Does Single-Point-AF Have A specific Application

Started Mar 16, 2008 | Discussions
worktolivelife Forum Member • Posts: 71
Does Single-Point-AF Have A specific Application

hi gents,

guess i'm missing something? but it seems to me to perform the same in Dynamic-Area-AF , the difference only being made by the servo AF choice. Is there a specific situation where you would strictly wish to select ...Single-Point-AF / Single-Servo-AF

Thanks and appreciate your time

steve

Deleted17 Senior Member • Posts: 1,899
Studio work?

I would use this combo in a studio with inanimate subjects. The pictures I take for sellling products on ebay are a good example.

OP worktolivelife Forum Member • Posts: 71
Re: Studio work?

hi,

i understand the general guide for useage but would there be any reason technically that would effect the image to not just use the Dynamic-Area-AF as it would appear to perform exactly the same in Single-Servo -AF

thanks

steve

sleeboss Forum Member • Posts: 52
Re: Studio work?

Sometimes, the contrast is so high between the foreground and the background that you can't get the exposure right and can't get the focusing right also.

In this case, I might put the camera on a tripod and single focus on the foreground and take a snap with proper exposure. Then single focus on the background second main focus which is on a sweet spot and snap a pic with proper exposure for that.

Then combine the two using photoshop using masking. You can get the specifics in any photoshop how-to book.

Now you'll end up with a great photo with all important areas in focus and in proper exposure.

Murray Bowles
Murray Bowles Senior Member • Posts: 2,084
Re: Does Single-Point-AF Have A specific Application

That is, is there a difference between AF-S+single-point and AF-S+dynamic?

If you believe the manual, it doesn't seem so. I wonder if the peculiar matrix metering behavior earlier threads have noticed when locking focus and recomposing (where it "remembers" something of the image at the time of AF-lock) happens in both modes.

Deleted17 Senior Member • Posts: 1,899
Re: Does Single-Point-AF Have A specific Application

As far as I know, dyanmic means that the point may change if the camera decides there is a better point to focus on. If you are trying to do a portrait and you want to keep one eye in focus, you don't want the camera jumping to the nose just as you shoot...

Jeff Diaz Contributing Member • Posts: 593
Choice depends upon how you use the camera

AF-S single point is best when the photographer and subject can stay still or if the camera is on a tripod and the subject is static.

AF-C single point AF is better if the photographer and/or subject are moving. Turn AF Lock On to off. The camera will maintain focus on the selected subject using the single selected AF point. If more AF points are needed, then switch to dynamic AF and select 11, 21 or 51 point.

I set my cameras to AF-ON to activate AF only. I leave the camera in AF-C. Press and release of AF-ON effectively functions like AF-S. Holding down AF-ON makes the system act in AF-C continuous AF.

Tim Updegrove Regular Member • Posts: 330
Fastest Focus Mode

I'm no expert but in other threads, it has been pointed out that single-point AF is the fastest focus mode since a single focus point is being feed to the CPU. 9-pt is the next fastest, and so on.

LuvLatins Contributing Member • Posts: 748
Re: Does Single-Point-AF Have A specific Application

Focus Tutorial Part 1

There are several concepts and controls at work here, and without an understanding of what each one really entails, it will be hard to accomplish the task at hand - to consistently get photographs with the intended subject in focus. OK class, let's begin. Just to make it simple, please place the focus area mode selector found under the Multi Selector on the back of the camera in the middle position - dynamic area AF. We'll talk about this in a minute.

Let's first consider the focus mode selector located on the front of the camera. It has 3 positions - C, S, and M. "C" stands for continuous - the lens will continue to focus as long as the shutter button is held down halfway (or one of the AF buttons are utilized, but let's not confuse the issue further). The only mode in which focus tracking will occur is the "C" or continuous mode. Here's an easy test: with the mode selector in the "C" position, focus on something close by depressing the shutter button half way. The green focus light in the viewfinder should come on, and the object will appear to be in focus. Now mash the button all the way down - you have taken a picure and it should be in focus - yahoo!!! Now press the button halfway down again and focus on your subject. Holding the shutter button halfway down, walk backwards 4 feet. You'll hear your lens AF working, and when you press the button all the way down and take another picture, it will also be in focus. Yahoo again! Now try this same test in the "S" mode. "S" stands for single - once the camera acquires focus, it will remain focused at that distance. We'll repeat the test. Of course the first picture will be in focus - hooray! But when you try the second half of the test, as you walk backwards you'll notice the green focus light stays on, but the autofocus will be silent, and when you snap the picture, your object will not be in focus. Don't despair, your camera isn't defective - that's the way single servo autofocus works. In "M" or manual - there is no autofocus, you have to manually twist the focus ring on the lens to get it to focus.

