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bulb763

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What makes a 50mm lens a 50mm lens? Or a 70-200mm a 70-200mm? Or any focal length lens for that matter... It's obviously got to be a measurement of something, but what? :thinking:
 
bulb763 said:
What makes a 50mm lens a 50mm lens? Or a 70-200mm a 70-200mm? Or any focal length lens for that matter... It's obviously got to be a measurement of something, but what? :thinking:
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Parallel light rays travelling perpendicular to the focal plane converge after they pass through a lens and meet at a point, equal to the focal length, behind the lens.
For your 50mm lens it's a simple construction but the zoom lens has additional moving elements that alter the distance.

Bob
 
Wikipedia is great for this sort of question....

"When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length."

In lay terms, the distance from the sensor/film to a particular point in the lens.
 
Canon Bob said:
Parallel light rays travelling perpendicular to the focal plane converge after they pass through a lens and meet at a point, equal to the focal length, behind the lens.


Bob
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Shouldn't that be horizontal Bob? :D
 
CT said:
Shouldn't that be horizontal Bob? :D
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Only if your shooting in portrait orientation :lol:

(Bulb...ignore my silly remark and don't be confused by it)

Bob

Edit...you were kidding CT....or have I missed something?
 
Canon Bob said:
Edit...you were kidding CT....or have I missed something?
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Partly pulling your leg Bob. :D

dj.
  1. Mathematics. Intersecting at or forming right angles.
  2. Being at right angles to the horizontal; vertical. See synonyms at vertical.
  3. often Perpendicular Of or relating to a style of English Gothic architecture of the 14th and 15th centuries, characterized by emphasis of the vertical element.
adv. In a perpendicular position.
n.
  1. Mathematics. A line or plane perpendicular to a given line or plane.
  2. A perpendicular position.
  3. A device, such as a plumb line, used in marking the vertical from a given point.
  4. A vertical or nearly vertical line or plane.

Most people I reckon would think of perpendicular as vertical, so it kind of sounded a bit like your were describing a periscope. :D
 
CT said:
Partly pulling your leg Bob. :D
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Ah, I'm with it now. A lifetime in engineering has given me the "two planes at a right angle" definition and orientation didn't figure in the equation.

It's sometimes difficult to look at how others would interpret the wording.

Cheers CT :thumbs:

Bob
 
I'm an engineer as well. perpendicular is at 90 to a plane. Is that vertical definition american?
 
who? said:
I'm an engineer as well. perpendicular is at 90 to a plane. Is that vertical definition american?
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No, my last 14 years were with a US company and they use the same definition.......one of the few things where they do :cuckoo:

Bob
 
All this engineering guff is extremely fascinating guys ... :suspect: ... but I don't think we have clearly answered the OP's original question in layman's terms ... :shrug: ... or for thickos like me in terms that are more than easily assimilated and stored away for future regurgitation in similar easily recalled terminology ... ;)




:popcorn:





:p
 
I think Bob actually did answer it Rog, but a lens works by converging the light rays at a point in the lens known as the nodal point, from where the rays then diverge again on their way to the sensor. It's the distance from the nodal point to the sensor/film plane, which would be 50mm in the case of a 50mm lens focused at infinity and so on through the various lens focal lengths.

Fairly simple to understand in the case of a fixed prime lens, and with zoom lenses, that nodal point moves depending on the focal length set on the lens.

Mirror lenses which are a fixed lens, and often substantially physically shorter than their focal length would suggest, achieve their focal length by bouncing the image off mirrors internally before finally going to the sensor.
 
Here's a simple explanation. There are some fundamental lies in it, but I don't want to go to into the quantum aspects of light.

Light rays travel parallel to each other, like this:

Code: 
================

Let's put a lens on the end... < is the lens, > is the light rays converging, < is them diverging as they cross over each other, and | is the sensor.

Code: 
================(><|

The nodal point of the lens is the point in between the ><, so the focal length of the lens is the distance from that point to the sensor plane.
 
CT said:
I think Bob actually did answer it Rog,
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You might well be right in high level terms Cedric ... :shrug: ... but high level is not necessarily going to clarify it for dummies like me ... :D


So, if I understand your explanation correctly and the following diagrammatic illustration from Blapto the measurement is purely a 'notional' one rather than a physical measurement from any part of the lens / glass to FP ... :thinking: ... :gag:






:p
 
It's not a measurement from any part of the lens you can see Rog, as the nodal point will be one of the internal lens elements. :)

They do actually mark the focal plane on the top plate of your camera - it's that symbol which is a circle with a line passing through it - the line marking the exact position of the sensor.
 
