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08-07-2016, 05:54 PM   #16
Pentaxian




Join Date: Feb 2009
Location: Arizona
Posts: 895
QuoteOriginally posted by Bob L Quote
Adding a diopter/close up filter effectively increases the focal length of the native lens
No it doesn't. It must be remembered that when shooting at macro distances, light rays are diverging when they strike the front element. Light rays from distant objects enter a lens in a parallel manner. What this means is that for a lens to image an object with diverging rays, it must extend it's focal length to accommodate this deviance from parallel. This can be done with tubes. Achromats don't extend the focal length to accomplish this. All the achromat does is retain the original focal length of the main lens in macro situations. It bends the diverging rays close to parallel so that the main lens can focus them. Sorry to be so wonky.


Last edited by desertscape; 08-08-2016 at 09:19 AM.
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