LuvLatins Contributing Member • Posts: 748
Re: Does Single-Point-AF Have A specific Application

Focus Tutorial Part 2

OK this was all about tracking. Now that we've decided that continuous ("C") is the only Focus Mode tracking can occur in, we'll look at the Autofocus Area Modes. These are selected by the Autofocus Area Mode Selector. Again, there are 3 positions. The lowest position is Single Point Mode. In this mode, autofocus is based on the information gathered by the single selected focus sensor. As the manual states, this is used for relatively static compositions. Strangely enough, if we repeat the test, as you move backwards, as long as the subject remains in the selected autofocus sensor the lens will continue to autofocus. So to some extent it will do AF tracking, as long as the subject remains in the selected focus sensor. The second mode (center position) is Dynamic Area Autofocus. Here you find more of what we really are looking for in autotracking because if the subject moves out of the selected sensor, the camera will use information from the surrounding sensors to attempt to keep the subject in focus. We have a choice of how many of the surrounding sensors we want to use: 9, 21, or 51. I've worked with this a little, and have found 21 works fairly well in my photography. However there are several factors to consider in making the choice - how far and fast the subject is moving out of the initial focus area, the amount of contrast, the nature of the subject, etc, etc, etc. Remember also we are dealing with 15 cross sensors in the central area, and only unidirectional sensors on the periphery. This gets into the Multicam 3500DX autofocus internals, and I haven't found any discussion of how it really works. Who cares - just mess with it and find what works for you. RTFM, page 269-270 for Nikon's recommendations on where to use which number of sensors. Remember also that using the Multi selector you can choose the group of sensors to use in 9 and 21 point settings. In Dynamic Area Autofocus there is one other option, called 51 point 3D tracking. Like standard 51 Point tracking, this will use the selected sensor, and all of the surrounding 50 sensors as the subject moves off the selected sensor. I haven't found any detailed description, but it seems that 51 point 3D uses the colors in the selected sensor area to assist in the focus tracking over the surrounding 50 sensors. It is said to work best with subjects moving horizontally across the frame. I haven't used this much; my results were not as good as with the other dynamic area AF settings. I keep thinking that if I understood it better, I'd get better results. In Dynamic Area Autofocus, you will always see the selected sensor (otherwise it would be unusable). You should always start with your intended subject in the selected sensor area. You can illuminate the selected sensor always with the setting in a6. The third mode is called Auto Area Autofocus. Here the camera chooses what is the subject, and focuses on it for you. It seems to work well for point and shoot applications, but if there are multiple subjects in an area, it may choose the wrong one. This mode is where the active focus points are highlighted for one second after focus, but only in the Single Servo AF mode ("S"). In the "C" mode you will never see a selected focus sensor because you have no control over it. There is no auto tracking in this mode as far as I can determine.

LuvLatins Contributing Member • Posts: 748
Re: Does Single-Point-AF Have A specific Application

Focus Tutorial Part 3 (Last Part)

OK, there you have it. Let's recap. To have any real auto tracking, the focus mode should be in the "C" - continuous mode, and the Autofocus Area Mode Selector should be in the middle or Dynamic Area AF position. Choose the number of sensors you want to use, and go out and start shooting. Hold the shutter button halfway down with the subject in the selected focus area. Continue to hold it halfway to maintain focus as the subject moves. Once you release the shutter button, you will need to regain focus on the subject by pressing the shutter button halfway again with the subject in the selected focus area. If the subject leaves the sensor area you will also need to let go of the shutter button and recapture the focus on your subject. Mash the button down all the way anytime you want to take a photo. Take lots of photos. It's fun, and actually it's the only way to learn how autofocus and tracking really works.

Sorry this was so long, but I don't think any understanding of the process could be attained with anything less.

Tom

Oops - after re-reading the initial post, and reading my response, I came up with a few other points. In Dynamic Area AF with anything except 51 Points (3D Tracking) selected, you will not see the selected sensor move. It uses information from the surrounding sensors, but the selected sensor doesn't change. In 51 Point 3D, the camera actually moves the selected sensor based on the colors it has gleaned from the initial focus point, and you can watch the selected sensor move as you blast away. It's sort of kewl. In high contrast situations with good color differentiation it seems to work well - again, this is best for horizontal motion across the frame. Tracking is all about maintaining

nawknai Senior Member • Posts: 1,043
Re: Does Single-Point-AF Have A specific Application

Murray Bowles wrote:

That is, is there a difference between AF-S+single-point and
AF-S+dynamic?

If you believe the manual, it doesn't seem so.

I didn't notice the difference at all with the D50, but I definitely know the difference when using a D300. If you use AF-C and Dynamic Area AF, you can pick the focus point and focus on a subject using this selected focus point. Lets assume the subject is small. If that small object moves off your focus point and into the neighbouring focus point, the focus stays the same and doesn't change.

