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

Started Apr 17, 2011 | Discussions thread
noirdesir
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Re: your assumption limits you
In reply to edwardaneal, Apr 27, 2011

edwardaneal wrote:

I understand your example, but it assumes that nikon does not place any of their cross type sensors to take advantage of f/2.8 and faster glass like Canon does.

As you know Canon has this feature in all of its DSLRs except for the very bottom of the line. in their mid range cameras like the EOS 20d the cross type sensor that enables this is in fact only active when faster lenses are attached, from what I have been able to find out in their higher end camera these sensors actually stay active all the time.

So tell me why do you make the assumption that none of the cross type sensors used in the various Nikon af systems have also been placed to be able to take advantage of f/2.8 and faster glass?

Two simple reasons:

  • (1) Only Canon talks about these f/2.8 AF sensors, nobody else does. Why would Canon trumpet its advantage but no other camera maker does it? Canon very openly tells you which of their AF points get additional precision from f/2.8 lenses and which AF points do not. Following your logic that Nikon and Canon have the same technologies at their disposal, if Canon only adds these f/2.8 sensors to some of its AF points so would Nikon. And why would Nikon then withhold the information which AF points profit from f/2.8 lenses?

This is only circumstantial evidence but pretty strong one. But there is more...

  • (2) You can very simply test whether there are AF sensors for other apertures beyond the f/5.6 ones by adding a blocking ring at the exit of your lens that blocks the f/5.6 ring, as indicated in red in the image below. People have done this with Nikon bodies and the result was no AF action at all. If there were f/2.8 sensors, they would not be blocked and the AF system would be able to focus.

What you provide as evidence that faster lenses have faster and more precise AF is also only circumstantial evidence as it is comparing different lenses, not the same lens manually stopped down to either f/2.8 or f/5.6. And that AF motor speed and accuracy can vary a lot between different lenses is not exactly an unlikely idea, nor is the idea that different optical qualities (think of it in terms of SLRgear-type blur indices) can have an influence on AF accuracy and speed that far-fetched.

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