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Author Topic: What advantage multiple auto focus points?  (Read 14995 times)

paulbk

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What advantage multiple auto focus points?
« on: October 26, 2008, 04:33:16 pm »

What's the advantage to using multiple autofocus points?

I shoot with a Canon ID Mark II (old but good).
Shooting Mode: usually shoot Aperture-priority to control the depth of field
AutoFocus: use only one autofocus point (usually center frame) and use the back star button (*) to focus
Works good.

Why does anybody need 19 autofocus points? What’s the advantage? What am I missing?

Seems to me that if you let the camera pick the focus point --by definition-- you’ve lost control.

p
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paul b.k.
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Tony Beach

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What advantage multiple auto focus points?
« Reply #1 on: October 26, 2008, 04:34:49 pm »

It facilitates AF tracking.
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paulbk

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What advantage multiple auto focus points?
« Reply #2 on: October 26, 2008, 04:55:19 pm »

Quote from: Tony Beach
It facilitates AF tracking.

I shoot a lot of GT/Prototype races using one autofocus point and auto tracking. Works good. If you use more than one autofocus point tracking may jump as you swing-pan to keep the target car in frame.

Perhaps, and it's a weak perhaps, if you keep the camera more or less "still" while autotracking maybe there's an advantage. I've found none.

respectfully,
p
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paul b.k.
New England, USA

Tony Beach

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What advantage multiple auto focus points?
« Reply #3 on: October 26, 2008, 06:34:01 pm »

Did you have a question, or are you just trying to stir up a debate?

Quote from: paulbk
Why does anybody need 19 autofocus points? What’s the advantage? What am I missing?

I have 45 AF points on my D300, 15 of which are the cross type and grouped in a 3x5 pattern in the middle.  For tracking purposes, if the subject I am trying to focus on is small and drifts off of the AF sensor I have selected, then it is usually picked up seamlessly by a neighboring AF sensor.  What's more, for the purposes of actually acquiring the target using additional information gathered from the surrounding AF sensors helps to prevent hunting.  Finally, using 3D Matrix tracking on my D300 I can focus and recompose by using the greater sensitivity of the center grouping of AF sensors to acquire the subject and then framing the target to the edge of the frame while all the time keeping the target accurately focused.

Quote
Seems to me that if you let the camera pick the focus point --by definition-- you’ve lost control.

You lose control as soon as you stop focusing manually -- and for the record, I often do focus manually.  You also lose control when you set the camera to Aperture Priority shooting mode, or use Matrix or Center-weighted metering, or use TTL flash modes.  Sometimes the world throws randomness into the photographic process, and whenever that happens you are losing control; at times like that it is not a bad thing to cede some control to the camera which can make decisions much faster than a human can.  Personally, I prefer an automatic transmission for my car, it's a lot easier to drive when I don't have to mess with a stick shift; likewise, Anti-lock brakes save lives despite wresting control from the driver at the most critical moments -- sometimes automation is a good thing.
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Ken Bennett

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What advantage multiple auto focus points?
« Reply #4 on: October 26, 2008, 08:34:06 pm »

Quote from: paulbk
Why does anybody need 19 autofocus points? What’s the advantage? What am I missing?


Well, I don't often have my subject dead center in the photo, so being able to use an off-center focus point can make things a little easier. The whole focus-and-recompose thing gets old after a while (and is inaccurate with wide angle lenses at close distances anyway.)
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The View

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What advantage multiple auto focus points?
« Reply #5 on: October 27, 2008, 01:09:30 am »

You use autofocus points to tell the autofocus what you want sharp in the first place.

Important for shallow field of depth as in portraits.

The more autofocus points are spread across the viewfinder, the easier it is to choose one that you use for the lead eye without moving the camera too much.

