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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i disagree
In reply to noirdesir, Apr 20, 2011

I disagree. The focus system may only need as much light as it can get with an f/5 or 5.6 lens in reasonable light and it may not focus faster or more accuratly with more light, but when you get that f/5 or f/5.6 lens in lower light the focus system can start to miss focus and hunt for focus where as if you had an f/1.4 or f/2.8 lens on the camera the lens would pass as much light to the af system in low light as the f/5 or 5.6 lens did in good light.

sometimes if the light gets too low my 70-300vr will hunt for focus. In these same lighting conditions my primes do not.

If you think I am wrong test it. mount a variable apreture zoom lens and get it in a low light situation at the long end where it is only f/5.6. by sure your AF assist is turned off - I would bet it is fairly easy to get it hunt and not focus well. Then try it with an f/1.4 prime in the same light on the same focus target.

go look here

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

a quote

"The data collected from AF sensors is used to control an electromechanical system that adjusts the focus of the optical system. A variation of autofocus is called an electronic rangefinder, a system in which focus data are provided to the operator, but adjustment of the optical system is still performed manually.

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.

Most consumer camera systems will only autofocus reliably with lenses that have a maximum aperture of at least f/5.6 , while professional models can often cope with lenses that have a maximum aperture of f/8, which is particularly useful for lenses used in conjunction with teleconverters."

noirdesir wrote:

Anything faster than f/5.6 (or probably f/5) is lost on the phase-detect AF (except on some Canon bodies which have a few special f/2.8 focus points). The phase-detect AF sensor in all Nikon bodies only sees light coming from the 'f/5.6 ring'.

One more detailed explanation is in this post:
http://forums.dpreview.com/forums/read.asp?forum=1018&message=17616077

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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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