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  Topic: Lou FCD's photo corner, Technics, equipment, photos...< Next Oldest | Next Newest >  
Wesley R. Elsberry



Posts: 4482
Joined: May 2002

(Permalink) Posted: April 19 2012,00:16   

Quote (midwifetoad @ April 18 2012,20:47)
For folks using auto-focus and auto-exposure, the transmission factor of the glass is irrelevant, unless one brand is crappy compared to another.

Movie makers need to work like Ansel Adams and control the luminosity of ever element in the frame. Or at least know how it's going to look.

That's why actors sit around for hours sometimes, waiting for the light to get right.


That's a bit too glib. Unless you are using a Nikon D4, your 500mm f/8 mirror reflex lens would not autofocus even if it had autofocus capability. Autofocus modules stop working somewhere around f/5.6 to f/6.3. The D4 applies some of its high ISO capability to its autofocus module, which will accommodate lenses out to f/8.

This more commonly gets encountered when using teleconverters. A lens that will autofocus fine on its own will often fail to do so when you add a 2x teleconverter and essentially add 2 to its maximum f-stop. [ETA: That's unclear... you have to treat the lens as being two more f-stops slower, so an f/4 lens with a 2x teleconverter would be equivalent to an f/8 lens, while an f/5.6 lens would be equivalent to an f/11 lens.]

Autoexposure shouldn't have that working limit, but you do have to account for other transmission losses if you want to use handheld light meters that aren't tied into the optical system at issue and are using sensors or emulsions with limited dynamic range. Losing a half-stop or more of accuracy simply is not acceptable under critical use conditions. Calibration is essential then, and that is what Ansel Adams was famous for in his advocacy of the Zone System.

Edited by Wesley R. Elsberry on April 19 2012,00:31

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"You can't teach an old dogma new tricks." - Dorothy Parker

    
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