AF question

<p>I feel as though I should know this but after wracking my brain I just can't figure out the answer...</p>

<p>We all know that EOS bodies can usually only autofocus with lenses that have max apertures of f/5.6 or wider (f8 for some pro bodies) so why can our cameras still autofocus properly when we have an ND filter or polariser attached?</p>

<p>Example: What's the difference between putting a 1 stop ND filter on an f/5.6 lens or using a 1.4x extender? Both reduce incoming light by 1 stop but one configuration will autofocus and the other won't.</p>

<p>The maximum aperture limit has nothing to do with the amount light. Aperture also limits the angle at which the light strikes the AF sensor. Wider apertures mean wider angles. Phase-detection AF sensors need light from two different directions separated by a certain minimum angle, and the smallest angle that the AF sensor can work with corresponds to aperture 5.6.</p>

<p>I don't think my 5D focusses with an 8 stop ND filter attached.</p>

<p>While we may think the AF/no AF decision should be made on the basis of the light in the scene, it is really a decision made by the camera based on information it gets from the lens or lens/extender combination. Since the filter does not communicate to the camera, the camera doesn't see a change, and continues AF.</p>

<p>Peter, it's so obvious now you mention it. That makes perfect sense, thank you.<br>

That would explain why I can successfully autofocus in a dim room with an f/5.6 lens but not outdoors in blazing sunshine with an f/8 lens. I got it now :-)</p>

<p>There is also the point that Canon simply blocks AF for lenses (or combination of TC and lens) when it drops below that. This is based on nothing else but the communication with the electronics in a lens - not the measured 'incoming' light. One can always argue whether that's a good decision or not - I think it is clear in the sense it makes zero promises beyond its specifications.It's a bit brutal, but fair. Either it works well, or it doesn't at all.<br>

There are plenty people (as far as I've seen) using TCs on consumer 70-300 zooms, thinking a TC is a good cheap way to reach 600mm. And on the other brands, AF pretends to work. I've used a f/6.7 combination, and AF does attempt to work - slow, erratic and unreliable. But at times, it worked fine enough. Unless you know that AF speed and precision are related to max. aperture (and frankly, many people do not), you'd end up very unhappy with the performance, "upgrade" the camera to find it doesn't solve the issue at all, and then feel like the manufacterer is at fault. Canon's approach might be less flexible, but no false promises.</p>