# The Naked Scientists Forum

### Author Topic: related to camera  (Read 3310 times)

#### qazibasit

• Sr. Member
• Posts: 344
##### related to camera
« on: 21/06/2005 10:13:37 »
well i want to know what do you mean by focal length apperture=F/2.8
and focal length 8.64 mm and what do you mean by exposure.

#### gsmollin

• Hero Member
• Posts: 749
##### Re: related to camera
« Reply #1 on: 21/06/2005 19:58:24 »
quote:
Originally posted by qazibasit

well i want to know what do you mean by focal length apperture=F/2.8
and focal length 8.64 mm and what do you mean by exposure.

A focal length is the distance from a lens to the point where the light from a distant source comes to a focus. The light comes through an objective lens of a given diameter. The "f-stop" numbers are the ratios of the focal length to the objective diameter. Therefore, a camera with an objective lens of 28.6 mm, and a focal length of 80 mm, is an f/2.8 camera.

The exposure time in a camera is the length of time the shutter is open to expose the film, or detector. A 1/30 second exposure opens the shutter for 33.3 milliseconds.

#### qazibasit

• Sr. Member
• Posts: 344
##### Re: related to camera
« Reply #2 on: 22/06/2005 17:06:38 »
so 80 mm camera is good or 35 mm camera is better and how it favours in the result.

#### gsmollin

• Hero Member
• Posts: 749
##### Re: related to camera
« Reply #3 on: 23/06/2005 17:34:13 »
We have to watch our millimeters. "f" ratings are all about focal length ratio to objective lens diameter ratio. In your original example, an f2.8 lens with a focal length of 8.64 mm has a diameter of 8.64mm/2.8=3.09mm.

The example I gave was about an f2.8 camera with an 80 mm focal length; a different case. I hope I didn't confuse you by changing the example case.

When you write "35 mm camera", I suspect you are referring to the film format. This is the other end of the camera. My example of the 80 mm lens was taken from a "35 mm camera", which is my camera, and is why I chose it.

Do you have a specific question about camera(s), or is this just generic?

#### qazibasit

• Sr. Member
• Posts: 344
##### Re: related to camera
« Reply #4 on: 24/06/2005 15:18:55 »
its just generic i was actually reading a photography book by some author and in that he was reffering his cam by saying that this snap was from 50 mm camera and this by a 100mm and this from a telelense and thats why i was asking you about this.

#### gsmollin

• Hero Member
• Posts: 749
##### Re: related to camera
« Reply #5 on: 25/06/2005 02:56:36 »
In camera-talk, when people say "100 mm lens", they mean focal length. Knowing the format of the camera, say 35 mm, the focal length gives you the magnification of the image on the film.

#### qazibasit

• Sr. Member
• Posts: 344
##### Re: related to camera
« Reply #6 on: 26/06/2005 11:39:05 »
ok thank i get this thing

#### The Naked Scientists Forum

##### Re: related to camera
« Reply #6 on: 26/06/2005 11:39:05 »