steenkeen 5%!
>
Looks like you may easily do so - without even knowing it. A true, Mike
Hatam style, wide freak would buy a dozen, measure them all and keep the
widest one. Or buy even more to find the best combination of actual AOV
and sharpness. :-)
My conclusion? I seldom frame perfectly and often crop at least
slightly. Just pick a lens, use it and don't fret. My 17-35 on FF is
pretty darn wide (whatever its actual fl), WAY cheaper and lighter than
either of those above, PTLens corrects it beautifully - and I'm a happy
camper.
Moose
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