I think I knew where the Focal Plane was and how it was marked cos that's in my instruction book ... ;)

Okay so you're saying it is actually a physical measurement then ... :shrug: ... as it is taken from one of the internal glass elements of the lens ... :nuts:


We're getting there ... :thumbs: ... aren't we ... :D




:p
 
It is actually a physical measurement - taken when the lens is focused at infinity. You're not getting there Rog - you're there mate! :D
 
Haha - that's just an age thing Rog. ;)
 
It doesn't have to be at a particular element. It doesn't actually have to be in the lens.

We have negative lenses too, so the light rays hit the lens parallel, and then diverge. If you draw a dotted line from each of the diverging rays, they'll converge in front of the lens, towards the object, and that's where the nodal point is.

Wikipedia actually have good ray diagrams:
http://en.wikipedia.org/wiki/Focal_length

Consider the fact that a Nikon 600mm f/4 only extends 445mm from the lens mount, and the lens mount to focal plane distance is 46.5 mm. 445+46.5 = 491mm, therefore the nodal point of the lens is actually in front of the lens!
 
who? said:
I'm an engineer as well. perpendicular is at 90 to a plane. Is that vertical definition american?
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Sorry to stray from the original point, but I was definitely always taught that perpendicular just meant at 90 degrees to a reference plane, regardless of whether it's horizontal or vertical :thinking:. Those definitions seem to suggest it's only vertical:shrug:. I'm very confused now.....

(I'm another engineer by the way.....)
 
I think the vertical reference only makes sense when you are lying on your back taking a photo of something directly above you..:lol:

Sorry, couldnt resist, ( ex engineer)
Allan
 
Fascinating stuff! I've read that wiki article some time ago and it made little sense to me then. I'll give it another read and see what I can make of it ;)

BTW I'm supposed to be an engineer too :nuts:
 
All these engineers showing up! I could tell this engineer joke, but then I'd have to ban myself for going off topic. :D
 
Blapto said:
Oh well, you were close enough to telling the joke for all practical purposes.:clap:
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Just this once then..

Three friends, a doctor, an architect and an engineer, went to their golf club on a Sunday morning to play their usual round of golf. Arriving at the first tee, they were held up by a large party of players teeing off. Each player kept missing the ball completely, taking huge divots out of the green, and when the ball was eventually hit, the whole procedure started again with the next player. After about an hour waiting, only half of the party had successfully teed off, at which point the three friends, becoming increasingly angry, returned to the clubhouse and complained to a club official who explained....

"Ah... that will be the coach party from the local blind institute. They only come here once a year, we don't charge them and people usually make allowances for them, but if you like I'll go and ask them to stand aside while you tee off"

The doctor was horrified! "Oh dear certainly not! They can take as long as necessary. I feel terrible about complaining now - I shall go to the bar and pay for a round of drinks for them when they get back."

The architect said "I feel terrible too, I shall also leave a round of drinks in for them, it's the least we can do!"

The engineer who had been looking very thoughtful throughout said...



"Why can't they play in the dark? :thinking:



:coat:
 
chris321 said:
Sorry to stray from the original point, but I was definitely always taught that perpendicular just meant at 90 degrees to a reference plane, regardless of whether it's horizontal or vertical :thinking:. Those definitions seem to suggest it's only vertical:shrug:. I'm very confused now.....

(I'm another engineer by the way.....)
Click to expand...

This is correct.

Perpendicular simply means at 90 degrees to something. Planes can be made from vector directions, so for simplicity, let's just consider two vectors (lines). A vector being simply a line with direction (and magnitude if necessary to know) inside a space defined by arbitrary axis directions ( X, Y and Z by convention)

For a vector to be perpendicular to another, there must be a 90 degree angle between the two. As for the direction of the orginal, these do not matter. Also, the two vectors do not even have to intersect at any point.

Hope this clears up any confusion :)
 
Wow, 31 posts just because I used the word "perpendicular".....could have saved a lot of server space if I'd just said "pointin' at" :(

Bob
 
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