Lenses a Techie Question

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ohdearme

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Hello, I am new to the forum just now. Thank you to the forum bosses for the welcome letter.

I always thought that the focal length of a lens is the distance from lens to the film, you can tell how old I am by my use of the word "film".

Yet physics websites on optics say that the focal length is distance from lens to the point of convergence, which is before the back of the camera (isn't it?)

I am confused. There cannot be two focal points can there?

Please help. Keep it simple.

Regards

Alan
 
I'm sure the mods can move this for you later on :)

Unfortunately I can't answer your question though
 
There's a good discussion on this here. It's about as simple as it gets, but might take a read or two to sink in!
 
but then film is not at the back of the camera either is it
 
In a very, very simple one element lens, the focal length would indeed be the distance to the film. However as lenses get more complicated in their construction then the actual distance could be almost anything!
 
Alan Clogwyn said:
In a very, very simple one element lens, the focal length would indeed be the distance to the film.
Click to expand...
Sorry Alan, this is only the case for an object at infinity. If you've not seen it yet, the link I posted above is a pretty good description of what's going on!
 
More accurately, the focal length is the distance from the principal plane to the focal point (for an object at infinity). The principal plane is where you would need to place a simple single lens to mimic the effect of the complex one. A complex 2000mm lens is considerably shorter than 2 metres long, whereas you would need to place a single lens of 2000mm focal length 2 metres from the film for it to focus.
 
Dear All

I now have a big headache with all this, but going through it slowly. Thanks to you all, I am glad I found this friendly forum.

But where IS the focal point? Lens to point of convergence or lens to film plane (where it is upside down)?

Alan
 
The point of focus is the same as the point of convergence, unless you start shooting in 3D - I think :shrug:
I would be interested to look at the optics websites you referred to as I could be wrong.

As people have said if you get a simple lens, hold it so that an image from outside the window is focused on a piece of paper then you can simply measure from the lens to the piece of paper to determine the focal length of that lens.

If you then put another simple lens in between the first lens and the paper you would have to move the paper to get the view back in focus and you can no longer simply measure from one of the lenses to the piece of paper to get the focal length.

Simply substitute film or a sensor for the piece of paper.
 
Okay. The headache is still there but it is the weekend so I will have time to read Cuchulainne's link, thank you Brian.

Kev, here is the link you asked for.

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/foclen.html

This site seems clear, as other physics websites do, that the focal point is at the point of convergence.

It occurred to me that the physics people and probably the opticians, actually have a different definition of the focal point.
Opticians: Lens to point of convergence
Photography people: Lens to back of the camera

That would explain it you know.
Nice weekend all, don't refract, sorry reflect on it too much!
Best
Alan
 
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