On 5/5/2011 7:36 PM, manuel viet wrote:
>
> Foremost, Panasonic made the wise move to go for the low hanging fruit for
> their first lens under their own brand. I won't be the one to blame them for
> the choice of a focal length matching exactly the sensor diagonal. This focal
> length is the easiest to work on, and the most natural. The mythology of 50mm
> being the 'normal' lens for 24x36 film is simply the result of Barnack picking
> up something that was already available in Leitz catalog when he designed the
> Ur Leica. Like the 3:2 ratio, this was an engineering decision, not a
> photographer's request.
I've always been interested in this question of the definition of a
"normal lens" as being tied up somehow with a "natural" field of view.
I've previously seen that defined as the angle of view defined by the
eye's central and sharp field of view as opposed to our much wider but
unsharp peripheral field of view. But I couldn't recall an associated
angle for the eye and attempts to find it on the web led to extremely
wide ranges.
The exception was the Wiki article for "normal lens" which gives the
typical definition for the focal length of a normal lens as about equal
to the diagonal of the film. But it further goes on to relate that to
angle of view using a print made from from an image taken by that lens
thusly: "A lens with a focal length about equal to the diagonal size of
the film or sensor format is known as a normal lens; its angle of view
is similar to the angle subtended by a large-enough print viewed at a
typical viewing distance equal to the print diagonal; this angle of view
is about 53° diagonally."
By that definition of film diagonal (which we all know) the normal focal
length for 35mm is 43.3mm. Also, according to the same Wiki article,
Barnak wanted a wider lens for the Leica but chose the 50mm because lens
technology of the day was unable to produce a wider angle lens with
acceptable performance. It was certainly an engineering decision but
whether it's consistent with already being in the Leitz catalog I don't
know but suppose that it doesn't really matter.
Incidentally, by the diagonal definition, the normal lens for 4/3
cameras should be 21.8mm which is very close to the 2:1 ratio often
stated as the "crop factor" between 4/3 and 35mm.
>
> I'm not too much a chart admirer, but one should try the lens comparator at
> dpreview, it's quite telling :
>
> http://www.dpreview.com/lensreviews/widget/Fullscreen.ashx?reviews=62,59&fullscreen=true&av=5.333,5.333&fl=20,17&vis=VisualiserSharpnessMTF,VisualiserSharpnessMTF&stack=horizontal&lock=&config=/lensreviews/widget/LensReviewConfiguration.xml%3F4
Very interesting. It most certainly is telling, especially for an f/2
vs f/2.8 lens. It was also interesting to see that the Panny does its
best work at about f/3.2 while the Zuiko is best at f/4.5. I sometimes
find it difficult to throw off the old 35mm notions of lenses usually
performing best between f/5.6 and f/11. That's a range where 4/3 lenses
are already showing very deleterious effects of diffraction.
Interestingly enough, if you shoot at f/8 or smaller the Panny loses its
advantage as the diffraction limited performance of the two lenses is
very similar.
Chuck Norcutt
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