Question about Focal Length

[sbug_adam]

What does it mean when a lens is 75mm-300mm..? My best guess is how wide the iris of the lens opens. The larger the opening the greater distance zoomed. am I even close?

Adam

[Craig Wilkins]

http://photography.about.com/library...def_focall.htm

let me know if this helps :)

[Hogleg 44]

Originally Posted by sbug_adam

What does it mean when a lens is 75mm-300mm..? My best guess is how wide the iris of the lens opens. The larger the opening the greater distance zoomed. am I even close?

Adam

It really has nothing to do with the iris, but the focal length of the lens. You also might think of it as an optical crop, ie; what you see at 300 is an 'optical' center crop of what you see at 75. ('optical'=no pixel loss, vs. 'digital') Also, regarding what you see in the viewfinder with a given lens of 'xx'mm is dependent on the "crop factor" of the sensor in different digital cameras vs what you would see on an actual 35mm film frame.

[sbug_adam]

Originally Posted by Craig Wilkins

http://photography.about.com/library/glossary/bldef_focall.htm

let me know if this helps :)

well, that kinda helps but I'm just curious where do they get the numbers from. Im gonna look for a disected camera online and maybe post it up..

thanks though!

Adam

[chloew]

If you have a simple convex lens (like a magnifying glass) and pass in a parallel beam of light, the lens will focus the light into a point at the focal length of the lens.

In other words, if you ever used a magnifying lens to focus the light of the sun (parallel light beams) to burn bugs or ants, the distance at which all the sunlight is concentrated into a point of light is the focal length of the magnifying glass.

Although a camera lens has many individual lenses it effectively does the same thing. The focal length quoted for a camera lens is the equivalent focal length of a single lens.

For longer focal lengths a smaller amount of the image is projected onto the film or CCD chip and therefore provides greater maginification.

[sbug_adam]

I totally understand now. thanks christian..!

adam

[Hogleg 44]

Originally Posted by sbug_adam

well, that kinda helps but I'm just curious where do they get the numbers from. Im gonna look for a disected camera online and maybe post it up..

thanks though!

Adam

Try this. maby it will help.

http://www.asterism.org/tutorials/tut06-1.htm