Monday, June 7, 2010

Understanding Focal Length and its Extremes

When purchasing a lens or deciding which lens is right to use for a particular situation focal length is something that is very important to consider.

So what is focal length?
Focal length (or focal distance) is defined as the distance of the lens to the point of focus.  It refers to how strongly the lens converges and diverges the light.  The shorter the focal length the stronger the light is bent, which brings objects that are closer into focus.  A long focal length is associated with a larger magnification, but also a narrower view. 

So why do more mm's mean a closer look?
The higher your focal length the closer your can zoom in.  This is because the longer your focal length (literally the length of your lens) the further away the focal point will be.  The opposite is true for wide angled lenses.  Here is an illustration from this great article:

Understanding and calculating focal length can actually get pretty in depth, way more in depth than I care to explain (partially because I despite calculus).  Here is an explanation from Wikipedia that explains focal length in great intellectual detail. 
In general, the focal length or EFL is the value that describes the ability of the optical system to focus light, and is the value used to calculate the magnification of the system. The other parameters are used in determining where an image will be formed for a given object position.
For the case of a lens of thickness d in air, and surfaces with radii of curvature R1 and R2, the effective focal length f is given by:

where n is the refractive index of the lens medium. The quantity 1/f is also known as the optical power of the lens.
 Does that make any sense?  No?  Good, because it doesn't to me either. 

So just how close can you get while being far away?  Check out these extreme lenses!

If you are interested in a super telephoto lens you can purchase an 800mm Canon lens for just under $11,000 at Adorama. It weighs a whopping 9lbs and allows you to shoot up to 20 meters away in clear focus.

Canon also makes a 1200mm lens that has been dubbed "The Mother of All Lenses".  It boasts being the longest telephoto lens ever made by Canon and is available used from B&HPhoto for a whopping $120,000.  Just to lease it will cost you over four grand.  The lens weighs 36lbs and is 33 inches long.  To move it once it is in its protective case you'll need a wheeled cart.  How far can you shoot with this baby?  At least two miles, but image clarity greatly depends on air quality at that distance.  For more information about this lens and some sample photos of it in action please go here.

So what is the longest and most powerful telephoto camera lens ever made? 

In the early 1960s Canon made a 5200mm lens.  It weighed an unfathomable 100kg and could shoot 18-32 miles away! The camera is actually that tiny little thing attached to the right of the lens.  So if you are interested in taking a close up shot of a lions eyeball from over 100 meters away this is the camera for you!   Here are some photos detailing just how incredible this lens is. 

That is about as extreme as it gets in the photography world when it comes to focal lengths.  Any larger and you are brushing up against the field of astronomy and telescope focal lengths which are measured in meters rather than millimeters. 

1 comment:

  1. Those lenses are awesome. When the digital revolution first came about, a pro guy who shot up in the Arctic came in to our store and traded in all his lenses as they weren't compatible anymore. That must've been painful!

    He traded in a Sigma 800mm lens. Staff perks allowed us to lease second hand equipment for personal use. I remember it being super heavy, fantastic quality but a little OTT for pics in the park!!


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