Respectfully,why multi af points?

Well explained As you point this out about front focusing it is as canon has and has always stated as far as I have always read in there manuals and white papers. The focusing will always focus to the nearest point.

This is easy to demonstrate I dont have the 7D but imagine it has a center point expansion area. If you use for macro center point and accidently have the center point expansion points on it will almost always front focus if pointing the lens at a flattish surface in a downward angle. Switch to center point alone and its as if it is a different focusing system ( which it is) and it will be spot on.

There is a lot more than to photography than buying an expensive camera , pointing it at something , having a rough idea or a guess at what settings are needed and hoping.

If you are using a camera and know exactly what setting you need in a given situation (as an experienced or trained photographer does know) and then things are not as they thought they where then something maybe amiss.

If before you are not sure of the exact result you will get then you are still learning and must expect and will get erratic results, which is fine and it is great to keep learning.

Tests that are shown here are more than dubious and do need to be taken as a bit of fun and a huge pick of salt .

It is always worth at first to err on the side that it may be user error unless you are very experience or had several years training

Tony_S said:

I've done some testing since I got my 7D, and I can confirm that using all focus points (something I would never do in normal use), I get erratic results as well. The camera seems able to pick up objects well outside of the obvious focus area and locks onto them, putting the desired focus off.

In my case I was focusing on a box sitting on the carpet, and although all of the focus points that lit up were on the box, it consistently focused a few inches in front on the carpet.

Focusing on the nearest object is fairly standard behaviour, and both my 40D and 7D manuals mention it when using all focus points, but I'm not sure if this ability to pick up objects so far away from the focus area is new to the 7D.

My own opinion is that this is a major cause of the problems that people are complaining about when they are using all focus points - I've seen a lot of photos that appear to be severely front-focused.

That's not to say that I don't believe that there are some faulty 7D cameras around as well, just that the more inexperienced (not trying to be patronising) user may get caught out when using all focus points.

Click to expand...

Well in my case I had both.

I didn't understand the focusing system very well, particularly the intended use of each focusing mode. I also happened to have a broken camera. The working unit solved the erratic focusing, but zone and 19 point worked the same as per design.

There are those on the forum who totally dismiss any focus complaints from a new user, regardless of the posted pictures.

Coming from the P&S world, it is intuitive to believe the 19 point focus was intended to be a general purpose focus mechanism. It is the default in full auto / creative mode, which I assumed was the interim modes to use while learning the camera. The manual could do a better job of visually showing the intended use of each focus mode.

It is still my opinion that ONE SHOT + 19 POINT MODE = GENERAL PURPOSE FOCUS, not just focus closest object. Ease of use issues should not be dismissed.

not hard to do, but hard to get it right (not front/back focusing), esp. w/

Umm, well then you might want to look hard at your technique. I've had great luck with it. Focus locks when you press the button half way down... you should be able to get great results and dead on focus as long as you keep the button down while you recompose. It's not hard at all.

BigP said:

not hard to do, but hard to get it right (not front/back focusing), esp. w/

Click to expand...

something abot geometry, the distance between lines in a triangle with an angle: x, y, z - where x & y (distance to object, and different heights of recompose) will result in the wrong focus distance.

In which case you can easily use the "joystick" button to reselect your focus point if necessary on the fly. Once you get used to it, it's pretty quick, and that eliminates any issues with recomposition distance.

BigP said:

something abot geometry, the distance between lines in a triangle with an angle: x, y, z - where x & y (distance to object, and different heights of recompose) will result in the wrong focus distance.

Click to expand...

Here's a pretty good discussion of this issue.

http://www.bobatkins.com/photography/technical/focus_recompose.html

Bottom line is that focus and recompose does result in some degree of back-focusing error. Frequently insignificant, but not always.

reginalddwight said:

Multi-AF points can be helpful for static subjects but probably are best suited for dynamic subjects in AI Servo or AI Focus modes.

The real puzzler is why some people favor shooting static subjects in AI Servo rather than One Shot. I just re-read Rob Galbraith's take of the 7D, and he says the following, "Static autofocus was excellent, using Spot AF, various AF points, AI Servo (we almost never use One Shot) and a small batch of different lenses."

Click to expand...

