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

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.

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.

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.

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.

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)?

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.


.

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