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2 lens questions
  
 
joanlvh
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p.1 #1 · p.1 #1 · 2 lens questions


a question , when I use a lens with minimum forces distance of 1.64, I can obviously use the lens at longer focal distance. I am finding that when I use a Nikon extension tube I can only focus at close distances, not at longer ones. Is this the way it should work, extension tubes make the lens good only at close distances?

also, if I want a 50mm autofocus using Sony Nex , is there any reason to buy the new 55mm FF lens as opposed to the 50 mm Touit which is only for aspc lenses? thank u joanlvh



Nov 01, 2013 at 12:31 AM
Ernie Aubert
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p.1 #2 · p.1 #2 · 2 lens questions


The answer to your first question is that adding an extension tube inherently has the effect you've noticed; it can focus closer, but you lose infinity focus. The longer the tube, the closer the lens will focus, and the more you lose when the focus is set to infinity.


Nov 01, 2013 at 03:09 AM
DanBrown
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p.1 #3 · p.1 #3 · 2 lens questions


Here's a nice explanation of extension tubes.


Nov 01, 2013 at 03:39 AM
jcolwell
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p.1 #4 · p.1 #4 · 2 lens questions


joanlvh wrote:
...also, if I want a 50mm autofocus using Sony Nex , is there any reason to buy the new 55mm FF lens as opposed to the 50 mm Touit which is only for aspc lenses? thank u joanlvh


Generally, a lens designed for the APS-C size sensor will have an "image circle" with a diameter that's just a bit larger than the diagonal of the sensor. That's shown for an APS-C sensor inside the image circle of an APS-C lens on the bottom right of the attached figure.

The situation with a full frame sensor and an APS-C lens is shown on the bottom left. This usually results in serious 'hard' vignetting, sort of like the example photo below of the "stupid bird", cropped by a circular mask about the size of an APS-C image circle.

(original stupid bird, http://www.fredmiranda.com/forum/topic/1250953/0#11902327)













Nov 01, 2013 at 02:45 PM
joanlvh
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p.1 #5 · p.1 #5 · 2 lens questions


thanks all


Nov 01, 2013 at 03:22 PM





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