AF at f8

[Roy Churchill]

Just wondering if there is any technical reason why Canon cannot put AF (centre point) at f8 on 1.6 croppers or is it just something that Canon are reserving for the one series.

[Axel Hildebrandt]

Interesting question, I'm not aware of a technical reason. My best guess is that it would achieve focus lock not as fast. If I'm not mistaken, 1D bodies have two processors and the other models have only one.

[Roger Clark]

Roy,
If you were doing photography back before AF in filom days with microprisms, by about f/8 they became pretty useless. Similar with AF systems. The AF system works on an out-of-focus section of the incoming light beam. By f/8 there is not much spread of the light to get that out-of-focus area (this is all inside the camera close to the focal point). It's a depth of field issue. So, while technically it would work, it would not be very accurate and that would cause a lot of complaints.

The 1D series cameras have more sensitive AF sensors and the specs for focus lock are tighter. Even with the 1D series, f/8 focus is only on the center AF sensor. That sensor is probably more and finer pixels to detect the tighter beam of light from the f/8 light cone.

Roger

[Michael Yessik]

I know nothing about Canon, but Nikon systems work such that all autofocus operates with the lens wide open, and it only stops down after AF is locked and you fully depress the shutter. I assume Canon is the same, so only the rated f number of the lens matters for AF, not the particular setting for your shot.

[Roy Churchill]

I know nothing about Canon, but Nikon systems work such that all autofocus operates with the lens wide open, and it only stops down after AF is locked and you fully depress the shutter. I assume Canon is the same, so only the rated f number of the lens matters for AF, not the particular setting for your shot.

Thank you Michael but you are missing the point of my query. Yes, I know that the AF operates with the lens wide open and only stops down when you take the shot. I am talking about AF when you have a f5.6 lens with a 1.4 tc attached (so wide open is f8). Canon one series will AF at f8 on the central focus point but non one series will not.

[Roy Churchill]

Roy,
If you were doing photography back before AF in filom days with microprisms, by about f/8 they became pretty useless. Similar with AF systems. The AF system works on an out-of-focus section of the incoming light beam. By f/8 there is not much spread of the light to get that out-of-focus area (this is all inside the camera close to the focal point). It's a depth of field issue. So, while technically it would work, it would not be very accurate and that would cause a lot of complaints.

The 1D series cameras have more sensitive AF sensors and the specs for focus lock are tighter. Even with the 1D series, f/8 focus is only on the center AF sensor. That sensor is probably more and finer pixels to detect the tighter beam of light from the f/8 light cone.

Roger

Thank you for your detailed response Roger - it makes sense. I guess from what you are saying is that it would be technically possible to have the same AF system on the smaller sensor but cost would prohibit it.

[John Chardine]

Canon of course could put an AF sensor in their xxD or 5/7D bodies that would work at f8 but they play the same game every other company does so that they maximise their markets for each model and reduce direct competition between models. Good analogies would be Apple's iPad versus the MacBooks (the former is close, but will never be, the latter). Same with Photoshop Elements and CS5.