ARShutterbug
Veteran Member
Every lens behaves differently, but the t/stop effect of aperture reduction is more-noticeable on larger and more-complex lenses. It is most-noticeable on my 70-200 lens, where the difference between whole f/stops can be around 1/3 t/stop, particularly during long-exposures.
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http://www.pbase.com/arshutterbug/
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http://www.pbase.com/arshutterbug/