lens sizes

Joined: 11 Oct 2005 Posts: 176 Location: philly

i know what the numbers mean, like 20mm, but wat exactly are they measuring?

Posted: Wed Mar 21, 2007 12:36 am

beats me.

P.S.
I like you avatar

Joined: 27 Oct 2004 Posts: 452 Location: Gloucester, UK

den9 wrote:

i know what the numbers mean, like 20mm, but wat exactly are they measuring?

The focal length of the lens.

Joined: 21 Oct 2006 Posts: 350 Location: Ontario, Canada

den9 wrote:

i know what the numbers mean, like 20mm, but wat exactly are they measuring?

The measurement, 20mm in this case refers to the focal length of the lens. A simple illustration of focal length would be the distant that you would hold a magnifying glass from a paper to burn a hole in it with the focussed sun's rays.

If the numbers on the lens are hyphenated such as 24-70mm, the focal length is a zoom with a variable FL.

Other numbers indicated filter size ie. (theta) θ 72mm and the aperture as a ratio ie. 1:2.8 which indicates a f/2.8 lens. If the ratio is hyphenated ie. 1:3.5-5.6, the aperture varies as the lens' focal length is changed (zoomed) from wide to tele.

Posted: Wed Mar 21, 2007 3:24 pm

Pturton how on earth did you get a theta out of the forum? thats very impressive!

still I don't think it's supposed to be a theta, it's just shorthand for diameter (a circle with the line through it representing the measurement).

Joined: 21 Oct 2006 Posts: 350 Location: Ontario, Canada

Mongoose wrote:

Pturton how on earth did you get a theta out of the forum? thats very impressive!

still I don't think it's supposed to be a theta, it's just shorthand for diameter (a circle with the line through it representing the measurement).

Guess I should have put my trifocals on when I looked at my lens 8-)

You are right, it is not a 'theta' but an oval or circle with a slash through it. Thinking thread diameter, theta made sense to me.

To get the θ character, I cut and pasted it from a Google search for "theta".

Another search on Google shows I should have used the diameter symbol (ø). The empty set math symbol could also be used Ø.

Joined: 11 Oct 2005 Posts: 176 Location: philly

i knew the numbers meant how far it will bring the image closer, im guessing focal length is how close the lense inside are, like 50mm apart.

how do u get zoom X's numbers

Joined: 02 Jan 2007 Posts: 190

I'm not sure if this is completely accurate, but I think the focal distance's "mm" value is obtained from the distance that the focal point is from the inner lens.

The zoom X's (times) numbers are calculated on the point and shoot camera's based on their inherent focal lengths. (ie: 40mm to 400mm = 10x zoom)

Joined: 21 Oct 2006 Posts: 350 Location: Ontario, Canada

A single element 20mm FL lens that is 20 mm from the film/sensor plane will interfere with the mirror mechanism.

A lens' focal point is corrected to focus on the film/sensor plane and keep it clear of the mirror.

If you are talking about "how far it will bring the image closer" the normal focal length for a 35mm FF is 50mm which approximates the view of the eye. A 200mm lens would make a subject appear 4 times closer.