Autofocus with ultrawide angles

[William]

Was'nt there an old saying, A poor tradsman always blames his tools , I shoot with the Sigma 10-20 on AF and never had a problem with DOF , Always sharp as a tack to the horizon , BTW I never look at the distance scale

[arthurking83]

according to the manual:

Best for: Candids and other non-moving subjects where you want to quickly focus on whatever is closest to the camera, or single moving subjects that you expect will move across the composition as you continue to shoot.
Not recommended for: Situations where you need precise control over the point of focus.

If Canon recommend this mode for candids, non moving subjects closest to the camera, or tracking a single subject moving across the frame, how about testing the feature on the types of uses Canon recommend, instead of the types of scenes Canon recommend against!

[in2fx]

I walked out to my balcony which overlooks a valley, the closest object is a tree 10 meters away off to the side.

With just the center focus point selected, it was consistantly selecting past infinity.

Maybe I am reading this wrong, but if you are using the centre spot with the tree off to one side, unless you pointed your centre spot at the tree to focus and then recompose the shot, then the focus would be on the other side of the valley where the centre spot was ?

[pmack]

wowsers, thanks for the extensive reply arthurking

So camera is set to single AF-S mode to operate in auto focus point mode. in just about all of the ten attempts at focusing, all were randomly focused. There may have been a few similarly focused distances, but the highlighted focus points always changed from one focus attempt to the other.

Are you talking about the distance corresponding to the different locations of the highlighted focus points, or are you talking about observing distance displayed on the lens scale. If the latter, what sort of readings were you getting, simillar to mine?

So what I assume now is:
Nikon have programmed some 'intelligence' in this focus mode(which I never use, other than for a few test runs to see what it does.. I'll explain another part this Nikon feature later)
So, back to the assumption, and because I'm not a Nikon DSLR software/firmware engineer, I can only assume base don my testing. If the lens is not focused on anything in particular, or is focused at a specific point, and this auto mode focus is required, it may focus to the closest available subject, but if the command is then to focus again, without any user input, it may then try to focus again to a different distance. After all, if the camera has already acquired focus(as it believes it has), then it should obviously try to focus to another point, if the command to do so again is made.

Hmmm very interesting assumption, makes sense to me. I didn't defocus my camera also, so this all seems pretty consistant.

With your test, I see nothing wrong with either your camera, nor your lenses. Only problem I saw was in the reason for the test in the first place? Why this test. if it was to simply see what was happening, then sure.. why not? I'd also do it.
But if part of your expectations were to see a probable random event with any sense of repeatability, then I hope know you can see.. why all these replies and posts directed at you that your testing is flawed. Because it appears to us that it is(based on what we thought you were expecting to find).

To be honest i completely forget the motivation for the test, maybe i was playing around with my camera out of curiosity sake, and noticed what appeared to be unusual bahaviour. I posted this topic to see if this was a commonly known behaviour or not. I expected the camera to be more repeatable with selecting infinity each time, but your hypothesis or assumption could be the explanation for why that is not the case.

I'll re-do my test after defocusing to see if i get the same results as you.
BTW, did you mention if you used a tripod or not?

One other part of the Nikon 3D focus mode, is that it also tracks by colour. So that when enabled, you can choose a primary target(making sure it contrast well against the rest of the scene) and not worry about subject position in the frame.
The target chosen is tracked automatically by the focusing system and the focus point moves about the viewfinder following the subject as best as it can.
This mode of focusing is hit and miss, where the subject moves quickly, but on occasion, it works really well(eg. a red subject against a green background).

The 7D has a very similar feature. Though i am not sure if it uses colour to follow your object.
You select your starting point, and it follows your target regardless of how you frame it (obviously as long as it stays within the area of the 19 AF points). I haven't tested it out on a proper moving target though so not sure how good it is.

Anyway thanks again, excellent post.
Oh and I am not saying that because of ricktas post.

[pmack]

If Canon recommend this mode for candids, non moving subjects closest to the camera, or tracking a single subject moving across the frame, how about testing the feature on the types of uses Canon recommend, instead of the types of scenes Canon recommend against!

Hmm suppose it depends how you interpret what they say, or more importantly how you are supposed to interpret it.
"non-moving subjects where you want to quickly focus on whatever is closest to the camera" could still be used to explain the scene at infinity. The scene close to infinity may still contain the closest objects to the camera. And "precise control over the point of focus", I think they are referring to control over which part of the frame it focusses on, rather than the accuracy. Although obviously you do get better accuracy with the one point anyway.

Still to test out the how Automatic point selection works in servo mode.

[pmack]

Maybe I am reading this wrong, but if you are using the centre spot with the tree off to one side, unless you pointed your centre spot at the tree to focus and then recompose the shot, then the focus would be on the other side of the valley where the centre spot was ?

Yes, i didn't recompose. I expected it to go close to infinity in that case, instead it went past infinity.
This was done to compare with the Auto point AF select result.

[pmack]

nice bump, champion.

btw, in the OP when i said it was selecting past infinity with center point auto focus, i think this is correct as the other day i found infinity on the lens to be slightly past the vertical infinity mark, so halfway across the horizontal line. so results are only bad when all focus points are selected, which would rarely affect me so no biggie

[Clubmanmc]

still testing your lenses??

i find the more you tell the camera to do, ie less auto settings as possible.. the more control over the image you have...

so setting it in auto with all the focus points on is kinda not really a test, specifically when the lens has such a short infinity setting...

M