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I'm very new to photography, and have been reading a lot about concepts and techniques. Yesterday, my brother gave my kid a gift of a professional photography session. I saw the photographer had Nikon cameras, and since I had met him before, and remembered he used to use Canons, I asked him why he switched. He said because of the focusing system.

While he was taking pictures of my kid walking towards him, I noticed the lens kept focusingn automatically without him doing a half-press of the shutter button. Now being very new to photography, I know that some cameras have an in-camera focusing system, and some others have it on the lens itself. I think he was using a Nikon D3S and a Nikon D700.

After asking him about the auto focus, he said he'd just keep the AF-ON button pressed, and kept shooting pictures. My questions are these:

  1. Is the AF-ON feature an in-camera focusing mechanism?
  2. If so, is there a Canon equivalent? Meaning, that it works just like that?
  3. If a camera has in-camera focusing, does it mean that it will automatically keep focusing the lens for you?

I've looked up a few camera specs, and noticed that Nikon has highest AF points (61 if I recall correctly), and just want to confirm if Nikon definitely beats Canon in this regard. I do recall seeing the spec of an upcoming Canon, and think that cam was going to have more AF points.

Thanks.

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The D3S does indeed have a mode where you can make it focus continuously. That is apparently what this photographer was using. On this camera, the focus detection and logic is in the camera, but the motor is in the lens. I don't know anything about how that relates to the Canon line, but I'd be surprised if they didn't have something similar in a similar level camera. –  Olin Lathrop Jun 14 '12 at 23:53
    
AF-ON isn't a feature, it's just a button that tells the camera to refocus. Every autofocus body Nikon makes, even the D40, has a continuous focus mode that will repeatedly focus as long as the shutter button is pressed halfway or the AF-ON button is pressed on bodies that have one. On lower-end bodies, the shutter release does double duty for setting focus and exposure; on higher-end bodies the functions can be split out among the shutter release and the AF-ON and AE-L (auto exposure lock) buttons. –  Blrfl Jun 15 '12 at 11:49

3 Answers 3

up vote 5 down vote accepted

This would be Canon's AI Servo auto-focus, which is their continuous servo focus option. It is available on all Canon DSLR's as far as I am aware. The other two options are Single Shot and AI Focus. On higher-end Canon bodies, you have the option of configuring one of the back body buttons to focus. On newer Canon bodies, there is usually a dedicated AF-ON button on the back of the camera already configured for this purpose. Pressing and holding the AF-ON button (or whatever button you choose) when using AI Servo drive mode will indeed allow continuous shooting with inter-frame focusing, subject tracking, etc.

When it comes to subject tracking, the camera can do some of the work, but you have to make sure you keep the subject in-frame. You also have to make sure that the subject is within an appropriate range of the active AF points, otherwise the camera will be unable to properly determine what needs to be focused or perform subject tracking. Proper continuous/servo AF technique with subject tracking and inter-frame focus adjustment requires skill, and just like any other skill, it has to be learned. You'll need to put in the practice to be able to effectively use any AF system, and the more hours, the better you'll get.

As for the highest AF points, it is actually Canon that has 61 points, on their two newest pro-grade cameras: The 1D X and 5D III. The Canon 61pt AF system is, spec wise, the best in the world at the moment, with 41 cross-type sensors and 5 double-cross type sensors, sensitive up to f/5.6. The Nikon AF system is 51pt, with 15 cross type, the center cluster of which is sensitive up to f/8, while the rest are f/5.6. Regarding whether one brand "beats" the other, that is truly irrelevant in the grand scheme of things. Both brands regularly leapfrog each other. At the moment Canon is the ISO and AF king (many early users of the 5D III have been amazed by the new 61pt AF system's capabilities and accuracy), where as Nikon is currently the Dynamic Range and Megapixel king. It is unlikely that these factors will remain the same for long, and after the next round of camera releases from both companies, the statistics will undoubtedly change again.

Don't use "who's best" as a factor for buying...that will never end well. Figure out what you want to do, what your budget is, whether you want to be able to share lenses with friends who have the same brand as you do, etc. The two Nikon cameras your friend is using, the D3s and D700, are very high-end cameras, and they cost a lot of money. Canon has similar cameras, like the 1D X, 5D III, 1D IV, etc. that also cost a fortune. If you are just starting in photography, you are looking WAY too high up the totem pole, and you couldn't possibly need a camera like any one of those listed right now. Look at the bottom range if you’re just starting. For Canon, that would be the Rebel series (xxxD numbers), and for Nikon, look at the D3xxx series. Buy a CHEAP camera body, as bodies come and go, and change every couple years. The true long-term value in photography is in lenses, and in that arena, Canon has a bit of an edge with a greater selection and a few very unique entries in their line-up, as well as some unique designs that no other company uses. Regardless, Nikon makes some excellent glass as well, and you can't go wrong with either brand.

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excellent and detailed reply, jrista. Thanks for the knowledge –  GR7 Jun 15 '12 at 20:05

Well you are asking a lot of questions here, and I'll address at least part of them.

The answer to the question you are asking in the title, is yes, Canon has an equivalent to the AF-ON. They also call it AF-ON. Not all bodies have this, but most of them do.

1.) The AF-ON feature is a feature of the body, but works in conjunction with the autofocus motor which can be located on the lens, or in the body depending on your camera setup.

2.) Yes Canon does have an option to track moving subjects similar to Nikon. Canon has two modes that can be used in combination with the AF-ON button, which are Al Servo and AI Focus. We have a great question that already addresses what the difference between the two modes are here: What is the difference between Al Focus and Al Servo autofocus modes?

3.) I think understanding the different modes that Nikon has available for autofocus will help answer this, see the following question: When to use a particular auto-focus mode

I wouldn't say that Nikon definitely beats Canon in focus points. It isn't just always about the total count, but the sensitivity and type of the focus points, which can become a much more technical topic.

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Not all bodies will have it, but on the 7D, 50D, 60D, 5D, 1D series etc, there is an AF-ON button on the rear of the camera just about where the thumb rests. This will allow you to focus and meter, without the risk of accidentally triggering the shutter. (I think the XXXD/Rebel series just have an exposure lock button where the higher end models have their AF-ON button).

To do what you want, combine the AF-ON button with the AI Servo auto-focus mode and keep the AF-ON button held, to track your subject.

Note that the Nikon I believe uses 51 AF points. Canon's latest models - the 1D X and 5D mark III have 61 AF points.

Re your comment about focussing systems being in the lens or body, note that this applies to Nikon only really. Some bodies do not possess a focus motor, meaning that some older lenses can't autofocus on them and you need the AF-S, AF-I, or D type lenses that have the motors built into the lens. In the Canon world, all EF and EF-S lenses have the focus motors in the lens itself anyway so there is no confusion here. (The only exception I can think of - aside from some 3rd party lenses - is the Canon MP-E 65mm, which is a VERY specialised lens, and is manual focus only).

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