This is one point why I dislike the autofocus systems in current full frame cameras. The focus points are all bunched up in the center.
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Craig Arnold

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What advantage multiple auto focus points?
« Reply #6 on: October 27, 2008, 04:09:31 am »

Quote from: paulbk
Why does anybody need 19 autofocus points? What’s the advantage? What am I missing?

p

Well obviously it makes action photography a whole lot easier. Much can be done with a single AF point if you are skilled enough, but if the camera can do the work for you why not?

(I personally am nowhere near skilled enough to get any reasonable number of keepers using single AF for fast-moving subjects that don't take up a significant part of the frame.)

One advantage I have noticed with my 5D when shooting with the 50L @1.2 is that very tiny movements by the subject or my hand will throw the optimal plane of focus off by enough that I have focussed on the nose instead of the eye for example. So using the centre+6 points in AI Focus really seems to help. If the subject moves forward an inch the AF system sometimes notices and adjusts the AF and increases the keeper rate.

But what I would really love is a very good 3-point system with a bigger spread to left and right than most of the current systems. And with only 3 points we could have eye-controlled AF back. It's really a portrait request more than anything. To be able to minimise focus-recompose errors and quickly select a close point for both portrait and landscape composition.

The nicest AF system I have ever used is still my Canon 50E, which has 3 AF points and eye-controlled selection, and an IR assist for low-light.

On the off-chance someone from Canon ever reads this: how about custom 5DII that you can order from the factory with different AF systems? Ala Leica's MP. Yes, yes I can see all the reasons why it is not likely to happen, but this is an internet forum, if I can't make stupid requests here - then where?
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Robert Roaldi

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What advantage multiple auto focus points?
« Reply #7 on: October 27, 2008, 01:39:10 pm »

Well, I thought it was a genuine question and not trying to stir up a debate, but that may be because I have the same question. I think the confusion in my mind is that I don't understand what algorithms the cameras are using when trying to determine which autofucs points to use.

This is more or less curiosity on my part (although I own an E-1). I don't have a camera with 20-30 focus points, so I have never read the manual that explains how those cameras work. (I have to add that my experience with camera manuals is that they don't often explain these things well anyway). Can anyone point to a link with a good general description?

I understand the idea of using a different focus point with a stationary target, rather than focus-lock and re-aim.

But it's not clear to me what happens when trying to track a race (rally in my case) car, say, something I have tried to do with my E-1 (but not often enough yet to be able to determine what it tries to do). If you lock on with a centre spot, and the car moves out of that spot's area, how does the camera know which other spot(s) to use.

I guess this is a way of saying that I (and maybe Paul) needs to read a primer on the subject. Is there one? My camera's manual is far from clear.
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BernardLanguillier

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What advantage multiple auto focus points?
« Reply #8 on: October 27, 2008, 01:54:33 pm »

Quote from: paulbk
What's the advantage to using multiple autofocus points?

I shoot with a Canon ID Mark II (old but good).
Shooting Mode: usually shoot Aperture-priority to control the depth of field
AutoFocus: use only one autofocus point (usually center frame) and use the back star button (*) to focus
Works good.

Why does anybody need 19 autofocus points? What’s the advantage? What am I missing?

As far as using the center AF point is concerned, aren't you missing all the cases where the point of focus of a fast moving subject with limited DoF should not be in the center of the frame for composition reasons? I would think that this is most of the time, no?

Beyond that, smaller subjects do clearly benefit from tracking with more AF points.

Cheers,
Bernard

Nick Walker

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What advantage multiple auto focus points?
« Reply #9 on: October 27, 2008, 08:13:57 pm »

Quote from: paulbk
What's the advantage to using multiple autofocus points?

I shoot with a Canon ID Mark II (old but good).
Shooting Mode: usually shoot Aperture-priority to control the depth of field
AutoFocus: use only one autofocus point (usually center frame) and use the back star button (*) to focus
Works good.

Why does anybody need 19 autofocus points? What’s the advantage? What am I missing?

