Lenses/ Optics

Phototoxin

I'm a bit rust on my optics part of physics.

I want to know if its possible to get a fixed lens so that when you look at it only things within say 50m are in focus. or 50m with a depth of fielf of a bout 5 metres.

is this possible and how do i calculate the focal length of what i need. is it 1/f= 1/u+1/v ? f=focal lenght, u= image and v = observer ? also how do i determine depth of field ?

essentially what i want to know is can you get something that when you look through it things at 50m +/-5m or so are in focus whereas everything else is not

OR

where you can look through something and then anything 50m or closer is in focus?

ga2re2t

I think this is the answer:

Basically, you can use a convex lens with a focal length of 50 m which will focus the light from your object 50 m away into parallel light. This can then be viewed by your eye or your camera.

A circular lens will focus a point object (e.g. an apple). A cylindrical-type lens should be able to focus a line object (e.g. a row of apples).

I have no idea about how to determine field of depth. If you're looking for a lens with a focal length of 50m +/- 5 m then you're looking for an imperfect lens. This might be good for field of depth, but your image will suffer in quality.

More info on lenses can be found on Wikipedia:
http://en.wikipedia.org/wiki/Lens_(optics)

Professor_Fink

ga2re2t wrote: »

I have no idea about how to determine field of depth. If you're looking for a lens with a focal length of 50m +/- 5 m then you're looking for an imperfect lens. This might be good for field of depth, but your image will suffer in quality.

Thinking about this, there are two solutions. The first is to use a lens with a special cross-section (i.e. not spherical, but rotationally symmetric). This would actually prevent you from bringing anything into perfect focus, but would even out the level of focus over some region. This seems a pretty bad option.

The better option would be to make a compound lens with an aperture, just like a camera lens.

SkepticOne

Phototoxin wrote: »

essentially what i want to know is can you get something that when you look through it things at 50m +/-5m or so are in focus whereas everything else is not

OR

where you can look through something and then anything 50m or closer is in focus?

The first answer is correct in general. If your focus is at 50 metres and you have a narrow depth of field then foreground and background objects will be out of focus but objects near 50 meters will be in focus.



In the above picture from Wikipedia the paw is out of focus as is the background whereas the face is in focus.



The Wikipedia article from which the illustrations above are taken also has formulas for calculating the depth of field.

Cheap compact cameras are often set to focus at infinity but have a large depth of field so don't require focussing for relatively nearby objects. Everything beyond a certain point (say 3 metres) is in focus.

The photography forum will have more information regarding depth of field and how to control it.

SOL

Well tbh I fell asleep during our optics lectures so I have no actual advice of a scientific or detailed nature, but from a photographers point of view:

http://www.dofmaster.com/doftable.html

is very useful, and you can easily obtain your required DOF with a 200mm camera lens,

What do you want to use the lens for>?

Phototoxin

it was to determine range.. say you had a rifle and you wanted to see if someone was within 50metres then you look through said eyepiece and if they are infocus they are in range. the 50m was just an example.

Professor_Fink

Phototoxin wrote: »

it was to determine range.. say you had a rifle and you wanted to see if someone was within 50metres then you look through said eyepiece and if they are infocus they are in range. the 50m was just an example.

Ah, well in that case you would be better looking at coincidence or parallax rangefinders. Rangefinder cameras do exactly what you want in a somewhat more accurate way than a simple lens.

Phototoxin

thanks fink