Re: Nikon D40 with 300mm lens AND teleconverter (Nikkor AF-S Teleconverter TC-20E II)


"Chris Malcolm" <cam@xxxxxxxxxxxxxxxxx> wrote in message
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Neil Harrington <neil@xxxxxxxxxxxxxxx> wrote:

"Wilba" <wilba@xxxxxxxxxxxxxxxxxxx> wrote in message
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Neil Harrington wrote:
Wilba wrote:
Neil Harrington wrote:
Wilba wrote:
David J Taylor wrote:
greg@xxxxxxxxxxx wrote:

Greg obviously does have *some* idea about focal lengths, so
explaining
the common 35mm equivalencies does not seem like a bad idea. He's
going
to run into the concept sooner or later anyway.

He does, it is, and he will, but he won't understand it from David's
explanation. :-)

I think he's better off understanding the perspective of a stated
focal
length on his camera, like 100/33 = 3x, rather than 100*1.5/50 = 3x.

Not really. The "3x" in your examples tells him nothing useful.

If we accept that statement at face value, it's equally true to say that
"100mm tells him nothing useful".

Since he's new to photography he may not be able to conceptualize that
angle of view *now*, but he will in time as he gets used to using
lenses
and becomes familiar with the equivalencies.

Are you telling me that when you think "18mm on a 1.5 factor camera",
you
are thinking predominantly of an actual angle? As a number of degrees?

Angles are measured in degrees, not millimeters. I'm familiar with the
field
of view, the "look," of a 28mm lens on a 35 for example. I happen to know
that that's about 75 degrees corner to corner, but I don't think of a 28
as
a 75-degree lens; I think of it as a 28 and I know what photos taken with
it
will look like. And with the 1.5x lens factor on a DX camera, I know that
18mm will be roughly the same.

[...]

I don't know about the "always" part, but generally speaking with short
lenses one naturally thinks in terms of field of view, while with long
lenses one thinks of magnification.

Until you get down to macro distances :-)

Right.


I've never met one who thinks in terms of halving or doubling their
angle
of view, but that's probably just me, and maybe something to do with the
way the image in the viewfinder "comes towards you", or "moves away from
you" when you zoom.

The terms in which most people think aren't necessarily the best terms
in which to undersrand what is going on. If they were we wouldn't have
had to invent science :-)

It's really a matter of visualizing the effects of different focal
lengths,
not "halving or doubling" anything.

Visualising the effects of different focal lengths with a particular
size of image plane, such as the very familiar 35mm film
camera. Switch to a medium format camera and you have to adjust all
the focal lengths. The only thing that stays constant with respect to
the effects of perspective projection of the view onto the image plane
with respect to changes of lens, format, etc., is the subtended angle
of the field of view. That's not how most people are used to thinking
of it, but it's the only geometric invariant.

The distinction didn't matter a hoot so long as most people who
changed focal lengths had 35mm film cameras, except for a few
professional with very expensive bigger stuff.

But nowadays with digital cameras the size of the image plane (the
sensor) varies all over the place as one easily adjusted technological
variable in a whole slew of engineering compromises and trade offs in
mass market cameras. So we have to shift to a new way of describing
the relationship between lens and perspective. The only natural
invariant parameter which falls out of the geometry of perspective
projection is the subtended angle of the field of view.

Indeed. Maybe it *would* be better if we started thinking of 75-degree
lenses and 63-degree lenses and so on, instead of 28mm (equivalent) lenses,
35mm (equivalent) lenses etc. But changing everyone's familar and
comfortable way of doing this would be one hellacious job. And of course it
would be complicated by smaller format DSLRs which accommodate full-frame
lenses as well.


So, define what you mean by perspective. I'm sure we're talking about
two
very different things. Let me know what you're talking about.

Sure. Just putting it in my own words (and I'm sure there are more
authoritative and concise ways of putting it), perspective is the
apparent
spatial relationships between the various objects in a picture that are
at
different distances from the viewer. Perspective always involves one or
more
vanishing points, though these are not necessarily within the picture and
not necessarily obvious -- without straight lines in the picture they are
usually not obvious, but same-sized objects will appear smaller as they
are
farther from the viewer position and will still follow the rules relating
to
vanishing points.

As mentioned earlier, some people insist that perspective depends
*entirely*
on viewer (or camera) position and has nothing to do with focal length.
As
proof of this they point out that a wide-angle photo can be cropped until
it
covers the same field of view as a long-lens photo, and the perspective
in
the part that remains never changes. This is of course true, but in my
opinion it ignores the fact that the entire picture contributes to
perspective -- and therefore there really is such a thing as "wide angle
perspective" as opposed to "long lens perspective," and anyone can see
the
difference between them. I regard this as a somewhat knotty area in
discussions of perspective, and both ways of looking at it are legitimate
in
their own terms.

The differences are most obvious ar the edges of the image, because
that is where the differences between the projective geometry of the
camera and the human eye and brain are at their largest.

The human eye (or rather the usual pair of them) has a field of view
of
about 180 degrees horizontally, which is even greater than that of a
typical fisheye lens. But the angle of view over which the eye can
see
things clearly defined is much smaller -- more like a long telephoto.

A very long telephoto! At normal reading distances, for example, most
people can only clearly see one or two words on a page of print, and
have to move their eyes to see more. The rest is hugely fuzzy.

We see things clearly over a wider range by scanning, which the
camera
lens cannot do in a single shot. For these and other reasons it is
meaningless to make such a comparison between the camera lens and the
eye.

Completely irrelevant, and another reason why I wouldn't talk about
FoV.

But you *are* talking about field of view. You're just calling it
"perspective."