I always use servo focus on my 50D (and 20D before that) assigned to the * button ( I use the * and not AF-ON, because my grip doesn't have an AF-ON button, and I do a lot of vertical shooting). Servo focus with the * (or AF-ON) button gives you the best of both worlds. You get one-shot focus, if you push and release the * button, and servo focus, if you keep the button depressed. I haven't used my camera in one-shot mode for over three years. Keeping the button depressed until you fire the shutter allows the camera to compensate for small changes in focus distance right up until you take the shot. In one-shot mode, if you lock the focus and then pause, even slightly, before taking the shot, the subject (or you) could move slightly. This isn't usually a problem, but can be when you are using very large apertures on longish lenses.
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ju_ju said:

This is easy to demonstrate I dont have the 7D but imagine it has a center point expansion area. If you use for macro center point and accidently have the center point expansion points on it will almost always front focus if pointing the lens at a flattish surface in a downward angle. Switch to center point alone and its as if it is a different focusing system ( which it is) and it will be spot on.

Click to expand...

Absolutely right. I confirmed this yesterday with a shot of a gull sitting on the local golf course taken with centre point focus with expansion. I got a lovely tack sharp photo of the grass about 2 feet in front of a blurry gull, even though DPP shows my centre focus point is right on the bird. Switching to just the centre focus point got me the correct focus.

There is a lot more to the focus settings on the 7D than it first appears, and there are definitely situations where using the wrong one (even though it might seem ok) will get you a result that you weren't expecting.

I'm even more convinced now that the majority of the issues raised on the forum are due to using the wrong focus setting. I'm not being patronising - I had exactly the same problems myself when I first started using it, until I realised what was going on.

bhollis said:

Here's a pretty good discussion of this issue.

http://www.bobatkins.com/photography/technical/focus_recompose.html

Bottom line is that focus and recompose does result in some degree of back-focusing error. Frequently insignificant, but not always.

Click to expand...

This is true but not as troublesome as some here think--I do it all the time because I prefer the extra precision of the center AF point. I only choose an outer point when I'm extremely close to the subject using a fast lens wide open.

Generally speaking you can safely focus-lock and recompose with the shallow DOF of a full frame sensor at a distance of 18 feet divided by the f-stop - a bit closer with a 1.6X crop sensor. You can be as close to 4 feet at f4 and you'll be fine recomposing as long as the original focus subject is still in the frame.

See this link for the formula:

http://www.stason.org/TULARC/recreation/photography/lenses-faq/16-If-I-focus-on-some-point-and-then-recompose-with-that-p.html

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TomFL said:

Well in my case I had both.

I didn't understand the focusing system very well, particularly the intended use of each focusing mode. I also happened to have a broken camera. The working unit solved the erratic focusing, but zone and 19 point worked the same as per design.

There are those on the forum who totally dismiss any focus complaints from a new user, regardless of the posted pictures.

Coming from the P&S world, it is intuitive to believe the 19 point focus was intended to be a general purpose focus mechanism. It is the default in full auto / creative mode, which I assumed was the interim modes to use while learning the camera. The manual could do a better job of visually showing the intended use of each focus mode.

It is still my opinion that ONE SHOT + 19 POINT MODE = GENERAL PURPOSE FOCUS, not just focus closest object. Ease of use issues should not be dismissed.

Click to expand...

I think the problem if you're coming from a P&S camera is that they have such a huge DOF that poor focusing isn't really noticeable a lot of the time, so a multi-point focus system might appear to work better. With 18Mpixels at 100% and the shallow DOF that an SLR gives you, you'll really notice if your focus is even slightly off.

I understand what you're saying, but the other problem is defining "general purpose focus". What do you want the camera to focus on? If it's a shot of your dog then you don't want the grass in front to grab the focus (i.e. you don't want it select the nearest object). If it's a shot of 2 people in front of a busy background and the camera is aimed between them to get them both in shot then you do want to focus on the nearest object. The camera can't "know" which part of the image is the bit you want to be in focus - hence all the advice about using a single focus point, it puts you in control.

You have to try and think how the camera could work. You have 19 points laid out across a screen. The camera is aimed at a room. There are many things in the room. The camera does not know what 'any' thing is, a book and a persona is all the same. There is no human brain in the camera. How does it know if you want to focus on any particular thing that is under one of those focus points? There are 2 things to consider. Focus is contrast based so it will focus on something that has contrasting lines and the camera (as per Canon) will try to focus on the closest thing thinking that is what you are trying to capture. The only way you can actually select what you want it to focus on is to choose a particular point and say I want what I point that on to be in focus otherwise the camera is guessing. Knowing that you may be able to trick the camera to focus on what you want using 19pts but the odds are in the cameras favour and against you LoL.

