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I've just updated my body from Nikon D40X to Nikon D300 and realized that the D300 has a AF-ON button on the rear of body. That button's feature do the same as half-pressing shutter button, which focus to the object.

I did some research and found that most entry-level and mid-range bodies do not have this button. I've read What is the advantage to back-button autofocus? but it does not answer my question.

In addition, how does this button help the photographer in taking picture? It's put into semipro or pro body. Why does a "PRO" need it.

I'll be happy if anyone can give some use cases which need an AF-ON button rather than shutter button, too.

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    You say you referred to a previous question, but you didn't find any suitable answers there. Could you specify what exactly is missing from those answers? Because to me your question looks as a duplicate of that one. – Saaru Lindestøkke Nov 28 '13 at 11:02
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    possible duplicate of What is the advantage to back-button autofocus? – Saaru Lindestøkke Nov 28 '13 at 11:10
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    Yes, I read that, but he doesn't say what those answers were lacking. The OP asks for advantages, the linked question contains those. Additionally a use case is asked, which is given in one of the answers of the linked question. The linked question contains the answers to all the questions the OP asks at this time, hence I flagged a duplicate. – Saaru Lindestøkke Nov 28 '13 at 14:58
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    Ok, encouraging sounds reasonable. @TuTran So should the question be rather: why do entry-level bodies don't have an AF-ON button? Is that where you're more interested in? Because the answers to the linked question already explain why it is important for a pro. – Saaru Lindestøkke Nov 28 '13 at 21:41
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    Possibly better to ask why Canon and Nikon don't bother with this button on their entry-level models? Sony's cheapest entry level (SLT) body has this button, and Pentax has it too. – Esa Paulasto Dec 2 '13 at 16:31
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The AF-ON button can be used as the sole means of activating focus on cameras with configurable buttons. There are many occasions where activating AF at the time the shutter button is activated is NOT desired, and in fact can result in less than optimal camera performance. Separating AF and shutter is of particular importance and usefulness when using AI-Servo or Continuous focus mode, where you are focusing constantly on moving subjects.

Personally, I use what is often termed "rear-button focus". I use the Canon EOS 7D, which allows me to customize the functions of a lot of my camera buttons. I reconfigured the shutter button to only activate metering and the shutter, and configured the * button to activate AF, and configured the AF-ON button to stand in for the original function of the * button (lock auto exposure). I use this particular button config simply because the * button is far more convenient for activation by my thumb than the slightly inset and farther away AF-ON button (as I use AE Lock much less frequently than AF ON.)

I am now free to focus to my hearts content without actually taking any photos, which can be useful when you just need to observe the behavior of a subject (in my case, usually birds and wildlife). Even though space is cheap, it is wise to take a shot only when your likelihood of capturing the right kind of behavior, the right kind of action, is high. A shutter-happy photographer can easily burn through 64 gigs of CF cards in a 6-8 hour period of time (trust me, I know first hand, as I was rather trigger happy when I first started doing bird photography.) These days, I usually get away with one, maybe two 16Gb CF cards over a similar time period.

There are other benefits to separating shutter and AF functions. The shutter button also activates metering and image stabilization if half-pressed. You have far more fine grained control over the behavior of your camera when you separate out the critical AF function from the most used button on the camera. You can activate IS/VR if you need it, or not, as necessary. You can activate metering with a shutter half-press, lock it in, then recompose, AF, and take the shot. This is almost an essential behavior with the kind of photography I do, where I usually meter off the sky first, lock in that exposure, then manually compensate according to the subject after re-framing.

Similarly, separating AF out to its own button makes "focus & recompose" a synch, where as with shutter+AF combined, you usually have to hold the shutter button half down to maintain your focus during recomposition (and then, only in non-servo modes.) Another side effect of using a dedicated AF button separate from the shutter button is that you can leave your camera in AI Servo all the time. If you need "single shot" behavior, simply hold the AF button until your subject is focused, release it, and take your shot or shots. When you need servo functionality, just hold the AF button down until you no longer need focus, and take as many shots throughout that period of time as you need to, even with continuous shooting modes.

Separating AF and shutter can also help reduce missfocus. It is often the case that you focus on a subject, capture some frames when it does something interesting, wait for the subject to do something else interesting only to have the camera suddenly start hunting for focus the moment you start shooting. Separating AF out to its own dedicated button allows you to lock focus, then keep the camera focused on that point until YOU, rather than the camera, decides to change it. This greatly reduces the chance of your camera behaving poorly and refocusing a scene that was already focused. If, for whatever reason, your subject moves, you can instantly refocus just by pressing your dedicated AF-ON button.

Fundamentally, AF-ON, in cameras that have configurable buttons, gives you a dedicated AF button that can be configured as the sole means, rather than an alternate means, of activating and deactivating AF. This, in turn, gives you more explicit control over your camera'e behavior.

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    Why not post/move this answer under the earlier question? – Esa Paulasto Nov 28 '13 at 13:14
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    @EsaPaulasto - I'm not sure it's a dupe, the OP mentions that it doesn't answer his question. It would be pretty suspect to have some say question X doesn't answer and close his question down by telling them to read question X... :D – John Cavan Nov 28 '13 at 14:32
  • I agree with John, OP clearly said he read the "dup", so I answered here. – jrista Nov 28 '13 at 16:39
  • Your answer is so interesting. It explains more clearly than I expected. And the way you take picture make me exciting to be a photographer. – Tu Tran Nov 29 '13 at 8:50
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The two main reasons are cost and the lack of demand for the feature from the primary purchasers of low end DSLR cameras.

Every additional button increases the manufacturing cost of a camera. In addition to the button and the electrical switch it activates there is also the additional consideration of some sort of seal between the button and the body. Even non-weather sealed bodies provide rudimentary protection from environmental dust and moisture. Then there is the increased number of inputs required to the camera's central processing unit and the need for additional circuitry to allow more buttons to communicate with the processor. Then there is the additional code needed to enable the camera to allow the user to modify exactly what each button does and does not do.

Although there are many exceptions, by and large most entry level DSLR users have never heard of 'back button focus' and probably wouldn't be interested in learning how to use it if their camera offered the feature. (The few that do want it can remap the Exposure lock button on many entry level cameras. If the manufacturers gave us everything in the cheapest model it wouldn't be nearly as cheap and why would anyone buy the more expensive ones?) The same group of purchasers who most often use Auto or various Scene modes rather than A/Av, S/Tv, M, or even P probably aren't willing to devote the time to learning how to remap any of their camera's buttons, much less practice enough to use them effectively. The vast majority would rather save a few dollars on a camera that doesn't include a feature they don't know about and wouldn't learn to use if they did.

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Control which is the reason for most professional features. This gives you a chance to decide when the camera focuses. Most cameras will disable autofocus on the half-press of the shutter-release after when AF-On is pressed. The same is often done by recomposing and half-pressing but that usually tied exposure to focus. Advanced users may want to often separate the two events and a lot of professional models allow this behavior to be tweeked.

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Lack of a dedicated AF-ON button does not prevent the use of the back-button focusing technique. I had a Rebel XS and I've used other Rebels that use one of the other back buttons (* I think) as the AF-ON button via a custom function.

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Mainly simplicity - it is easier to press the AF-ON button to lock focus/exposure.

Also, it is easier to press a button at the back than to half depress a shutter button.

Lastly, there is also habit - if you are used to back button focus, you will prefer it. (I use the back button to focus on my 5D MK II all the time but I also retain the shutter button focus.)

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