If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
edwardaneal
Senior MemberPosts: 9,101Gear list
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Re: And Wikipedia is wrong in this regard
In reply to noirdesir, Apr 22, 2011

forget focus speed - if you are correct please tell me why my f/5.6 lens hunts for focus and sometimes cant even lock focus in low light yet my primes dont hunt and easily lock focus in exactly the same light

I have tested this many times and with every variable aperture lens I have owned and I have always been able to duplicate it

there is a nice article on auto focus systems and specifically phase detect autofocus systems on lensrentals.com here:

http://www.lensrentals.com/blog/2010/07/how-autofocus-often-works

and here is a quote from the section specific to phase detect

" The Effect of Lens Aperture

No matter what the sensor type, however, it will usually be more accurate with a wider aperture lens . Remember, during autofocus the camera automatically opens the lens to its widest aperture, only closing it down to the aperture you’ve chosen just before the shutter curtain opens. Phase detection autofocus is more accurate when the light beams are entering from a wider angle. In the schematic below beams from an f/2.8 lens (blue) would enter at a wider angle than those of an f/4 lens (red), which are still wider than an f/5.6 lens (yellow). By f/8, only the most accurate sensors (usually only the center point on the more expensive bodies) can function at all, but even then focus may be slow and inaccurate. This is the reason our f/5.6 lenses stop autofocusing when we try to add a teleconverter, which changes them to f/8 or f/11 lenses."

and another quote from the same article

" As mentioned above, lens aperture can also affect phase-detection autofocus accuracy . Usually this matters little, because the smaller aperture lens will have a greater depth of field. There is a point, however, where the aperture is too small for the sensor to autofocus accurately, usually at f/5.6 or f/8. (Remember that the camera automatically opens the lens aperture to maximum during autofocus, so the aperture you’ve set the lens at doesn’t matter, it’s the maximum aperture the lens can achieve that matters.) It also is the reason that f/2.8 lenses can sometimes autofocus in more difficult conditions than lenses of lesser aperture .

noirdesir wrote:

edwardaneal wrote:

http://en.wikipedia.org/wiki/Autofocus

The speed of the AF system is highly dependent on the maximum aperture offered by the lens . F-stops of around f/2 to f/2.8 are generally considered optimal in terms of focusing speed and accuracy. Faster lenses than this (e.g.: f/1.4 or f/1.8) typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light.

And Wikipedia unfortunately is wrong in this. Notice that it gives no technical explanation of how phase-detect AF works at all. No technical explanation as why things are as it claims them to be.

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My kit - D200, 10.5mm f/2.8D, 35mm f/1.8G, 50mm f/1.4G & 70-300VR
SB800, SB600 and other misc lighting equipment

Lenses worth mentioning owned and sold– 12-24 f/4, 17-55 f/2.8, 35-70 f/2.8, 80-200 f/2.8, 20mm f/2.8, 35mm f/2, 50mm f/1.8, 50mm f/1.4D, 60mm f/2.8D, 85mm f/1.8, 105mm f/2D-DC, 180mm f/2.8, 300mm f/4D-ED

 edwardaneal's gear list:edwardaneal's gear list
Sony Alpha NEX-7 Sigma 60mm F2.8 DN Carl Zeiss Touit 1.8/32
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