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Author Topic: Minimum Aperture  (Read 3120 times)

gferdinandsen

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Minimum Aperture
« on: September 28, 2012, 08:44:48 AM »
What goes into making the minumum aperture?  I mean whay does my 70-200 go down to f/32 but my 50 only goes to f/16?  Why can't the 24-70 go that extra stop down from f/22 to f/32?
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Minimum Aperture
« on: September 28, 2012, 08:44:48 AM »

AmbientLight

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Re: Minimum Aperture
« Reply #1 on: September 28, 2012, 09:30:57 AM »
I am not a lens developer, but as far as I can make out it appears to be a direct result of lens specifications. A lens optimized for large apertures will not necessarily offer very small ones. Your 50mm lens is a prime example of this. Then again some lenses offer extremely small apertures, but I doubt that those lenses have been designed for minimized apertures. It is just possible without much ado to deliver extremely small apertures with some lens designs (for example tele lenses) and it is more difficult for other lens designs with shorter focal lengths (for example standard range zooms like the 24-70mm). There are some articles on CanonRumours about lens design, which will shed more light on this topic compared to the tiny bits of information I can provide.
« Last Edit: September 28, 2012, 10:04:55 AM by AmbientLight »

neuroanatomist

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Re: Minimum Aperture
« Reply #2 on: September 28, 2012, 09:51:22 AM »
I mean whay does my 70-200 go down to f/32 but my 50 only goes to f/16?  Why can't the 24-70 go that extra stop down from f/22 to f/32?

Where do they start?  Consider: 50/1.2 to f/16 is 7.33 stops; 70-200/2.8 to F/32 is...7 stops.  Given the physical size of the blades, and the number of blades, there's a practical limit to how small the hole can get.  So, your 70-200/2.8 closes down to a minimum iris diaphragm diameter of 2.2mm at f/32, the 24-70/2.8 is 1.1mm at f/22 and for it to go to f/32, the iris diaphragm would need to be 0.75mm in diameter.

Your 50mm lens is a prime example...

*rimshot* Ba dum bum.
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IIIHobbs

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Re: Minimum Aperture
« Reply #3 on: September 28, 2012, 01:50:25 PM »

Your 50mm lens is a prime example...

*rimshot* Ba dum bum.

I see what you did there.

quartzie

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Re: Minimum Aperture
« Reply #4 on: September 28, 2012, 02:45:57 PM »
It's typically a matter of image quality - and depends on lens design and intended use.

It has to be said that a suitably small aperture affects image quality because of diffraction patterns when the opening gets small enough. This can be optimized when the lens uses a high quality optical formula (such as the 70-200, but for worse lenses, the diffraction noise effectively precludes such small apertures, as image sharpness drops drastically.

koolkurkle

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Re: Minimum Aperture
« Reply #5 on: September 30, 2012, 05:22:00 AM »
There is a pretty good explanation of diffraction for photographers at http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm.

Videoshooter

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Re: Minimum Aperture
« Reply #6 on: October 02, 2012, 01:19:47 PM »
For an iris opening of a certain constant size, the f-stop will be lower for a lens with a longer focal length. This is because f-stop is the ratio of iris size to focal length. Practically, this means that most telephoto lenses will have a smaller minimum f-stop than most wide angle lenses.
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Re: Minimum Aperture
« Reply #6 on: October 02, 2012, 01:19:47 PM »