9, 11, 21 or 51? How to decide?

Started May 13, 2012 | Discussions
ckon
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9, 11, 21 or 51? How to decide?
May 13, 2012

Maybe this is dumb question time, but ...

Our cameras give us many options regarding focus settings, but finding out "why" has been a challenge to me. Ever since I got my first Nikon DSLR, I've looked all over the web for why the user should select a particular NUMBER of focus points.

Recommendations for setting the number of points are easy to find, but I've never seen an explanation as to why, not even from the experts. I don't recall EVER reading about why 9 points is better than 21 or vise-versa and I've never seen a clear-cut difference in my results to help me understand what's happening when I select one instead of the other.

It seems like a greater number of points would give the camera more focusing options, but most of the experts recommend a smaller number and/or group. Is there anyone who can explain to me why I should use 9 points instead of 11, or say, 21? I'd really feel like my education took a giant leap if I could come up with an answer to this one. TIA.
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abcdefghijklmnop
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

In one of the videos Moose Peterson has done, he explains why he uses 21pts with his D3s. In a discussion with the Nikon engineer that was in charge/worked on the af system, he was told that the system was optimized for 21pts.

This is all, I have ever seen on why anyone selects one number of points over another.

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ckon
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

Thanx - at least that's a start.
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kliang
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

I'm curious too, and I thought choosing more points just meant more coverage of the area for AF-C to have more points to search with automatically, and for the user to have more points to select for both AF-S/AF-C . Fewer points = fewer areas to search and hence faster potentially. I'm curious as to different reasons and my understanding might be incorrect.

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GMack
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

My "theory" is that the more AF selections are for Action, maybe face recognition too, and how the camera picks among those is some sort of "fuzzy logic" that maybe no one but the programmer at Nikon knows.

You can use Nikon's ViewNX2 and you may see 'a bunch' of red focus squares, but never really know how the camera selected among them 'the best' one(s), or averaged them in some fashion, or the one you really wanted (and maybe a missed AF shot too?). In a single mode, you see one square so you know. Nothing more confusing than seeing a lot of them and then trying to figure out what the camera was really thinking.

I opt for the lesser amount (I'd be happy with only 'one' good one) as I shoot slow objects and not action. Landscapes and portraits are generally not moving around a lot to command a whole viewfinder's worth of AF sensor picks. Probably depends more on what you are shooting as to how many you need.

Aside, if your camera does have the AF alignment issues reported on here, would you really want to use 51 of them with some of the outer ones being bad? How they come into play in the camera's fuzzy logic may result in a fuzzy picture.

Mack

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kenwj
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

My experience is that fewer points give you faster reacting focus and the focus doesn't hunt as much.

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gnagel
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

I go with 21 points most of the time. But, I believe that the AF system is more responsive with fewer points. The trade off is that with fewer points it is more difficult to ensure that one of the points will land on the subject--especially if it is moving.

With 9 points, AF will only track a small portion of the frame. With 51 points, AF will continue to track throughout most of the frame. Once a subject leaves the AF tracking area, the camera will no longer attempt to achieve focus. So, I usually choose something in the middle (21 points) unless I'm very confident that I can subject will remain in a small segment of the frame.

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corneaboy
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

Thank you so much for raising this issue. I was beginning to think I was the only one frustrated by the lack of information in the manual and on the net.

Is the reason some recommend the 9 points over 51 because using 9 points is faster? What are you giviing up? Does it make a difference whether you are tracking something big, like a horse, or a very small bird darting about in an irregular path?

You go to A7 in the menu to choose between 11 and 51 points. The array for the 11 points has the same spacial coverage but with wider spacing between points. Ok, that makes sense. Then on the camera you choose 9,21 or 51 pts.

The display looks like you wind up with the same result, whether you pick the 11 or 51 point basis.

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Morrestry
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

For me doing people/portrait photography, I found that trying to select through all 51 points to select the one I wanted took too long, as I'm trying to select the proper focus point and speed is a factor.