In my mind I absolutely not thinking about or talking about FoV. I'm
talking about how my view of the scene in the viewfinder changes with
focal length. I don't ever think of that in terms of angles. Perspective
is a bad choice of word to describe it, but I'm keen to see your
definition anyway.

The terms in which you think of it aren;t necessarily the terms in
which you ought to be thinking of it in order to be most easily able
to translate your descriptions across the kinds of imaging tecnology
change we're faced with everyday in discussing digital camera
photographs.

I've never seen anyone argue that a 100mm is "normal".

I said "most natural," not "normal." And as I said it's a fallacy anyway.
People have used the argument to explain why 100mm (or thereabouts) is
the
ideal f.l. for portraiture (on a 35), and I can't count the number of
times
I've seen someone claim that it's "most natural" because it duplicates
the
magnification of the human eye, i.e. that the image in the viewfinder
looks
the same size as when seen with the naked eye. It simply isn't true, or
at
least it's never been true on any SLR I've tried the comparison with.

"Normal" is almost meaningless. For head-and-shoulders portraiture a
100mm
lens does provide a natural look and a 50mm would be regarded by most
people
as too short. For scenery, landscapes etc. a 50 is often too long.

It's to do with how we use our eyes. When we study a person in detail
we're usually at a distance where we take in the head and shoulders
within a narrow angle of view, and our attention is focussed within
that narrow view. The 70-100mm lens on a 35mm camera duplicates
that. If you use the standard 50mm lens then the perspective
projection makes the nose a little too big. Faces are things we study
in very careful detail, and we're sensitive to the unnatural look of
very small changes in proportion.

Whereas when we look at a landscape we let our eyes roam over a wider
field of view, and we're much less fussy about perspective distortion
of trees and rocks etc at the edges of the image. So a much wider
lens, such as 28mm on a 35mm film camera, is regarded by many as the
natural landscape (or cityscape etc.) lens.

It's also
too long by the traditional rule which says normal focal length should be
about equal to the diagonal of the negative.

Which is simply an ad hoc rule of thumb. As all of us have got more
used to seeing camera images every day, and photographers have
adventurously pushed at the edges of the envelope of acceptability,
we've all become more tolerant of the perspective distortion artefacts
at the edges of wider angles of view. And the bigger noses :-)

People got used to the idea of
50mm being "normal" simply because most camera makers accepted the Leica
standard f.l. as their own. But there have been rangefinder 35s with
(non-interchangeable) shorter lenses for almost as long as 35s have been
popular. In the early 1950s the popular Kodak Signet had a 44mm lens. The
Bolsey I think was about the same. Most of the Japanese RF 35s that began
to
flood the country had 45mm lenses. The trend since then has been for
fixed
focal length lenses on compact 35s to become shorter and shorter, various
models being 42, 40 and 38mm. The last compact 35s I owned (Konica Big
Mini,
Yashica T4 and Olympus Stylus Epic) all had 35mm lenses. All of those
were
"normal."

The 50mm lens on the 35mm camera was simply a fixed lens general
purpose compromise between a natural looking head and shoulders
portrait and a natural looking landscape. We look in different ways at
those two things.

Also, with SLRs there had to be room for the mirror to swing. In the early
'50s when I first became interested in photography, fast German SLR lenses
like the Biotar generally were 58mm, fast Japanese lenses a few years later
usually 55mm. Standard lenses for SLRs had to become a bit more retrofocus
to get to 50mm.


Actually even with 35mm SLRs, I always preferred the 35mm lens to the
50mm
as a "normal." To me it always seemed the 50 was too long for some things
and too short for almost everything else.

Exactly. It depends on how we usually use our eyes to look at the
subject of the photograph, i.e. over how much angle they normally roam
in order to see it.

Not necessarily, but the concept of 35mm lens equivalence is now well
established and everyone uses it, not just for DSLRs but for all kinds of
digital cameras. In fact it actually started with APS cameras, which were
considered to have a lens factor of 1.25x.

It's rather like the power of light bulbs. Older folk among us are so
used to tungsten wire electric bulbs that newer light technologies
often express their light output in terms of tungsten-equivalence
wattage.

Yes, that's an excellent analogy.


By the time our children have grown up the "normal" output of
a 100 watt tungsten electric light will have become a meaningless
idea. For better understanding of what goes on we'd be better shifting
to a definition of light output which actually focussed on what was
relevant, the amont of light, instead of an accidental equivalence of
a particular technology.

The same goes for 35mm film equivalence. Now that there are so many
different sensor sizes out there I'm fed up having sensor size
described in opaque irrelevant terms derived from vidicon technology,
so that I then ahve to find what the approximate so-called "crop
factor" is, which is a silly term derived from what happens when you
stick a lens designed for a 35mm camera onto a digital camera with a
smaller sensor, which is what I have to divide focal lenmgth with to
discover the 35mm equivalence, which unless I'm an experienced old
35mm film SLR photographer is meaningless anyway.

It's all a mess because nobody can be bothered with the fundamental
geometry of what's going on and just wants rules of thumb which help
them to stumble from a vague oversimplification of yesterday's
technology to today's.

If I want to be able to jam my camera into the corner of a room and
see all four walls at once I know I want a slightly greater field of
view than 90 degrees. In deciding which of the several cameras I could
borrow would do the job wouldn't it be easy if all their lenses were
labelled in terms of angle of view? :-)

--
Chris Malcolm cam@xxxxxxxxxxxxxxxxxxxx DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

My apologies for the untrimmed reply, but I thought it all so interesting
and relevant I couldn't decide where to trim. :-)

Neil


.

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