Choosing one focus point is more accurate. Why is spot metering considered the most accurate? Because the actual focus element is larger than the box you see in the screen. Like I had said, the camera uses contrast to be able to detect if something is in focus or not. Sometimes you look at something, place the focus box on top of what you want in focus (an eye for example) and when you check the picture you might see that the eye was not perfectly in focus but the nose was. It could be because the more was still under one of the actual focus elements and the camera focused on that, the nearest thing in its view. That is where spot is better as spot is smaller than the box and would have an easier time to get what you want in focus. Ok then... why not always use spot? Good question. I mostly do but there are times that you might be trying to focus on something that does not have enough contrast lines in it to capture focus. That is where the fill single focus point may be better because there is a larger area for it to try and achieve focus. Same idea with AF expansion or group focus. There are many different types of focus to choose from because no one type is perfect for all situations. They give you a choice to use the one that fits the job.
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So, if I am shooting kid football and kid soccer w/ the 7D (and 70-200 2.8 IS) is the best bet to use Single Point AF and AI Servo?

Not Spot, not AF point expansion.

Thanks

Eadward_ said:

So, if I am shooting kid football and kid soccer w/ the 7D (and 70-200 2.8 IS) is the best bet to use Single Point AF and AI Servo?

Not Spot, not AF point expansion.

Thanks

Click to expand...

Definitely not spot, according to the literature distributed by Canon on the purpose of spot focus. I would have thought expansion would be OK, but I've only read the literature, and not tried the camera.
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BigP said:

something abot geometry, the distance between lines in a triangle with an angle: x, y, z - where x & y (distance to object, and different heights of recompose) will result in the wrong focus distance.

Click to expand...

Have a look at my site, I would say that 75% of my standard to wide angle shots are done with the focus, re-compose method....works for me.

Clint
http://clintdunn.zenfolio.com

Eadward_ said:

So, if I am shooting kid football and kid soccer w/ the 7D (and 70-200 2.8 IS) is the best bet to use Single Point AF and AI Servo?

Not Spot, not AF point expansion.

Click to expand...

I have not shot those sports but for Hockey, Single Point or expansion works and set the C.Fn for AF Speed to slow (either the extreme left or one over to the right of it). Trial and error. Get a feel for what works best for you. I have also used (center) zone sometimes in hockey and it has worked.

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Michael Kaplan
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See my profile for equipment list

...it's basically because people expect the AF system to be able to do things all of the time that it is only really capable of delivering on when the circumstances are appropriate. There is a reason that higher end cameras have multiple AF modes and even custom functions to tune them, and that reason is that some modes and settings will perform much better in certain scenarios than others.

The master knows his tools and will select the appropriate AF mode and other settings to give themselves the best chance of getting the shot, the novice will pick the one that makes their life the easiest and then blame the camera when it doesn't work. If you want the best results, you have to take control of the camera and set it up appropriately for the task at hand and not let the camera make the decisions for you, because sometimes it's going to get it wrong. In some cases, that can even mean fiddling around with the custom functions before a shoot, and even during the shoot if conditions change.

Andy

Got a link to Canon's literature?

i

Alastair Norcross said:

Eadward_ said:

So, if I am shooting kid football and kid soccer w/ the 7D (and 70-200 2.8 IS) is the best bet to use Single Point AF and AI Servo?

Not Spot, not AF point expansion.

Thanks

Click to expand...

Definitely not spot, according to the literature distributed by Canon on the purpose of spot focus. I would have thought expansion would be OK, but I've only read the literature, and not tried the camera.
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Alastair
http://anorcross.smugmug.com
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Click to expand...

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i
...
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Ian Worthington said:

Got a link to Canon's literature?

Click to expand...

Here you go:

http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=3049
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Alastair
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Well there is no doubt a general purpose AF algorithm is very difficult. P&S does much better (face recognition, etc.) because they have the entire image to evaluate and can be slow. The 7D only has 19 points and must be very fast.

I believe it could be better. For example I would prefer it first attempt to focus on the center AF point, if this point does not meet a high enough contrast threshold, move further out, and repeat.

I admit it, I'm lazy. If they could hit focus on the "intended object" 90% of the time, that would be useful. There is also a consideration for the target user for this camera, Canon may have rightly dismissed P&S upgraders who are supposed to go the Rebel route first. I'm a gearhead, so I want good equipment, even if I don't need it.

Ultimately I will adapt to the camera and not look back. Use single point mode unless you are doing BIF or something special.