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

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".

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.

The conventional thing is to refer to digital camera focal lengths in
terms of 35mm equivalencies.

Correct, and that's exactly the problem - the photographer is not thinking
in terms of the actual focal length they're using or how that will look
through the viewfinder. They're fudging it into something else that
correlates with what you see.

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.

Correct. That's how most people do it. That doesn't mean it's the best way
for everyone all the time. Same with most conventions. :-)

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? I
just don't think that way.

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? (With the same camera, if you change one you change the other by a
fixed ratio, right?) If so, why does conceptualising in terms of one always
beat doing so in the other?

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. 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.

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.

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.

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". 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.

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?

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

Not with the ME Super here on my desk.

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


.

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