Aperture/focusing distance changes at different FL for zoom lenses?

Started Mar 21, 2013 | Discussions thread
revio Senior Member • Posts: 1,731
Re: Aperture/focusing distance changes at different FL for zoom lenses?
It's not that the aperture closes down, but that the focal length increases.

Of course the aperture closes down! As the focal length increases, the aperture closes.

That's what it means when the number goes from 3.5 to 4 to 5.6 etc!

Remember that the f-number is the ratio of the focal length to the entrance pupil. Take for example the very nice M.Zuiko 40-150 mm zoom. At 40 mm, its relative aperture is 4.0 => 40/4.0 = 10 mm. At 150 mm, its relative aperture is 5.6 => 150/5.6 = 27 mm. No closing down there.

Closing down physically or not, the usable F:no, seen as "usabel light input", decreases as F:no gets changed from 3,5 to 5,6 for example

F:5,6 demands a longer shutter time (or "slower speed", if that is better understood) than F:4.

Most amateur users don´t even have to know that aperture expressed as F-number is a ratio of entrance pupil vs f-lenght, but, of course, it´s never bad TO know 

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