If I was doing more landscape shots and had the time to properly set up the shot, the 51-point would be a no-brainer.

Although I really wish the D800 focus-point spread was wider.

-Kyle

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reginalddwight
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Rationale for choosing # of AF points in AF-C
In reply to ckon, May 13, 2012

User's manual of D800/E (p. 93):

9: Predictable movement (runners/cars on a track)

21: Unpredictable movement (football players)

51: Quick, unpredictable movement (birds)

I advise choosing the lowest effective number of AF points to hasten focus acquisition and tracking.

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DavidGBK
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Re: 9, 11, 21 or 51? How to decide?
In reply to Morrestry, May 13, 2012

I understood that the 9, 11, 21 51 set up refers to the number of focus points the AF will use around the main focus point you have chosen.

So a smaller number means the quicker the AF can react to movement. So for 9 it only checks the 9 points around the main point.

Regards

David

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corneaboy
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Re: Rationale for choosing # of AF points in AF-C
In reply to reginalddwight, May 13, 2012

No offense Reginald but your answer is the problem.

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reginalddwight
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Re: Rationale for choosing # of AF points in AF-C
In reply to corneaboy, May 13, 2012

No offense taken. Free advice is just that.

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Island Golfer
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

The focus points detect contrast and track the movement of the subject you are photographing. If you have a staionary subject, all you will need are 9 points, as nothing is moving out of the area that the 9 points covers. But, if you have a moving subject (i.e.; a fidgety child, a bird in flight, etc.), the subject is moving faster than you can recompose your focus. So, using more focus points will lay out a larger focus area pattern in which the subject can move and still be in-focus. For a complete explaination of how the focusing system works, read this article:
http://mansurovs.com/dslr-autofocus-modes-explained
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truview

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corneaboy
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 13, 2012

I think ckon is asking for a more comprehensive response than what is being quoted from the manuals, which don't answer his question if you read carefully.

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JediLight
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Re: 9, 11, 21 or 51? How to decide?
In reply to Island Golfer, May 13, 2012

Island Golfer wrote:

The focus points detect contrast and track the movement of the subject you are photographing. If you have a staionary subject, all you will need are 9 points, as nothing is moving out of the area that the 9 points covers. But, if you have a moving subject (i.e.; a fidgety child, a bird in flight, etc.), the subject is moving faster than you can recompose your focus. So, using more focus points will lay out a larger focus area pattern in which the subject can move and still be in-focus. For a complete explaination of how the focusing system works, read this article:
http://mansurovs.com/dslr-autofocus-modes-explained
--
truview

This makes sense. What would be good to know though is what is the disadvantage of using 51 points all the time? On the above it sounds like 51 points is better as more chance of capturing movement focussed correctly.

There must be some disadvantage however to this otherwise the camera would only have 51 points as the option.

I am assuming that the reason that you choose less focus points is that it gives you more chance of the camera only focussing on what is in the centre of the focus point group (when this is desirable), and hence it not acquiring focus on something else you were not expecting it to.

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ckon
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Re: 9, 11, 21 or 51? How to decide?
In reply to corneaboy, May 13, 2012

Thanx for the responses so far - let me put a finer point on it if I can.

I think that most of us who've been using this system for any length of time have something of a grasp on how it works, but I/we are missing the specifics on how to decide exactly which setting to use under whatever photographic challenge we're facing at the moment. Distant BIF: 51 points because it's a small subject and harder to localize in the frame? Nearby bird (larger in the frame): 9 points so the focus doesn't wander? What about what's in between? This is probably pretty much how I decide, but I'd be more comfortable with the decision if I had a better idea of where the system would work the best.

Again, TIA and thanx for a good discussion.
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sssesq
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Re: 9, 11, 21 or 51? How to decide?
In reply to JediLight, May 13, 2012

JediLight wrote:

I am assuming that the reason that you choose less focus points is that it gives you more chance of the camera only focussing on what is in the centre of the focus point group (when this is desirable), and hence it not acquiring focus on something else you were not expecting it to.

