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Old 3rd May 2010, 06:13 AM
Dave miller Dave miller is offline
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Quote:
Originally Posted by Mike O'Pray View Post
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If I may make an observation which I hope doesn't get this post off topic. I have noted that with my El Nikkor 50mm lens and leaving the analyser probe on exactly the same spot and if I get a reading of say 8 secs at f8 then opening up to f5.6 doesn't give me 4 secs exactly. It is close but always slightly more than 4 secs.

I can only think that in reality my f5.6 doesn't quite have a linear relationship with f8. In other words I am assuming that my f5.6 is not quite double the aperture of f8.

Could there be another explanation?

Mike

The lens aperture control is a mechanical device built with an allowable tolerance; therefore the steps must be regarded as approximate until tested and proven. In addition older lens may have aperture cams worn to the extent that an appreciable difference may be found in reaching a setting in an upwards direction to that reached when closing the lens down.
Time control on the other hand (if made by an electronic timer) should be much more accurate and repeatable, hence my preference for relying on the timer rather than aperture for exposure control.
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