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


"Wilba" <wilba@xxxxxxxxxxxxxxxxxxx> wrote in message
news:fm72ms$ntb$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Neil Harrington wrote:
Wilba wrote:
Neil Harrington wrote:
Wilba wrote:
David J Taylor wrote:
greg@xxxxxxxxxxx wrote:

I see there's more to focal length measurements than I realised!

Yes, a simple rule is that with cameras like the D40, multiply the
optical focal length (the one on the barrel, and the speicifcation)
by 1.5 to get the equivalent filed of view, so the 18-55mm kit lens
has the same FoV as a 27 - 82mm lens on a film SLR, and the
70 - 300mm lens has a 105 - 450mm FoV.

That's misleading. Let's stick with focal length, and not confuse
Greg with FoV.

Greg says he's new to photography, so 70-300 and 105-450 are equally
meaningless, so I don't see that you've done him any favours anyhow.
:-)

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".

Sure it does, if he's familiar with the effects of different focal lengths
and can visualize them with respect to the subject. If he's not, he
eventually will be. Presumably he already knows that magnification is
proportional to focal length, so "3x" tells him nothing new or useful.


What exactly do you expect him to do with the "3x"?

The same as you expect him to do with "100mm". The difference is, with a
magnification he may be able to relate it to other experience, like
looking through telescopes, binocculars, magnifiers, etc.

As I've pointed out, you are completely mistaken in trying to make that sort
of comparison. Your "3x" or any other "x" in connection with a camera lens
WILL NOT give him the same result as that power in a telescope or
binoculars.

[ . . . ]

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.



Your "3x" on the other hand would never be of much use to him.

Angle of view and magnification are inextricably geared to each other,
right?

Right.


(With the same camera, if you change one you change the other by a fixed
ratio, right?)

Right.


If so, why does conceptualising in terms of one always beat doing so in
the other?

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.


The last thing a beginner needs is to start speaking or thinking in
non-standard terms.

When someone says "Xmm lens on a 35mm camera", I visualise that in terms
of what a two-dimensional rendering of the scene would look like from 50/X
times the distance, or X/50 magnification.

Why think in terms of a 50 at all? I don't. I have a pretty good idea of
what the picture will look like with a 28, as opposed to what it will look
like with an 85 or a 135. Why on earth start with visualizing the results
with a 50 and then somehow dividing from that, which is what you apparently
are saying?


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.

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


Greg, on a 35mm camera a 50mm lens gives a perspective similar to the
human eye,

That simply is not true. There is no way to equate any focal length to
"a perspective similar to the human eye" since the eye sees in an
entirely different way from a camera lens -- and anyway it has been
argued with
some justification that perspective depends on camera position, not
focal
length, though that is only partly true.

I'm not talking about FoV, I'm talking about perspective (e.g.
http://en.wikipedia.org/wiki/Perspective_%28visual%29). You're barking
up the wrong tree.

You don't seem to understand what perspective is, with or without
Wikipedia aids.

Yeah, that doesn't make sense to me now. :-)

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 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.
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.

You have it above, or at least a first draft. :-)


Try this. Look at a scene and put your 35mm camera between your eye and
the scene, so that you are now seeing the scene through the viewfinder.
At what focal length does the scene appear to be not magnified (neither
enlarged nor reduced in the viewfinder)?

Depends on the camera's viewfinder system. The argument you're using here
has been used by others to "prove" that a 100mm lens (on a 35) is the
most natural, i.e. does not change magnification from that of the naked
eye. This is fallacious for a 100mm lens just as your trying to use the
same argument for a 50mm lens is fallacious.

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

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.

The photographer at the hospital where I worked always had a 28 on his
Pentax. For him that was the "normal" lens. And I remember seeing published
photos by a pro who almost always used a 21mm lens as his "normal" -- he
insisted that was the f.l. that best duplicated the way the human eye sees
things (he was mostly an outdoor events photographer).


My experience, having done the experiment as described, is that 50mm gives
a "normal" view, as described in the literature I read when I got that
camera.

Try it again. This is very hard to do accurately. You have to have both eyes
open of course, and the viewfinder image is somewhat displaced vertically as
well as horizontally. Use a zoom lens and change the focal length while
turning the camera slightly from side to side. See which way a distant image
moves relative to what's outside the viewfinder. If it moves in the same
direction you're turning the camera, the f.l. is too short; if it moves in
the opposite direction the f.l. is too long. When it doesn't move at all
you're at unity magnification, i.e. the same magnification as the naked eye.
Do this very carefully; the object is to find the point at which there is
absolutely no movement one way or the other. With my Minolta 700si it's just
a little short of 70mm on the zoom ring. With my Nikon D80 it's a little
above 45mm.

Again, this really doesn't show anything about what's "normal." It will vary
with the magnification of the camera's viewfinder system.


Most people who try this silly argument evidently have never actually
conducted the experiment themselves. The result depends entirely on the
magnification of the particular camera's viewfinder system.

"Entirely"? The focal length of the lens doesn't matter at all?

Of course it does. I'm saying the focal length at which you get apparent
unity magnification (no magnification compared to the naked eye) depends
entirely on the magnification of the particular camera's viewfinder system.


On a 35mm SLR, the f.l. with apparent unity magnification is usually
around
70mm, ...

Not with the ME Super here on my desk.

Have you actually tried it at about 70mm? As I said, it's very difficult to
do the comparison accurately. In any case it does vary somewhat with the
camera.


.. but varies from one camera to another. The variation is even greater
with DSLRs.

Of course. Hence my reply to myself.

IOW, the only thing that changes when you interpose the camera between
your eye and the scene is that you now see the scene through a frame.

That focal length provides a "normal perspective", and that's what I'm
talking about.

Nonsense. It shows nothing whatever about "normal perspective" (whatever
you imagine that is). Again: the results will vary according to the
individual camera's viewfinder system. Do you really think the focal
length for "normal perspective" for any picture changes with different
viewfinders?

OK, let's say that my experiment is irrelevant to any camera other than my
ME Super.

You seem to be arguing that Greg is better off to learn to think of his
view through an 18mm lens on a D40 as a "28mm" view, rather than as an
"18mm" view.

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.

However, the equivalencies obviously are more useful to those of us who are
familiar with focal lengths associated with 35mm cameras. If Greg is not, I
don't think there's any particular reason for him to become concerned with
them, but he will surely be seeing them mentioned often in the literature
and might as well have some understanding of what they're about.


I'm not sure that's the best advice. Other than compliance with
convention, why is it?

Just the fact that it *is* a convention makes it worth adhering to. You
could measure distance in stadii or room size in cubits, but few people
would grasp what you were saying since those units are not the conventional
ones nowadays.

Neil



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