Seems to me that if you let the camera pick the focus point --by definition-- you’ve lost control.

p


For a large object such as a racing car, one AF point will suffice provided the user accurately keeps the sensor, on the same, or an adjacent area of contrast. On the 1D MKII the seven cross types are crammed at the centre of the frame - mainly vertically positioned, in no-mans-land - poor layout for my working method - I rarely use the central AF sensor as it encourages sloppy composition and the 'crop' brigade. Canon positioned the cross type points in the centre of the frame as their research showed that the majority of sports images were printed vertically, hence the cross hair sensors being strewn vertically - bad decision AFAIC.

The MKII's outer, non cross type sensors are vertical only types and therefore need horizontal contrast patterns. This is problematic on certain subjects, for example tennis players wearing white shirts where you are relying on folds in the shirt for contrast. Most of the professional sports shooters that I worked alongside, equipped with 1D MKII's, when using an outer AF sensor, had the the CF17 AF option set to +7 surround Af sensors - if the outer AF  sensor selected was not presented with enough contrast, one of the surrounding 7 Af points would always come into play.

As soon as I stopped shooting action, and photographed static subjects (portrait) I would turn off the CF17 and revert to only one AF point.

A tip - Canon's super-telephoto lenses keep better beat with sport using a tracking sensitivity of -1, and yet the 70-200mm F2.8 IS lens, due to it's USM motor, works better at +1 for action.

If a subject is moving very erratically CF17 expand to 7 AF points can be useful. I would say that 80% of the time I use only one AF sensor to track movement. I personally don't like the camera making decisions on my behalf but there are circumstances where multiple AF sensors allow the photographer to react to sudden movements and reframe the image in a split second. For example a tennis player framed against a plain background (Wimbledon has plain green backgrounds) where there is very little likelihood of one of the 45 AF multiple points being thrown off the player onto a background that has, little, or no contrast, to fool the AF.

Canon 1D MKIII

As all of the 1D MKIII's 'selectable' 19 AF sensors are cross types (require F2.8, or faster lens, for 18 outer AF points to remain X type) I would select a single AF sensor, often off centre, and found all of them to be very accurate with 400mm F2.8 and 500mm F4 lenses, wide open, even with a subject close to the lenses nearest focusing limit - almost jumping into the lens hood  - the only problem is that the 1D MKIII, even my upgraded sub-mirror assembly replacement was inconsistent, missing less taxing moments that my previous generation AF systems would nail in their stride.

I always tweak the AF settings to suit the subject and discipline to hand, sometimes this may involve activating additional AF points to assist me capture erratic, or very low contrast, subjects.
« Last Edit: October 28, 2008, 03:43:47 am by Nick Walker »
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paulbk

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What advantage multiple auto focus points?
« Reply #10 on: October 27, 2008, 09:46:41 pm »

Nick,
Much thanks for the information and tips. My action shots are mostly GT/Prototype races. These are road races and I generally have a wide range of flexibility in my position. My favorite position is near the center radius of a downhill turn. Normally use Canon 70-200 mm 2.8/L. These cars are moving 100+ mph and I’m standing by a fence post 200-300 feet from the track (that’s not far). Framing composition is secondary. I have enough work just to keep the target car in frame and focused as they jockey through the course. I focus lock as the target car enters the turn and machine gun shots all through the turn. The target is always moving horizontally, first moving away (as it approaches the apex), then closing (as it leaves the apex), then moving away as it leaves the turn. Lots of fun. And about half the shots are usable. Drivers love them. I live about an hour from Lime Rock Park race course.

Using multiple AF points I’m never sure where the camera will lock and focus. The center point with CF17 expand to 7 AF points seems to work well.

Thanks again,
paul
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paul b.k.
New England, USA

Misirlou

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What advantage multiple auto focus points?
« Reply #11 on: October 27, 2008, 11:15:49 pm »

I have a 50 f1.4 that has such narrow depth of field that focus-then-recompose will not produce a sharp image, and you can easily see that in the finder. Maybe the problem is that its projected focal field isn't flat. But in any case, you have to focus on the subject wherever it ends up in the frame.
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