However, if you're using AF-C and Single Point AF, and your subject moves off your focus point and to a neighbouring point, the focus will change. The camera will focus on what is now on your selected AF point. This wouldn't happen in Dynamic Area AF. If your small subject moved to the next focus point, the camera wouldn't refocus onto whatever is located at your selected focus point.

Try it.

This is useful for tracking moving objects. If you're panning, do you want your camera to refocus every single time your subject isn't located at exactly the focus point you selected? No, of course not. As long as your subject is at, or very close to the focus point you selected, the focus shouldn't change.

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Lighthog Contributing Member • Posts: 572
Re: Does Single-Point-AF Have A specific Application

I thought I understood your 3-parts Tutorial, but then I am not sure. Could you please describe the specific custom setting for the AF-C; AF-S; and the shutter release? The focusing issue that I am most concern with is candid shots (meaning there will be movement) using 1.4 lens wide open at, say at 6ft distance from subject. The DOF of 1.4 lens is just an inch when focus on the eye with other most likely out of focus. What's the best approach?

Lighthog

digitalnoise Contributing Member • Posts: 868
Re: Does Single-Point-AF Have A specific Application

I blew some shots yesterday because I didn't switch to single point focusing.

Shooting a cityscape the autofocus locked with a cluster of a few points on a palm tree about a kilometer away. Even at f8 the distant background was just out of focus. Had I taken more control I would have locked onto something farther away and gotten the results I wanted.

That's the most common problem I've seen - autofocus grabbing something too close for the effect desired.

Jeff Morris
Jeff Morris Veteran Member • Posts: 3,296
Thanks LUVLATINS !

Your explaination of the autofocus system was well done. I have almost always used single point for most of my work because I did not trust auto tracking. I will give it a try and see how it works.

I have used Continuous Autofocus with single point with excellent results.
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Lighthog Contributing Member • Posts: 572
Re: Does Single-Point-AF Have A specific Application

LuvLatins can you answer above my question please. Or any one else care to comment. Thanks.

Lighthog

Wrateman Forum Member • Posts: 80
Re: Does Single-Point-AF Have A specific Application

digitalnoise wrote:

I blew some shots yesterday because I didn't switch to single point
focusing.

Shooting a cityscape the autofocus locked with a cluster of a few
points on a palm tree about a kilometer away. Even at f8 the distant
background was just out of focus. Had I taken more control I would
have locked onto something farther away and gotten the results I
wanted.

That's the most common problem I've seen - autofocus grabbing
something too close for the effect desired.

That's precisely what was happening to me when shooting basketball, too many arms/bodies & the ball. I would get the ball in focus, another player, etc. instead of my main subject.

I resorted to AF-C & Single (using either the center one or one off center when shooting portrait in order to keep a cross hatch sensor). Occasionally I'd try 9 Pt dynamic.

It's interesting that Dave Black raves about the 51 pt Auto/3D, but I haven't found it helpful in my shooting so far. Maybe for a big race car moving across the frame.

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Chris

N8535Y Veteran Member • Posts: 3,205
Does Dynamic even work in Single Servo Mode?

It is my understanding that Dynamic does not even work in Single Servo mode and only works in C mode.

If my understanding is incorrect, please point me to the page of the manual which says that Dynamic will work in Single Servo (S) mode.

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Bob

Murray Bowles
Murray Bowles Senior Member • Posts: 2,084
"until a lock can be obtained"

My reading of the manual is that if you set AF-S, "dynamic area" behaves the same as "single-area". Except maybe "If the subject is moving when the shutter release button is half pressed the camera will focus track until a lock can be obtained." The behavior of that focus tracking might be influenced by the AF area mode.

JasonOdell Senior Member • Posts: 2,282
Re: Does Single-Point-AF Have A specific Application

Good post, Tom.

I think a lot of people get confused with the term "tracking". The primary job of the Dynamic AF modes is to keep at least one active AF sensor on the target as it moves LATERALLY. This is different than an object moving towards you or away from you. Multiple AF points help the photographer keep at least one active sensor on "target", but they don't have any bearing on tracking performance of an object moving towards the camera. You can track a moving target in single-area mode (provided you are in continuous servo mode), as long as the sensor stays on target.

To get good tracking of targets that move towards you, then you need to consider how to set up custom setting a4, the infamous "lock-on" setting. With Lock-On set to "normal", there is a delay in activating AF tracking if the subject moves towards you (or away from you) suddenly. Once a subject is moving towards you at a fairly constant rate, then the predictive tracking system works very well-- the AF system "guesses" as to where the subject will be when you click the shutter and adjusts accordingly. Predictive AF fails with subjects that move erratically or unpredictably. For these subjects, setting a4 to "OFF" can help tremendously. The downside is that with a4 "OFF", you increase the chance that a subject other than your intended target becomes acquired by mistake (like background objects).

Cheers,
Jason

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