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

Started Apr 17, 2011 | Discussions thread
noirdesir
Forum ProPosts: 10,953
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Re: And Wikipedia is wrong in this regard
In reply to edwardaneal, Apr 22, 2011

edwardaneal wrote:

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

Since it is physically impossible for the extra light an f/2.8 lens provides over a f/5.6 lens (in reality the cut-off will be slightly higher than this), there must be another explanation, eg, the faster lens being sharper in the relevant f/5.6 ring than the slower lens or the slower lens having slower and less precise AF mechanics and electronics.

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

Again correlation is not causation, in particular if the proposed explanation is physically impossible.

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).

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 that only applies to certain Canon bodies which have distinct f/2.8 AF sensors. Think about it, why would Canon explicitly declare that a limited set of their AF sensors produces higher accuracy with f/2.8 lenses but other sensors do not? It is simply that some their sensors use the f/2.8 ring and others the f/5.6 ring. No other camera makes a similar claim.

Have a look at the link and images I posted in another post her today, tell me what you don't understand about it. And have a look at Marianne's posts on the subject, for those not believing all the technical descriptions of how PD-AF works she has done some tests that show that when you block the f/5.6 ring, no light reaches the AF sensor.

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