I have a very specific question for a very specific reason.


Are cross type AF points really that much better in still shooting at f4 or smaller??


I own two Rebels, the 450D and the 600D(t3i) and I've been happy with both EXCEPT that I get a fair amount (say 20 of 100) of really soft images that just aren't salvageable. My main lenses are a 24-105, and a 70-200 f4 IS, both of which show this problem. I suspect it has to do with the contrast detection AF but I'm not really sure.


I'm considering the T4i but I honestly don't see much advance over the T3i except for, possibly, the 9 cross type focus points. Since I shoot mostly models and mostly in single point AF (in portrait orientation with the top point on the eyes), I wonder if the cross typeAF point would be noticieably better with f4 lenses.


The video capability means nothing to me.


Little help please from them what understands this stuff.

Originally Posted by JohnGalt_ny ... I own two Rebels, the 450D and the 600D(t3i) and I've been happy with both EXCEPT that I get a fair amount (say 20 of 100) of really soft images that just aren't salvageable. My main lenses are a 24-105, and a 70-200 f4 IS, both of which show this problem. I suspect it has to do with the contrast detection AF but I'm not really sure. ... The video capability means nothing to me. ...

The "Contrast Detect AF" should only engage during video acquisition. In any of the still image modes those cameras, the 450D/XSi and the 600D/T3i, use passive SIR (Secondary Image Registration) autofocus and either single line or cross type AF sensors. (Passive SIR is a type of electronic rangefinder autofocus.)

Single line sensors are only sensitive to either horizontal or vertical edges, depending on the orientation of the sensor. If the sensor is a horizontal orientation, then it is sensitive only to vertical edges, for instance.

A single cross AF sensor is simultaneously sensitive to both horizontal and vertical edges. A dual-cross sensor, like that in the center of the 650D/T4i, is simultaneously sensitive to vertical, horizontal and oblique edges. A very similar AF sensor, if not the same, exists in the Canon 40D/50D60D mid-range cameras. I believe that the combination of dual-center-cross AF, plus 9 cross-type sensors, plus the Digic 5 image processor, will probably all add up to a very satisfying body for casual shooting.

You are correct in that with your constant aperture f4 lenses the center AF sensor is a cross type.

If you are not using an external flash with an AF Assist light for indoor shooting then purchasing a flash with an AF Assist light would help your current bodies dramatically. Otherwise, switching to constant aperture zooms with f2.8 maximum aperture will activate the high-precision center focus dot of many Canon cameras.

I can absolutely guarantee you that an off-center cross-type AF point will be WAY better than a regular off-center AF point. The question is, ...do you get cross-type AF with your lenses and body combo? I don't know about the 650D but some Canon cameras may simply not give you cross-type AF off-center unless you're using an f/2.8 lens or faster. That is what I would check into... (I couldn't tell if Ziggy was saying yes or no on that matter...)

=Matt=

Originally Posted by Matthew Saville I can absolutely guarantee you that an off-center cross-type AF point will be WAY better than a regular off-center AF point. The question is, ...do you get cross-type AF with your lenses and body combo? I don't know about the 650D but some Canon cameras may simply not give you cross-type AF off-center unless you're using an f/2.8 lens or faster. That is what I would check into... (I couldn't tell if Ziggy was saying yes or no on that matter...) =Matt=

Good points.

I should have concluded my statements. Because a cross type AF sensor is sensitive to more edge angles than a line AF sensor, it will cover more subject matter, i.e. it is twice as good as a line sensor in general subject matter.

The AF module in the Canon 650D/T4i is sensitive with all cross-type AF points to f5.6. This means that the Canon EF 70-200mm, f4L IS USM may be fitted with an EF 1.4x teleconverter and still autofocus with all 9 cross-type sensors.

To summarize: Yes, the Canon 650D/T4i should be a significant improvement over the Canon 600D/T3i in terms of autofocus. With more cross-type sensors and with a much faster Digic V processor (versus the Digic IV processor in the 600D/T3i) I expect that the major testing sites should corroborate a major improvement in AF performance.

I cannot stress enough the importance of a flash with an AF Assist light, like that in the Canon 580EX series, the 430EX series, and the Sigma DG Super series flashes. The Canon 580EX is by far the best with a very strong and large AF Assist pattern. This makes an incredible improvement in keeper rate in low light, but a significant improvement in good light too.

John, I'm going to jump in here with a suggestion rather than an answer. I know you say you like the smaller form factor of the Rebels, but I can only say that with the sheer amount of shooting you do.... it's time for a camera with more focus points, more accurate focus points and a few other bells and whistles.

