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Photography Question 
david 
 

focal length


i am quite new to photography,but have started a course at night school.can you please help with a simple way to work out focal lenght.i have read two or three books but cant seem to get my head round this


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2/22/2006 2:44:16 PM

 
Bob Fately   David, focal length is basically a measurement regarding the optics of a given lens. Think of a schematic drawing of a simple lens - with a tree on the right, the lens in the middle, and the film (or chip) on the left. Imagine those lines representing the light rays, coming from the top and bottom of the tree, cross-crossing inside the lens, and landing on the imager on the left side. Got that picture in your head?

Okay, focal length is simply the distance from where the light criss-crosses to the imaging plane. That's all it is. It is usually measured in millimeters (MM), though can be measured in inches (or, indeed, light years - any measure of length will do).

Now, what focal length implies is the angle of view that the lens in question provides. a shorter focal length has a wider angle of view than a longer focal length, on the same size imager. Take that sketch I described and imagine moving the lens to the left - then redraw the lines so they start at the top and bottom of the imager's line and criss-cross at the middle of the lens - you will see that the angle is wider than when you draw the lens further to the right and draw two lines at that distance.

A long focal length lens has a narrow angle of view, meaning that it brings in, or magnifies, whatever it does see.

A normal lens is generally defined as one with a view that more-or-less mimics the human eye. Now, what constitutes a "normal" lens depends on the size of the imaging system. For a view camera with 4x5 inch sheet film, a 150MM lens is pretty much normal. For a medium format film camera, with 2-1/4 inch film, an 80MM lens is normal. For a 35Mm film camera, a 50MM lens is normal. For most Digital SLRs, a 35MM lens is normal. As the imaging device gets physically smaller, what constitutes "normal" gets smaller as well.

A telephoto lens is one longer than a normal lens. A wide angle is whatever is shorter in focal length. A zoom lens is one that allows you to change the focal length (usually by twisting a ring) - so a 70-300MM is a telephoto zoom, and a 28-200MM is a wide-angle-to-telephoto zoom.

Does that help?


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2/22/2006 3:24:48 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
 
 
  Focal Length Illustration
Focal Length Illustration
© Justin G.
Miscellaneous Does...
 
 
Diagram illustrating Bob's points.


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2/22/2006 3:47:42 PM

 
david    bob thank you very much for your quick reply to my question.you have made things a lot claerer to me, I can know progress further.thank you once again


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2/22/2006 4:20:49 PM

 
Bob Fately   Well, let's thank Justin as well for finding that schematic I was trying to describe. Talk about a picture being worth a thousand words...


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2/22/2006 5:54:42 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  I actually just drew it up in Paint. It's free to have for anyone I'll post the link.

www.betterphoto.com/uploads/processed/0026/0602221549021focal_length.jpg

just click the link and then you can right-click and save, in any browser. I see bob that you are always answering the focal length questions! so you can have it! lol.


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2/22/2006 6:01:14 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  dang it the first link is broke. try this.

www.betterphoto.com/uploads/processed/0026/0602221549021focal_length.jpg


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2/22/2006 6:01:59 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  alright still not working..

www.betterphoto.com/uploads/processed/0026/0602221549021focal_length.jpg


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2/22/2006 6:02:41 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  third times the charm, that one works!


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2/22/2006 6:03:37 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  third times the charm, that one works!


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2/22/2006 6:03:37 PM

 
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