Exactly, I use 21 points. When shooting BBall players from the baseline, players that are not of interest will frequently bop in and out of the frame. I don't want them, I want the one that I have (hopefully) exactly on the center point. I also have the lockon target on my D700 set to number 2, (shorter or something like that). 9 points is too few at close in rapid movement (miss completly) and 51 is too many as Jedi pointed out.

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2wheel
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 14, 2012

Humbly offering a response...

I'm primarily an action shooter... Bicycle racing, motorcycle track days, snowboarding, adopted greyhounds at speed etc. So, my response is based on my experience in these environs and also from countless discussions and examinations with other sports shooters. If it's of consequence, I'm shooting with a D4 (And a D300.)

First, Nikons focus system is highly variable and dependent on a concert of settings including "Focus Tracking with Lock-On", "11 or 51 Focus Points" as well as aperture for DOF. etc. To utilize the system properly, one must balance several AF and other features. It's not just about how many/few points to use as a default and requires practice for your type of shooting or a particular situation.

Nikon's AF system in upper end DSLR's (D300 and above) utilizes a primary focus point for all focus modes except for the dynamic modes. (Which I'll exclude in the following examples) The number of points selected refer to the helper points around the primary or target focus point. The primary and helper points define an area of the frame to be considered in-focus for an exposure. i.e. 9 points will prioritize a smaller area of the frame than 21 points.

If one wished to have the entirety of the frame considered to be in-focus, one might choose 51 points. (And something above f/8) If one preferred to have only one area of the frame in focus and a narrow DOF, one might choose a single point and f/2.8.

Note that the above and below examples are illustrative only and I am not recommending that this is the only way to marry AF points and DOF.

Scenarios:

Consider a bicycle race where one might want to choose a particular rider in a pack of 50+. The pack may be moving directly at you at 30mph prior to entering a turn. Your planned exposure(s) will be mid-turn at which point, the pack will be moving from right to left. If you wish to capture a particular rider, upon initial acquisition, amongst the field of bright colors and high contrast options in the frame, you might only be able to see that riders face and nothing else. Thus, your initial target is very small and moving very quickly toward you. By choosing a single AF point, the AF system will acquire the subject rapidly and track that rider as they approach the turn and begin moving across the frame. The likelihood the that your chosen focus point will remain on that rider and continue to adjust focus as they both close distance and change direction is higher than if you were to utilize, say, a 21 (Or more) point array. In which case, you would be telling the AF system that it's OK to consider other near, but not precise, targets to factor into the exposure.

Conversely, if you were to desire that the entire pack be in focus at the same point in the turn, you might allow some flexibility in the AF system to select any rider or riders and choose 21 points. In this scenario, you might also chose a smaller aperture which is consistent with what you are looking for in the final image: A larger portion of the pack in-focus.

While I don't shoot BIF, I imagine it's not dissimilar to shooting a single snowboarder moving quickly and somewhat erratically on a mixed background of snow and trees. Typically, I'll chose 9 or 21 points as I may want more of the rider in focus than just his/her face. Again. I'll marry the broader AF point array with a smaller aperture.

I could envision favoring the 21 point as a birds head is a much smaller target than an entire human and, if I were to pan too quickly or to slowly, could allow the AF system to jump to another object in the frame.

Now, I have not experimented fully with 3D tracking and really can't comment on its effectiveness in either of the above examples but, I just can't imagine it as an effective tool for the bicycle race scenario. BIF and single snowboarder? Maybe.

Note that I have experienced the FPS of the D300 to outrun the AF system. For each exposure in say, a 6fps burst, the AF system might have to re-acquire the subject. If ones panning technique is off by just a small fraction, the AF system might jump to the tree line behind the rider for one exposure in the burst and then back to the rider once the panning has caught back up with the rider.

I've not yet had the chance to test the D4's superior AF system in the same situation.

One could go on and on with more examples but the above should suffice to make general point.

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faterikcartman
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Re: 9, 11, 21 or 51? How to decide?
In reply to ckon, May 14, 2012

There can be only one (not for action or movement).

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