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

Started Apr 17, 2011 | Discussions thread
SteveL54 Senior Member • Posts: 2,709
Re: And Wikipedia is wrong in this regard

There's no argument that several of Canon's AF sensors have elements designed to take advantage of an f/2.8 lens. All the references you site are either Canon based or influenced by Canon's literature. So they naturally state the advantage in AF operation of f/2.8 lenses over f/5.6 lenses.

But the context here is Nikon AF performance. Have you found Nikon sources claiming improved low light AF performance with bright lenses?

Nikon designs their CAM 3500 AF sensor to work at f/5.6.


"If all 15 cross-type sensors were able to demonstrate their full performance with any NIKKOR lens without exception, the camera would easily capture a subject,...

...indeed, that’s what we accomplished: All 15 cross-type sensors retain the same outstanding level of performance with any NIKKOR lens as fast as f/5.6** "

The entrance pupil of an AF system has to be specifically designed to accept light from a very limited region of the main lens. This can be f/5.6, f/2.8 or any aperture. The problem with AF modules that are designed to use f/2.8 require f/2.8 lenses to even work. Canon gets around that by intermixing or staging AF sensors with differing f-stop compatibilities.

Did you study the PDAF diagram in that Wikipedia article that you referenced? It's from a Nikon Patent. The figure and the patent identify the pair of apertures (73 and 74) within the AF unit that restricts where it takes light in from the camera's main lens. A pair of images from opposing sides of the main lens are compared to determine the defocus amount.

The main camera lens's exit pupil is the large disk labled 30. The limited regions that the AF system views through that lens from this exit pupil are represented by disks 31 and 32. Increasing the exit pupil of the main lens (by mounting a brighter lens on the camera) will not increase the light collected through regions 31 and 32.

Canon and Nikon have taken different appoaches. Each has its advantages and drawbacks.

The tests Marianne and I have done back up that Nikon's AF central sensor is designed to work at f/5.6. Could there be a little bleed-over into the AF system's entrance pupil from areas outside the f/5.6 region of an f/4 lens? Maybe, but there's not going to be any dramatic difference with an f/2.8 lens.

I've used f/5.6 and f/2.8 lenses in darkened environments on my D70 and D300. I have not noticed an increased tendency of the f/5.6 lenses to hunt over the f/2.8 lenses.

edwardaneal wrote:

exactly how I feel - more light = more contrast - but you refuse to hear it

More contrast? Brighter lenses operating at full aperture actually have reduced contrast due to the increased aberrations. They are just brighter.

They appear brighter to the main sensor which is designed to accept light from a wide angle. The AF modules are designed to accept light from a fairly restricted angle, hence most of that light doesn't get into the AF detectors.


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