A question about the lens equation?

Question

A person finds it necessary to focus on an object at 120mm in front of a camera lens, they need to insert an extention tube of length x between the lens and the camera find the minimum length of this tube. So far I know that 1/v = 70mm, 1/u = 175mm and 1/f = 50mm how do I find x? Also what is the effect of this tube on the furthest object that can be imaged by the camera? Thanks in advance

Answer 1

The Gaussian lens formula, for thin lenses, is: -

1 = 1 + 1

_ .. _ .. _

f.....u....v

Where 'f' is the focal length and 'u' is the distance to the object from the lens, with 'v' as the distance of the image from the lens (strictly measuring from the lens's centre).

if v is set at 120 mm, with f at 50 mm  - then the equation becomes: -

1/50 = 1/120 + 1/u

Thus: -

1/u = 1/50 - 1/120

Hence: -

u = 120 x 50/(120 - 50)

u = 85.71 mm.

If v was originally 70 mm then the extension tube has a length of 50 mm (120 - 70).

Thus: - x =50 mm

And the object is at 85.714 mm (rounded to 3 DP).

I hope this is of some help!

Sorry - I missed the minus sign the first time around!!!

Answer 2

Not very clear what you have been given to calculate your answer.

It seems as though you have been told that your camera has a lens to film distance of 70mm and a lens of focal length 50mm.

To focus an object 120mm away with a lens of f=50mm you will have an image distance of 85.7 mm. (Using lens equation 1/f = 1/u + 1/v). This is too long for it to focus on the film by 15.7 mm so you will need to move the lens 15.7mm out - the length x of your extension piece. You can now calculate the object distance using the same formula for the new image distance of 85.7 mm.

Answer 3

You need to tell us what you were given...not what you "figured out".  

A converging lens can do 6 things...(do=object dist.)

1) If 0<do<f The lens is a magnifying glass (image vert./erect)

all the rest: image real and inverted

2) If do=f The lens is a "Stop light"...NO IMAGE

3) If f<do<2f The lens is a projector

4) If do=R=2f The lens is a Xerox machine

======================================...

YOU HAVE THIS ONE---

5) If do>R The lens is a camera and f<di<2f

Now you need to tell us what they provided (other than 1/do = 1/120) so that we can help you.

======================================...

6) At infinity, all the rays parallel to the lens' principle axis are projected to the focus (used bt boy/girl scouts to start camp fires.

The diverging lens has only one case. All di are erect, smaller than life, virtual (and in the equation...MAKE SURE f<0).

Your equations... 1/do + 1/di = 1/f and magnification = -di/do

di<0 image virtual, m<0 and image inverted

Also remember (to avoid needless memorization)...

Convex mirrors are like diverging lenses and

Concave mirrors (make up or shaving mirror) are like converging lenses.

gl2u in Physics.

I'll check back tomorrow.  If no one has done it, I will.

-Fred

Answer 4

Yeah.  Sounds about right... what you questioned.  Although... you need to brush up on your writing skills.

They are putting you to shame.

Do they not teach this in science?  Paragraphs and stuff?

Communication is far more important than trying to understand some minds' science.

Learn English, then come back more confident!