# Focal length and focus point

• A
supermankid
First of all, Let me present some of the things I found from research. (considering convex lens)
Interpretation 1 => Focal length
is the distance from the center of the lens to the point where all rays converge.

Interpretation2 => Focal length
is the distance from the point of convergence to the place where image is formed.

1) I understand both but cannot combine two to make sense.
2) What determines the sharpness of the image.
In case of interpretation2, where is the best place to put the screen or sensor to get the best focused image.
In this case, the focal length could be any based on location of the screen, is not it?

Please save me from these dilemma...

First of all, Let me present some of the things I found from research. (considering convex lens)
Interpretation 1 => Focal length
is the distance from the center of the lens to the point where all rays converge.
All rays?

Interpretation2 => Focal length
is the distance from the point of convergence to the place where image is formed.
All images?

In this case, the focal length could be any based on location of the screen, is not it?
The focal length is constant. What does the image distance depend on, other than focal length?

Gold Member
First of all, Let me present some of the things I found from research. (considering convex lens)
Interpretation 1 => Focal length
is the distance from the center of the lens to the point where all rays converge.

Interpretation2 => Focal length
is the distance from the point of convergence to the place where image is formed.

1) I understand both but cannot combine two to make sense.
2) What determines the sharpness of the image.
In case of interpretation2, where is the best place to put the screen or sensor to get the best focused image.
In this case, the focal length could be any based on location of the screen, is not it?

Please save me from these dilemma...
One way to deal with this would be to read what is actually written about the term. You have taken a number of short cuts between what you have read and what you have written. For instance, you have missed out one essential word from the first line in this link when you talk about "rays".
Then look at the Lens Formula in this link and spot what you have left out in your explanation.
You need to be rigorous if you are trying to teach yourself Physics.

supermankid
supermankid
One way to deal with this would be to read what is actually written about the term. You have taken a number of short cuts between what you have read and what you have written. For instance, you have missed out one essential word from the first line in this link when you talk about "rays".
Then look at the Lens Formula in this link and spot what you have left out in your explanation.
You need to be rigorous if you are trying to teach yourself Physics.

It is a very simple question without any in depth explanation.
Could you please say why some photography websites mention focal length as distance from point of convergence to the sensor.

On the attached image. Is both the definition correct?

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Gold Member
I wouldn't see any point in putting a camera sensor in the position shown in the second, lower diagram. The image would be out of focus for a distant object and the diagram doesn't actually specify what object distance the sensor position may be chosen for. A very confusing diagram and I don't understand what's meant by "the point at which light rays combine in the lens. Which light rays? It reads like a very strange optics convention at the very least.
PS I cannot say why "some websites" make any statement. Any old random website is as likely to be misleading as it is to be accurate and helpful. If you want to be sure of a website then use one that's recommended by a reliable Forum like PF or, better still, use a textbook that's used in schools.

Staff Emeritus
First of all, Let me present some of the things I found from research. (considering convex lens)
Interpretation 1 => Focal length
is the distance from the center of the lens to the point where all rays converge.

Interpretation2 => Focal length
is the distance from the point of convergence to the place where image is formed.

1) I understand both but cannot combine two to make sense.
2) What determines the sharpness of the image.
In case of interpretation2, where is the best place to put the screen or sensor to get the best focused image.
In this case, the focal length could be any based on location of the screen, is not it?

Please save me from these dilemma...

"Focal length" is a physical property of the lens, independent of how it is used or where the image and object are.

"Focal point" is where the image is focused.

Those two are usually not the same EXCEPT for when the object is very far away (infinitely far away). Look at the thin lens equation and apply this to when the object is infinitely far away.

Zz.

supermankid and sophiecentaur
supermankid
I wouldn't see any point in putting a camera sensor in the position shown in the second, lower diagram. The image would be out of focus for a distant object and the diagram doesn't actually specify what object distance the sensor position may be chosen for. A very confusing diagram and I don't understand what's meant by "the point at which light rays combine in the lens. Which light rays? It reads like a very strange optics convention at the very least.
PS I cannot say why "some websites" make any statement. Any old random website is as likely to be misleading as it is to be accurate and helpful. If you want to be sure of a website then use one that's recommended by a reliable Forum like PF or, better still, use a textbook that's used in schools.
I clearly need to study on this but I think some "physics experts" also might need to learn to understand simple question and answer then straight foreward. Otherwise general public will always be confused by simple maths and physicist will only know numbers and not the real life situations. Thanks for the time and information to all.

supermankid
"Focal length" is a physical property of the lens, independent of how it is used or where the image and object are.

"Focal point" is where the image is focused.

Those two are usually not the same EXCEPT for when the object is very far away (infinitely far away). Look at the thin lens equation and apply this to when the object is infinitely far away.

Zz.
Thanks for making this clear...I think some photography websites needs to hire some optics expert to amend their website...

supermankid
I clearly need to study on this but I think some "physics experts" also might need to learn to understand simple question and answer then straight foreward. Otherwise general public will always be confused by simple maths and physicist will only know numbers and not the real life situations. Thanks for the time and information to all.
Multiple websites provide this info (together with illustration)
May be you can have a look once...are these illustration totally wrong ?

Gold Member
need to learn to understand simple question and answer
The difficulty (or worse) of a question is not always obvious to the questioner. It is tempting to blame the person who gives the answer but there is an acronym GIGO which is often applied to information technology.
May be you can have a look once...are these illustration totally wrong ?
Those illustrations are extremely misleading because they do not refer to a single convex lens. If you read the article, you will find that they refer to the "centre" of the (multi-element) camera lens but they do not define what they mean. They are not 'wrong' but you seem to have misinterpreted them (not your fault). This is a good example which proves what I said about reading a textbook first. We need to learn to walk before we try to run.

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supermankid
Gold Member
Thanks for making this clear...I think some photography websites needs to hire some optics expert to amend their website...
All they need to do is to put in a caveat, warning readers of the large gulf between the way they work and the basic optics we learn at School. That link you gave was about actually using a camera and doesn't discuss basic physics.

supermankid
Mentor
"Focal point" is where the image is focused.
In all the textbooks that I've used (both intro physics and intermediate-level optics), the focal point of a lens is the point where incoming light rays that are parallel to the len's optical axis are focused to (in the case of a converging lens) or diverge from (in the case of a diverging lens).

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

With a point object not at infinity, which doesn't produce incoming parallel light rays, the outgoing rays come to a focus at (or diverge from) some other point besides the focal point.

sophiecentaur and supermankid