Trying to understand AF point sensitivity?

[mark_stephan2]

<p>I have a 5D and 40D and I've read that the center point is a cross type sensor. Is it a cross type sensor when I use my 24-105L? Is it a cross type sensor with my 50/1.8 II? I've read that it's only sensitive to lenses that are f/2.8 or faster, is this true?</p>

[steve_dunn2]

<p>It varies from one model to another, and I don't know the details for either of those bodies. For instance, my 7D's central sensor is cross-type even with f/5.6 lenses, which is pretty common for non-pro EOS bodies with cross-type sensors. It does have an additional feature; with lenses that are at least f/2.8, it switches into high-precision mode. I think the central sensor on my previous 20D had similar features but it's a few years since I've had that and my memory may be faulty. Some bodies, particularly pro ones (since they're more likely to be used with fast lenses more often than consumer bodies are), need a faster lens, usually f/4 or f/2.8, to enable the cross-type sensor; with slower lenses, they revert to only line-type sensors.</p>

 

<p>The manual for my 7D describes how this works. Hopefully the manuals for your bodies do, too. Failing that, the Wikipedia page for the 40D says "Wide 9 point AF (all points are cross-type with the centre point having an additional f/2.8 sensitive cross-type sensor placed diagonally)" but a quick poke around didn't find me similar detail for the 5D. FWIW, since the 5D's AF sensor was based on that of the 20D, chances are fairly good it shares the same features on its central sensor.</p>

[JosvanEekelen]

<p>Canon's Chuck Westfall used to write about subjects like this on <a href="http://www.digitaljournalist.org">www.digitaljournalist.org</a> but that site seems to be down, perhaps for a long time. The internet archive has the site backupped quite well, see <a href="https://web.archive.org/web/20070609092802/http://www.digitaljournalist.org/">https://web.archive.org/web/20070609092802/http://www.digitaljournalist.org/</a> for an example.</p>

<p> </p>

[PuppyDigs]

<p>The 5D and 5D MKII have a cross-point in the center, with 8 single axis points around it. If you use Ai-servo, there is a cluster of assist points that activate around the center.</p>

<p>All the points on the 40D are cross points but the center point is more sensitive. What happens when you use a F 2.8 or faster lens is the center cross kicks in a higher level of accuracy. </p>

[mark_stephan2]

<p>Thank you for the explanations. I've only used Canon gear for a few months and appreciate your help! I now have a better understanding of how the AF points work.</p>