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Barbara & Tom Latino
 

How to Understand Lens Specs


Please share with me what lens specs mean. For example: 105MM 1.2.8D and 18-70 1:3.5-4.5G
Thank you.


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1/23/2006 7:11:44 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  #1

105MM is the focal length. This is how far the glass is from the focal plane where the film or chip sits. 1:2.8 is the maximum aperture. F/2.8 is the maximum aperture you can go to on this lens. Not sure what "D" is, usually the letters are different from one manufacturer to the next.
#2
18-70MM, same thing but a zoom. The focal length can be anywhere from 18-70mm meaning the glass at the short end is 18mm from the focal plane and 70mm on the long end of the focal plane. 1:3.5-4.5 meaning the maximum aperture at 18mm is f/3.5 and the maximum aperture at 70mm is f/4.5.
Hope this helps.


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1/23/2006 7:28:11 PM

 
Bob Fately   To expand on what Justin has stated:

The focal length of a lens is the measurement from the image plane (the film or chip) to the point at which the light rays "cross over" - think of the high school schematic of how a lens works with the tree (the subject) in the distance and light rays drawn from the top and bottom of the tree to the cylinder representing the lens, where the rays cross-cross. The distance from that cross-cross point to the film is the focal length.

The significance of focal length has to do with the angle of view - a longer focal length has a narrower angle of view, meaning that it "sees" less from side to side than a shorter focal length lens. But, to fill the frame, that means that what it does see is magnified.

The human eye apparatus basically has about a 45 degree angle of view; this is what you'd consider "normal". As it happens, for a 35MM film camera a lens of 50MM focal length has about that same angle of view, and is thus considered to be a "normal" lens. For other film or chip sizes, the "normal" lens is a different focal length - so, a medium format camera (like a Hasselblad) has an 80MM lens as it's "normal" lens - the angle of view on 2-1/4" film for an 80MM lens is about the same as a 50MM lens on a 35Mm film camera.

I know, it sounds a bit confusing - much easier to describe with a pen and paper.

Anyway, in the 35MM world, lenses shorter than 50MM focal length are wide angles, and longer than 50MM are telephotos. More MM effectively means more magnification. Using a 50MM lens as "normal", this means a 100MM lens is about a 2 power magnifier (or 2X), a 300MM lens would be 6X, etc.

Zoom lenses, like te 18-70, allow you to change the optical distance from that criss-cross point to the film or chip (even if the lens doesn't get physically longer, internally that's what's happening.

The two lenses you describe sound like Nikon lenses, the 105MM is also capable of focusing very close, and is referred to as a Macro lens. That is, while most lenses cannot get you close enough to make the image recorded on the film equal in size to the actual subject, a Macro type lens can focus close enough to do that.

The zoom you mention is a wide-angle -to-telephoto lens; it is designed to be more of an all-purpose lens for general photography.

I hope that wasn't too confusing...


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1/23/2006 8:50:41 PM

 
Justin G.
BetterPhoto Member Since: 7/13/2004
  Wow thanks Bob for the info and I do appreciate you "expanding" on my words without making me sound like an idiot. I didn't realize it was where the rays crossed, I always thought it was the physical measurement of the glass for some reason. Thanks again.


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1/23/2006 9:15:56 PM

 
Jon Close
BetterPhoto Member Since: 5/18/2000
  Regarding "D" and "G" - these specify certain lens attributes in the Nikon system. "D" lenses (aka AF-D) communicate focus distance to the camera for use in calculating TTL flash exposure. "G" lenses also communicate focus distance, but do not have an aperture ring on the camera. This limits their use to later model autofocus bodies that set the aperture electronically with controls on the camera.

See http://www.nikonians.com/html/resources/nikon_articles/other/compatibility.html for more information.


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1/24/2006 6:35:37 AM

 
Jon Close
BetterPhoto Member Since: 5/18/2000
  Arrrggh. Regarding "G" lenses, "... but do not have an aperture ring on the camera." should be "... but do not have an aperture ring on the LENS."


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1/24/2006 6:37:42 AM

 
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