• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Difference between focal length and the distance from the lens

Lens and distances

I do use images as help when working theses out. going to try and scan theses in but tryed my best just using pc for now. 0 = lens F = focal length

1) a camera with a 50mm focal length lens is focused on a persons face 1m from the lense. show that the film will be about 53mm behide the lens.
(focal length = 0.05)
(-) (?) (1m) (+)

Simple knowledge says 1/v=1/u +1/f i then rearranged that to make 1/u=1/1 - 1/ 0.050

= 20....... not 53 mm, very confused :S

2) with a zoom lens you can vary the power of a lens. show that a lens power of about 24 dioptre is required to focus a flower that is 250mm in front of the lens on a piece of film that is 50 mm behide the lens.

im extremly confused on this one. I yet again used 1/v=1/u+1/f rearranged it to make 1/u = 1/v - 1/f or 1/u= 1/0.25 - 1/24 = 3.95 :S not 50

I would think ive just messed up the equation in this one.

3)A lamp 400mm from a lens is in focus on a screen 400mm from the lens on the other side of the lens. show that the lens has a power of 5 diopters and a focal lenth of 200mm. Explain why the lamp must be moved nearer to the lens to project its image further away.

im unable to find out how they got 200mm focal lenth from there i can work out the power. 1/0.2 = 5. the wording of this question is extremly bad.

additional help

Just a couple of quick question that arnt on my homework but would like to know for background info.

whats the difference between focal lenth and the distance from the lens and why are they so different? what happens if it is not the same as the focal lenth? it is burred?

Cheers people, Thanks for looking

P.S sorry for bad spelling i have dyslexia:)
Last edited:


Homework Helper
1/u=1/1 - 1/ 0.050
Put all distances in the same units. Are you sure about that formula? I seem to remember 1/Di + 1/Do = 1/f where Di is the distance from the lens to the image, Do is the distance from the object to the lens and f is the focal length.

Better look up "dioptre" for #2. You haven't used it correctly.

#3 looks clear to me. Same formula.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads