Chuck Norcutt wrote:
>I have long believed the definition of "standard" focal length to be
>based on the diagonal of the film frame. Not knowing where I picked up
>this piece of wisdom I searched the net and quite to my surprise what
>should I find but the works of our own John Lind on the "Science of
>Photography" at: <http://johnlind.tripod.com/science/sciencelens.html>
>
>
Yes, I'm quite familiar with that idea, I'm sure it goes back a very
long ways. It doesn't make much real world difference, given the
variation in fls from nominal, but I personally feel that the diagonal
standard has failings when applied to formats with very different aspect
ratios. I don't know of anyone who uses their camera regularly for
diagonal shots, so the diagonal measure must be a compromise measure to
weight both axes. It seems to me that most shots, at least in the
formats we are talking about, are horizontal. Further, when shooting
vertical subjects, one turns the camera and uses the long axis AOV
anyway. Even 19 mm won't be any use for this behemouth unless I turn the
camera <http://galleries.moosemystic.net/Yosemite/pages/02043A06.htm>.
So, from a personal point of view, I think that the long axis is as good
as, if not better than, the diagonal length for determining "normal" fls.
>"The standard or "normal" focal length is one that approximates the
>perspective and, to a lesser extent the field of view (not including
>peripheral vision), of the human eye. The accepted standards spanning
>different sizes of film formats is a focal length approximately the same
>as the diagonal of the film plane, or a diagonal angle of view between
>50° and 55°. In small format 35mm the 24 X 36mm diagonal is about 43mm.
> Lenses with focal lengths between 40mm and 55mm are considered
>"normal" with 50mm overwhelmingly the standard length."
>
>
That's a pretty broad range. Yet there are those on the list for which a
"normal" is shorter or longer than that, so maybe the whole idea is
silly anyway except for marketing. One could argue that the "normal"
lens for a SLR in the last few years is a 28-90 mm zoom, based on what
was sold with them.
>Back in the 60's when Japanese fixed lens rangefinders were much more
>common than SLR's it's my recollection that your camera was much more
>likely to have a lens of about 42mm focal length and thus very close to
>the film diagonal.
>
I"m sure that was one factor. I suggest that the desire to keep the
length of the lens protrusion from relatively compact bodies short was
another significant factor. Standard lenses for Japanese interchangeable
lense RFs were, like others, 50 mm.
>I suspect that the addition of the mirror box forced
>lens designers to opt for a longer focal length as standard and some,
>such as Minolta, even went to 58mm as standard. I think the 50mm
>"standard" is only there to avoid the expense of having to use a
>retrofocus design on the lens that comes with every camera. That's just
>my speculation but I consider it a reasonable guess to account for what
>I think was a transition of what's considered "normal".
>
>
This strikes me as just plain wrong, unless there was some failing in
lens design technique very early on (yet they were making these lenses
for RFs...) and no one was smart enough to correct it soon. The famous
"pancake" lenses from several major makers are all in the close vicinity
of 40 mm. Maybe Topcon, with a 55mm register distance couldn't do 40mm,
but Nikon, Oly and others, at 46.5 mm or less, did. This is the minimum
fl for conventional, symetrical, non-retrofocus, lenses for most 35mm
SLRs. Take a look at the classic 6 element symetrical design of the
Zuiko 40/2
<http://olympus.dementia.org/eSIF/om-sif/lensgroup/40mmf2.htm>. As John
notes, 55-58mm standard lenses were fairly common in early 35 mm SLRs. I
think the Topcon Super D I used so long ago had a 58 mm lens too. I
think it just turned out that 50mm "worked" for most users of these
cameras. The 35 mm SLR is after all, a very different beast than its MF
and compact RF predecessors
>I suggested the 21/2 since the diagonal on the E-1's sensor is 21.8mm.
>
>
Huh! Ah, I see! I just took the measurements from the graphic in
dpreview review of the E-1 comparing the sensor size to others. Looking
more deeply, I see that they used raw, not effective, dimensions, so my
calculated diagonal and the horizontal dimension I used were a bit too big.
Even so, the 54.8° diagonal and 45.0° horizontal AOVs of 21 mm on 4/3
are still wider than the 46.8° & 39.6° of a 50 mm on 35 mm film. The 50
mm AOVs of 46.8° & 39.6° on 35 mm film are, in fact, much closer to the
48.8° & 39.9° of a 24 mm lens on a 4/3 sensor.
All a silly mathematical exercise, but I couldn't resist, as I already
had a spreadsheet with all the calcs in it.
Moose
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