40D - F2.8 at center - what about the others

Hi, I never knew that the center focus point for the 40D had a spec of F2.8. That is not an issue, because my lens has that ability, but what about the other pionts. Does this imply that when I select anyting but the center I at F2.8, I run the risk of crap results?

My experience says that I am, and I wondered why? The next question would be: What is the safe F-stop for the other points: F4?

Or should I read this differently?

David Ransley wrote in post #6603860
Hi, I never knew that the center focus point for the 40D had a spec of F2.8. That is not an issue, because my lens has that ability, but what about the other pionts. Does this imply that when I select anyting but the center I at F2.8, I run the risk of crap results?

My experience says that I am, and I wondered why? The next question would be: What is the safe F-stop for the other points: F4?

Or should I read this differently?

Hummm, I may not understand fully what your trying to explain here or ask, I am still learning much. I shoot with a 70-200f/2.8, my camera is set with center point as well as spot metering (If it matters)

I shot all sports like this day and night. Now I just posted (LINK) about how unusable the 40 is for night sports (Day is perfect)

I am very interested in understanding your question form others

All it means is that the centre cross focus point gives higher precision focussing with lenses of f/2.8 and faster. All the others work with lenses f/5.6 and faster, so no worries. It's only a problem if your lens is slower (higher f-number) than f/5.6.

What the literature says about the 40D Center AF sensor is:

"Unique hybrid cross-type AF point; world's first high-precision AF point with both horizontal and vertical high-precision; additional standard precision, cross-type sensors for lenses from f2.8 to f5.6"

What this means is the the center AF is more precise that the other sensors when used with lenses f/2.8 and faster. It doesn't mean that the other sensors are bad, but simply that the center sensor has a more capability.

As long as you're using a lens faster than f/5.6 then all the AF sensors should work as standard cross-type AF sensors. Above f/5.6 AF focus abiliity is degraded.

I have noticed that my 40D is sometimes not that accurate if I use the non-center points in lower light conditions. Hence the research and I even had it back at a service center. Some threads say that the center is up to 3x more accurate. I am all over the canon site to look for numbers on this to understand the limitations. It may be that I am expecting too much from the non center points. In general I like the camera a lot and sit back in awe looking at the RAW image files I get and I am working towards a higher keep rate.

What I meant by the F4 part in my question was:"Should I always stay faster than F4?" From what I see in the answers are that as long as the lens is faster than F5.6, I should be OK.

David Ransley wrote in post #6604030
What I meant by the F4 part in my question was:"Should I always stay faster than F4?" From what I see in the answers are that as long as the lens is faster than F5.6, I should be OK.

That's correct, except that as long as the lens is faster than f/5.6 the sensors should all be of equal precision (exposure metering and AF are done with the lens at maximum aperture). Using a lens of f/2.8 or faster will enable the maximum precision of the center AF point.

Yeah, your camera always focuses wide open so even if you are shooting at F/22, it won't matter.

The 40D AF system is awesome. Coming from a 30D, I swear those outer points perform better than the centre 30D point. So really, none of the points are particularly slouchy. At least, that's in my experience. In fact, I almost NEVER use the centre point because the outers perform so well. And I shoot 90% in dark conditions. Using a 580EX AF assist light helps a TON in focusing. Even if you are shooting without flash, just turn the flash firing to OFF, but still keep it powered on to use the AF assist lights.

Yes, I realise the always wide open part. My fear was more around lens speed, but some threads I have seen went on about inferior ability on the non-center points. Your answers settle my mind. It also helps with the selection of new lenses. If you need quality, go for the faster ranges.

In some instances I see my 40D returning results that are not what I expected. Low light and focusing AF provides soft images. To test I used live view for accurate focusing and the results are fine. Sharp and very usefull. I want to do some front focusing tests to see if that is an issue. My rule now is: If the light is bad, don't be in a hurry to take the shot.

yeah, the 40D/17-55 combo blows me away with the focusing speed and accuracy. Wow that pairing is amazing. It destroys my 24-70L/1Ds mark III combo in focus speed. The lens does make a difference I guess. I'm using my 24-70L less and less as a result.

To All
Can I ask what type of results your getting in evening and night shots (With out flash)

As my 40d adventure continues, I read deaper into the cross center focus. I adjusted acrodingly, but with this thread i am not understanding what f stopwould have to do with it.

I can shot with my 70-200f/4 and /2 in the day time with precises sharpness. The 135 as well, but the 24-85 is very particular at what fstop is best suited mainly 9 - 12 is when it shines.

As with the orignal question, the 40 (at least mine) is laboring to maintain focus with a f/2

17-55 F2.8L - I will look at that, because the 28-70 F2.8 I have is just too long in some instances. The 1.6x CMOS of the 40D provides range when not required and I normally need to back up. This is difficult, but in other scenarios it works great at the 70mm side.

I did find that my UV filter (keeps the dirt out) had an effect on quality. Results are sharper with thew new UV, but I sometimes wonder if it is not better to take it off for critical shots.

Do you think the UV will mess around with AF?

For One shot, nothing much can compare to the speed and accuracy of a 40D w/ 17-55, though I am surprised that it beats the 1DIII/24-70 combo. I would think that the body's better AF would compensate for the slower focus of the 24-70 over the 17-55. The 17-55 is one of the fastest focusing lenses that I've ever shot with.

But to the OP, what everyone else has said is accurate, with the 40, all points are sensitive to 2.8 glass. As long as you're under 5.6, everything should be as good as ever. Even when higher, AF will still be great, just not as fast.

David Ransley wrote in post #6604262
17-55 F2.8L - I will look at that, because the 28-70 F2.8 I have is just too long in some instances. The 1.6x CMOS of the 40D provides range when not required and I normally need to back up. This is difficult, but in other scenarios it works great at the 70mm side.

I did find that my UV filter (keeps the dirt out) had an effect on quality. Results are sharper with thew new UV, but I sometimes wonder if it is not better to take it off for critical shots.

In my opinion, which is shared by many, the 17-55 is the best lens for a crop sensor, period. Amazing piece of glass.

As for the UV, you have a $1k lens, don't skimp on the filter and put a bad piece of glass in front of all that good glass behind it. I shoot with B+W MRC UV filters on all my lenses, and the IQ is awesome. (buy them from hvstar.net)

Thanks all, that settles it then. As long as the lens is faster than 2.8, you are in the right spot for good AF on all the points.