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


"Wilba" <wilba@xxxxxxxxxxxxxxxxxxx> wrote in message
news:fm6nke$g26$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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. What exactly
do you expect him to do with the "3x"? The conventional thing is to refer to
digital camera focal lengths in terms of 35mm equivalencies. For this, all
he needs is the actual focal length and the lens factor, 1.5x in the case of
Nikon DX cameras and others using the same sensor size. So he knows that,
say, 18mm is very close to the angle of view of a 28mm lens on a 35, which
is a familiar wide angle standard.

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. Your "3x" on the other hand would
never be of much use to him. The last thing a beginner needs is to start
speaking or thinking in non-standard terms.


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. What the focal length of a lens controls (for any given format) is
angle of view. That has a bearing on perspective (though you'll find many
who argue it doesn't) but does not define perspective. When you say you're
"not talking about FoV, [you're] talking about perspective" you're talking
nonsense.


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."


so a 300mm lense will give you a view like a 6x telescope.

Again, not true. A 6x riflescope typically has a field of view of about
17 feet at 100 yards. A 300mm lens on a 35 has a field of view of about
43 feet at 100 yards, or two-thirds that on a DX camera.

Not talking about FoV (see how confused you all get when you do?). Talking
about perspective.

Again: calling angle of view "perspective" does not make it so.


For a D40, you will get a "normal" perspective through a lens around
50/1.5 = 33mm focal length.

The 50mm focal length was traditionally the "normal" for a 35mm camera
primarily because that's the focal length Barnack used for the original
Leica, but it was an odd choice and no one seems to know for sure why he
made it. The usual standard for "normal" has been a focal length equal to
the diagonal of the negative, which is about 43mm in the case of a 35. In
recent decades shorter lenses than that have been the rule for compact
35s with fixed focal length lenses, 35mm or so being the usual thing. Any
of these is as "normal" as any other, since "normal" is in the eye of the
beholder.

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.

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. On a 35mm SLR,
the f.l. with apparent unity magnification is usually around 70mm, but
varies from one camera to another. The variation is even greater with DSLRs.


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?

Neil


.

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