A simple question

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I then see
50 meters of fence?

I think this sounds right... although there may be differences if I took this to
extreme distances or mm's.

Thanks!

In article ,
wrote:

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I then see
50 meters of fence?

I think this sounds right... although there may be differences if I took this
to
extreme distances or mm's.
Sounds right to me too. Think similar triangles in front of and behind
the lens.

It might fail when you get so close to the fence that the front bits of
the lens are further from the optical centre of the lens than the fence
is and you scratch it on the barbed wire.

--
To de-mung my e-mail address:- fsnospam$elliott$$
PGP Fingerprint: 1A96 3CF7 637F 896B C810 E199 7E5C A9E4 8E59 E248

Elliott Roper wrote:
In article ,
wrote:

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I then see
50 meters of fence?

I think this sounds right... although there may be differences if I took this
to
extreme distances or mm's.
Sounds right to me too. Think similar triangles in front of and behind
the lens.

It might fail when you get so close to the fence that the front bits of
the lens are further from the optical centre of the lens than the fence
is and you scratch it on the barbed wire.

If you are standing at the end of the fence, and viewing the length of
it at a 5 degree angle...

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)

wrote:
If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I then see
50 meters of fence?

I think this sounds right... although there may be differences if I took this to
extreme distances or mm's.

Thanks!

Go to http://www.tawbaware.com/maxlyons/calc.htm and
check it out using the "Dimensional Field of View
Calculator" that is a little more than half way down the
entire page.

Assuming you mean that you are standing where your view
is perpendicular to the line of the fence, with a full
frame camera at 100 feet as an example, here is the
horizontal distance for a set of different focal
lengths:

25mm 144'
50mm 72'
100mm 36'
200mm 18'
400mm 9'

--
Floyd L. Davidson http://www.apaflo.com/
Ukpeagvik (Barrow, Alaska)

On Mon, 12 Mar 2012 01:11:09 +0000, Elliott Roper wrote:

In article ,
wrote:

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I then see
50 meters of fence?

I think this sounds right... although there may be differences if I took this
to
extreme distances or mm's.
Sounds right to me too. Think similar triangles in front of and behind
the lens.

It might fail when you get so close to the fence that the front bits of
the lens are further from the optical centre of the lens than the fence
is and you scratch it on the barbed wire.

Thanks!

And I'll watch out for sharp wires...

On Sun, 11 Mar 2012 17:29:20 -0800, (Floyd L. Davidson) wrote:

wrote:
If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I then see
50 meters of fence?

I think this sounds right... although there may be differences if I took this to
extreme distances or mm's.

Thanks!

Go to http://www.tawbaware.com/maxlyons/calc.htm and
check it out using the "Dimensional Field of View
Calculator" that is a little more than half way down the
entire page.

Assuming you mean that you are standing where your view
is perpendicular to the line of the fence, with a full
frame camera at 100 feet as an example, here is the
horizontal distance for a set of different focal
lengths:

25mm 144'
50mm 72'
100mm 36'
200mm 18'
400mm 9'

Thanks for the info and the linc!

wrote in message
...

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I
then see
50 meters of fence?

I think this sounds right... although there may be differences if I took
this to
extreme distances or mm's.

Thanks!

Approximately, yes, if you are far away, although there are more detailed
calculations. Be aware, though, that some 18 - 200mm zoom lenses are only
200mm focal length for distant objects, and when doing close-up work the
effective focal length is reduced to nearer 135 mm.

Cheers,
David

On Mon, 12 Mar 2012 06:55:22 -0000, "David J Taylor"
wrote:

wrote in message
.. .

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I
then see
50 meters of fence?

I think this sounds right... although there may be differences if I took
this to
extreme distances or mm's.

Thanks!

Approximately, yes, if you are far away, although there are more detailed
calculations. Be aware, though, that some 18 - 200mm zoom lenses are only
200mm focal length for distant objects, and when doing close-up work the
effective focal length is reduced to nearer 135 mm.

It's not just 18-200mm zooms. Most internal focus lens will have their
true focal length reduced when they focus closer than infinity.

Steve

On Mon, 12 Mar 2012 06:55:22 -0000, "David J Taylor"
wrote:

wrote in message
.. .

If I take a picture of lets say a fence line, and 100 meters of fence is
showing, and I used a 100mm lens - if I then use a 200mm lens would I
then see
50 meters of fence?

I think this sounds right... although there may be differences if I took
this to
extreme distances or mm's.

Thanks!

Approximately, yes, if you are far away, although there are more detailed
calculations. Be aware, though, that some 18 - 200mm zoom lenses are only
200mm focal length for distant objects, and when doing close-up work the
effective focal length is reduced to nearer 135 mm.

Cheers,
David

I know about the close focus issue, and frankly it ****es me off that the
manufacturers don't include a distance spec for that!

I was going to get rid of a cheap ($160) 300mm lens I have until I tested it
against my other lenses that can do 300 (a 28-300, a 50-500) and I found it's
nearly my best lens for short distance telephoto. I guess being a prime has a
bit to do with it.