Possible silly question about focal lengths

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-Rob-

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A non-photographer asked me this question recently expecting me to know the answer but I didn't.

Where the focal length of a lens is say 50mm what exactly does that refer to? I'm assuming it is the distance from one point to another but from where to where? I'm assuming it's not something within the lens because 500mm lenses aren't 500mm long so it must be an external measurement. I'm also assuming it's not the point at which the lens focuses on an object because again, a 500mm lens will focus on something more than 500mm from the lens.

I've tried a Google search and got answers but they were forumlas that meant nothing to me. Is anyone able to explain it in plain English?
 
A 50mm lens will focus two parallel beams of light together 50mm away from the lens centre. For practical purposes, anything in the distance which we consider infinity focus (60' +) can be considered parallel.


Steve.
 
In a simple lens a 50mm one would come to a focus 50mm behind it.
On a long focus lens of 500 mm it would again focus 500 mm behind it.

Telephoto lenses are of special optical construction that allows the construction of a shorter lens than a long focus one, but giving the same image size.

Wide Angle lenses for SLR cameras are Retro focus ( reverse telephoto)
This permits them to be mounted further away than their apparent focal length so as not to obstruct the Mirror... It makes them larger than they would otherwise be.
 
-Rob- said:
I still wouldn't class that as a plain english explaination but I suppose it's clearer than most. So the measurement is an internal distance then?
Click to expand...

- really?

"Very simply, it is the distance from the lens to the film, when focused on a subject at infinity." Substitute sensor for film.
 
-Rob- said:
I still wouldn't class that as a plain english explaination but I suppose it's clearer than most. So the measurement is an internal distance then?
Click to expand...

Yes. It's a measurement generally speking from the centre of the lens to the focal plane, i.e. the film or sensor. It's measured behind the lens put another way.

Some lenses like mirror lenses achieve big focal lengths such as 500mm or more which are actually longer than the lens, but this is achieved by bouncing the light back and forth in the lens before it goes to the sensor to achieve the neceessary focal length.
 
py6km said:
- really?

"Very simply, it is the distance from the lens to the film, when focused on a subject at infinity." Substitute sensor for film.
Click to expand...

Now that is the very simple plain English I was looking for.

I assume this measurement is from the good old days where a camera lens consisted of a single piece of glass but now that lenses have multiple elements the rules have changed slightly.
 
It is actually a measurement of the type of lens, not necessarily anything to do with photography, which is a collection of lenses.

When talked about in terms of a 'lens'
What it actually relates to, is at which distance would a point source of light, be converted into a parallel beam of light.
This is a simple lens like a magnifying glass. It can be reversed, so if your magnifying glass is a 50mm focal length lens, you hold it 50mm (5cm) above the ant, in order to focus the suns rays (as parallel as you are likely to get) onto the ant.

For a collection of lenses, commonly called a camera 'lens', then it describes the magnification inherent within the collection.
For a sensor at a fixed distance X from the end of the collection, you always need to have the image to be focused, in order to view it. So the initial few lenses which the light passes in to, might have the light bent outwards, with convex lenses, to make the image larger, whilst the elements nearer the rear of the camera might bend the light inwards, concave lenses, in order to provide a sharp image.
For much more of an explanation, it would require going into the refractive indices/differences between glass and air, and dioptric measurements.
Consider, almost all humans have the same ability to focus the 'lens' at the front of their eye.
However, some (like myself) have eyes which are not as deep, giving them myopia, short sighted-ness, they cannot bend the light enough to get a sharp image.
If you were to try my glasses, which are at about -9 dioptres, apart from giving you a major headache, you would see the world being much smaller than you are used to. The front plane of the glass makes the image smaller, whilst the second plane of glass focusses it to be closer than it really is.
 
Another question to ask is does a 50 mm lens have a focal length of 50mm.

This is actually unlikely as 50mm will only be a nominal value.
It might be any where from around 49mm to around 52mm.
there is a discrepancy in all focal lengths, but it may be worse the longer the lens.
 
Plain English, the focal length of a lens is the distance from the 'optical centre' to the sensor, when focused on infinity.

With a simple lens, the optical centre - technically the rear nodal point in this calculation - is the same as the physical centre, but with complex modern designs, the optical centre is often actually in mid air outside the physical lens. It is behind the lens in retrofocus wide angles, as used in all DLSRs, and this is necessary to give space to clear the mirror mechanism; in telephotos it is in front of the lens, to make them smaller.

In practise, the focal length is used to define the magnification of a lens, which is in turn used to measure the angle and field of view.

Edit: Another useful thing about focal length is its part in defining the f/number, which is necessary for exposure setting and depth of field calculation. Eg, a 100mm lens which has a maximum aperture that is 36mm in diameter, will be f/2.8, ie 100 divided by 36 equals 2.8. With longer prime lenses like a 100mm macro, you can pretty much see this and measure it roughly with a ruler, but it's more difficult to do visually with zooms and wide angles.
 
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