I stuck with my xsi for ages - great little camera - and liked the smaller form too, although I found it was SO small I had to use it with a grip. Thus, when I went first to the 50d and then to the 7d, those actually felt smaller ungripped.

However, the main point here is that the AF is so much better. Plus, you can change your settings so much faster with the thumbwheel (that was the real thing I wanted - I had it on my film camera and really missed it). Most importantly for me, MORE FOCUS POINTS. It makes an enormous difference for me with accuracy + composition + shallow depths of field, because I can use the extra points. Yes, the center one is still more accurate, but even so...

Have you looked at a 60d? It's really not a big camera - and is attractively priced - and I have to wonder if its features would give you a much bigger boost than you expect....

Just my unsolicited 2c.

WOW thank you Ziggy. The toughest thing to do is explain the complex to the simple minded and you did it great.

Matthew, I had checked out the Av limits and as Ziggy points out, the off center points that I use can react to f5.6 so my f4 lenses would engage the cross types even though the center point is optimal at f2.8.

Diva, you know your comments are always solicited and I always listen. Maybe I'll rent a 60D or a 7D and try them out. I particularly like the "zone AF" on the 7D. HMMMMM You're gonna cost me more money again.

Zone af is a waste of time for portraits IME - I never use it. Single point for me. BUT... you have WAY WAY WAY more points to choose from! Instead of having to move the camera, or guess, or compromise, you just wheel around to the point that is exactly over the eyes. This is the feature that stops me from buiying one of the well-priced 5dII's around right now - I just am not prepared to go back to 9 af pts ever again!

You know I'm a fan of my 7d, but check out the 60d next time you're in a store - you can at least play with it at Best Buy or Costco or whevever and see what you think. It has the same sensor as the 7d, a lot of good features and I think it's a little less chunky than the 7. You could do a lot worse than that, I think, and it's well under $1k...

Will do Diva.

If you review my stuff for the past 12 months, you KNOW I listen to everything you say.

I can't add much to the details of these specific cameras, but I can say that for sports at least cross-type AF is *much* more accurate.

Regarding zone AF (or AF expansion) my experience has been poor. Basically, with AF expansion I get fewer totally OOF shots, but also far fewer totally in-focus shots. I've had the best luck in using a single AF point, but it does take some practice for shooting fast moving subjects.

Originally Posted by JohnGalt_ny I have a very specific question for a very specific reason. Are cross type AF points really that much better in still shooting at f4 or smaller?? I own two Rebels, the 450D and the 600D(t3i) and I've been happy with both EXCEPT that I get a fair amount (say 20 of 100) of really soft images that just aren't salvageable. My main lenses are a 24-105, and a 70-200 f4 IS, both of which show this problem. I suspect it has to do with the contrast detection AF but I'm not really sure. I'm considering the T4i but I honestly don't see much advance over the T3i except for, possibly, the 9 cross type focus points. Since I shoot mostly models and mostly in single point AF (in portrait orientation with the top point on the eyes), I wonder if the cross typeAF point would be noticieably better with f4 lenses. The video capability means nothing to me. Little help please from them what understands this stuff.

I agree with the others but you should be able to use the outer focus points for portraits (get the AF confirmation). If you're trying to focus on the eyes then it sounds like you need more AF points on the 60D or 7D

Originally Posted by divamum John, I'm going to jump in here with a suggestion rather than an answer. I know you say you like the smaller form factor of the Rebels, but I can only say that with the sheer amount of shooting you do.... it's time for a camera with more focus points, more accurate focus points and a few other bells and whistles. I stuck with my xsi for ages - great little camera - and liked the smaller form too, although I found it was SO small I had to use it with a grip. Thus, when I went first to the 50d and then to the 7d, those actually felt smaller ungripped. However, the main point here is that the AF is so much better. Plus, you can change your settings so much faster with the thumbwheel (that was the real thing I wanted - I had it on my film camera and really missed it). Most importantly for me, MORE FOCUS POINTS. It makes an enormous difference for me with accuracy + composition + shallow depths of field, because I can use the extra points. Yes, the center one is still more accurate, but even so... Have you looked at a 60d? It's really not a big camera - and is attractively priced - and I have to wonder if its features would give you a much bigger boost than you expect.... Just my unsolicited 2c.

I agree with what divamum said regarding the additional focal points. I didn't feel comfortable using this feature on the 7D at first, as I was still learning a great deal, but now that I understand it, I use single focus on a variety of points at any time. At first switching your focal point is a hassle, but eventually it's like driving a manual transmission...you just do it and don't think about it.

For action, I'm still not sure what I'm most comfortable with, but if the subject is just sitting there...there's no reason not to plant the focus exactly where you want it, and not worry about recomposing.