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Photography Question 
Linda Buchanan
 

Sweet Spot of a Lens?


Does anyone know how to find out what the "sweet spot" is on different lenses I own? I have tried searching but I am not really finding the information. Thanks so much.


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4/11/2008 10:27:03 AM

 
Mark Feldstein
BetterPhoto Member Since: 3/17/2005
  Hey!
Actually, every individual lens may be just slightly different than other lenses of the same focal length and manufacturer. This is something you can easily determine by individual trial and error.
The ideal way is to set up a focusing target at a predetermined distance like 8-10 feet. Put the camera on a tripod, preferably with some kind of leveling device to ensure it's square and plumb to the focus target.
Then focus on the target and use the depth of field preview to stop the lens down to f8.0. You'll probably find the sharpest focus somewhere in the range of about f8.0 f11.5. Check the target and snap exposures starting at those stops. As you climb towards f16 through f22, again check the depth of field, and don't be surprised if the target seems soft or less sharp than it was at, say, 11.5. If it's sharper there or even down to f8 or 8.5, that would be the sweet spot for that particular lens.
Okie dokie? ;>)
Mark


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4/11/2008 11:19:35 AM

 
Linda Buchanan   Thanks Mark, you are always so good to help me, I will try that this weekend. Another dumb question - I have dof preview on my camera, but it is a little difficult for my old eyes to see. I should also get the correct answer viewing these on the computer, right?


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4/11/2008 1:08:10 PM

 
Alan N. Marcus  
Hi Linda,

Mark is on target as always --
Just to confuse you a little:

Camera lenses are converging lenses. Stated a different way, camera lens redirect light and cause light rays from the subject to be bend (change direction) inward. If we trace the path of rays as they transverse a lens and continue till they intersect the digital imaging chip’s surface, such a diagram resembles an ice cream cone. The apex or tip of the cone just kisses off on the surface of the light sensing chip.

Now consider that rays that pass exactly through the center are not redirected. They travel in a straight line until they n strike the digital chip. On the other hand, light rays coming from the periphery (edge) of the lens must be bent (redirected) significantly inward. It is these peripheral rays that are the most likely to miss their intended target. Like a line of shooters at the 50 yard line all firing a volley at a single target in the end zone, the likely result will not be one perfect circular bullet size hole but a larger hole with scalloped edges and a few stray misses. The shooters near the sideline are disadvantaged, they are further from target and thus more likely miss.

As you stop down the camera lens aperture this act blocks peripheral rays. Thus sharpness improves because the percentage of bull’s eye’s versus misses’ increase.

As you continue to stop down sharpness continues to improve. However, soon a higher percentage of rays than before will be just brushing by the edges of the blades of the Iris diaphragm aperture. These glancing blow with an obstacle also cause light rays to go astray. Thus, after a point, of stopping down, sharpness deteriorates. This action is known diffraction. While stopping down does increase depth-of-field, nevertheless sharpness deteriorates at the smaller apertures.

Each lens is different as to it’s sweet spot. As a rule of thumb it is likely to happen when the lens is about two to two or maybe two and a half f/stops down from maximum.

Alan Marcus (marginal technical gobbledygook)
Ammarcus@earthlink.net


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4/11/2008 3:19:47 PM

 
Mark Feldstein
BetterPhoto Member Since: 3/17/2005
  As thorough as always Alan. Thanks !

One thing I might add, Linda, is that your focusing target should be well lit with continuous lighting, say a photo flood or two set at 45 degree angles to the thing and not close enough to set it on fire. That extra gazoft in your lighting will make it easier to see edge to edge top to bottom of the entire frame inside the camera. Get the picture? ;>)

A pleasure as always Linda.
Mark
==========================
Go CUBS !!!!


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4/11/2008 4:47:16 PM

 
Anonymous    The sweet spot is between the focus point and the other surrounding focus points. So if you are off your mark a little your focus will still be good. This only works with higher end cameras where you have 45 focus point close to together. If you pick a focus point on your outside line forget it or on a model where they are to far from each other.

Sarah


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4/13/2008 8:47:27 PM

 
Alan N. Marcus   Every lens has a so-called “sweet spot”. This is an aperture setting that yields maximum sharpness – resolution – contrast. The “sweet spot” is an optical phenomenon that is unrelated to the camera’s focusing mechanism. All lenses have a “sweet spot” this is true for applications apart from cameras.

Not to say that focus points are anything other than marvelous. In the distant past we focused under a black cloth hood with magnifier. Our cameras featured focus scales that forced us to guess at subject distance and then we set the focus dial. Our cameras evolved to include and optical rangefinder and we carried an handheld model to help us with the guess. When the twin lens reflex camera was introduces, the upper lens served exclusively for composing and focusing, it was a twin brother to the taking lens, we loved them. When the SLR’s came out we tossed away our rangefinders and we fell more deeply in love.

Now days, cameras with 47 point of focus are magnificent. Next likely cameras will be mentally coupled to your brain. All you will need to do is look out at the vista, think about a well composed well exposed ---- throw the camera up to your eye and presto --- a prize winner.

Alan Marcus (truly marginal technical gobbledygook)
ammarcus@earthlink.net


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4/14/2008 9:15:08 AM

 
Rich Collins
BetterPhoto Member Since: 6/24/2005
  Lots of great answers. I just thought I'd add a site which perhaps helps to answer your question from a technical perspective as well. This review of lenses will enable you to see the actual results of this reviewers tests. I know this may not be exactly what you asked but it does include elements of your inquiry & I think you'll find it quite enlightening from a practical sense.Here is the review:
http://www.wlcastleman.com/equip/reviews/17-40/index.htm

And here is the main site:
http://www.wlcastleman.com/


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4/15/2008 12:44:57 PM

 
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