back button auto focusing.

<p>i have just recently heard about "back button auto focusing". are a lot of users familiar with this setting, and in fact, using it? what are the advantages/disadvantages for this setting? i am using the canon 5d mark II. thanks.</p>

<p>I'm not sure about the 5DII, since I don't have one, but on all the EOS bodies I've owned, it's done by setting Custom Function 4 to "1". It's my default setting because it ensures that the camera will only attempt to focus when I want it to, not when I push the shutter release. I can't imagine not using it when employing a tripod, for instance. It does slow down the process of focussing and taking a picture I suppose, but that isn't usually an issue for the kind of photography I do.</p>

<p>I love it because I can autofocus when I want to, and not be forced to when I'm about to take a photo. Often my selected point for focusing is not the same as the AF points in my composition. I'm not very proficient with birds in flight, and that's the only time when I like to switch back to focusing with the shutter button.</p>

<p>On my 5D mk I, I usually have CF4 set to 1. You'll just have to try it to see if you like it. It works well for me, though it did take practice to get quick at it.</p>

<p>I use it most of the time. The obvious advantage is you focus and shoot a frame or multiple frames without loosing your intended focus. Its just something you need to play with, I find I like it and use it about 75% of the time now.</p>

<p>Try it out, I think you'll like it, it separates focus from shutter depression. I use it 99% of the time, apart from times (like Stephen's example above) when you can't afford to have a lag, and your subject is centered.</p>

<p>The 5Dmk2 (along with the 40D, 50D, and 7D) has a dedicated "AF On" button (just to the left of the "*"). This obviates the need to change the function of the "*" button with a custom function (although you can do that too - at least on my 7D - in case your thumb is already trained...).</p>

<p>I switched about five years ago and I would never go back. It took me about a day to get used to it, and now my thumb just automatically hits the "*" button when I think "focus". Highly recommended, esp. for AF-Servo.</p>

<p>thanks for the responses to my question. which of the custom functions should i use? is custom function '' 1 " the best?</p>

<p>Also using the button the back is the best way to focus when shooting video with either the 7D or 5DII</p>

<p>Not to hijack the thread, but this function basically allows you to focus in advance, and then wait for the scene to be "right" when you press the shutter button?</p>

<p>Geoff, you said you use it for landscapes, and can't imagine not using it when using a tripod...why is that? It seems in both cases, you have the time to wait for focus when pressing the shutter button.</p>

<p>Jay, to answer out of turn, many times the point of focus in a landscape isn't where the camera is pointed. So using the AF to set the exposure and then compose and not worry about the shutter release refocusing can be critical.</p>

<p>I first got used to the back button autofocus on my H2 and found it invaluable when doing aerial photography. I could set it periodically on something that would read and then shoot without worrying about the "search". I was very glad that my dSLR also had this feature.</p>

<p>The biggest thing to get used to is just remembering that the shutter release doesn't focus, but once you get used to it, it is the absolute best way to shoot IMO. I might feel differently if I were to shoot sports action, but for other things, it is the best practice.</p>

Back focusing is nothing new, I began using custom F4 with my Canon EOS-3 film camera which allowed me to use the "*" button in the back of the camera to Auto Focus instead of using the shutter.

 

I was having major problems with the EOS-3's super sensitive shutter as compared to my other cameras, so I switched to using this function for a while. The only thing is that it threw me off sometimes since I had to constantly remember to press this button if I wanted the AF to kick in.

 

I finally figured out that by using the tip of my finger to control focusing with the shutter button on my EOS-3, I would not mishaply trigger it, so I switched back to using the shutter to AF which was more natural for me, but this is nothing new. All EOS cameras allow for this.

<p>I use CF4 setting 1 because I hate it when I push the shutter button and the camera doesn't take a picture. When shooting candids in low light, which I do a lot, the camera can't always focus. When you're set at CF4 setting 0, if the camera isn't sure it has the right focus it won't take a picture. You can switch the lens to manual focus, but that takes too long for candid shooting. And if the light is good in some parts of the room and bad in other parts, you'd have to constantly flip the lens from MF to AF and back. Much much easier to just divorce the focus and shutter buttons so they can be controlled independently. You get all the benefits of AF while being able to instantly return to MF when you need to. And when you press the shutter button, the camera always takes a picture.</p>

<p>A lot of people use the CF-IV : 1 settings incorrectly, there are five choices and unless you understand what they do and how they interact then it will result in frustration and much AF "inaccuracy".</p>

<p>For example, CF-IV : 1:1 actually stops the camera focusing when you push the AF button. It initiates focus and metering with the AF button but stops AF when you push the AF button. To set the camera up to separate the buttons like most people assume, and want, so metering is done with the shutter button and AF controlled only by the AF button you need to set it to CF-IV : 1:2 or CF-IV : 1:3.</p>

<p>This is from the 1Ds MkIII menus but the custom functions work the same for both cameras.</p>

<blockquote>

<p>CF IV–1: Back-button AF control <br />1-0 — Shutter button and AF-On button both activate AF & metering <br />1-1 — AF & metering at shutter button; AF-On button now locks focus <br />1-2 — Metering only at shutter button (no AE Lock); AF-On button starts AF <br />1-3 — Exposure locked at first press of shutter button; AF at rear AF-On button <br />1-4 — AF & metering at shutter button; rear AF-On button disabled <br />The new AF-On button at the rear of the camera gives EOS-1D Mark III users a full-time method to activate AF. This Custom Function <br />allows changes in its operation, and also allows the user to remove AF operation from the shutter button if they desire. C.Fn IV-1-3 is <br />the equivalent of C.Fn 4-1 on previous EOS cameras (“back-button AF” with AE Lock at shutter button), and C.Fn IV-1-2 is the same <br />as C.Fn 4-3 on previous cameras (“back-button AF with no AE lock). Note that with C.Fn IV-2 (see below), you can use the AE Lock <br />button on the EOS-1D Mark III instead of the AF-On button. <br /><br /></p>

</blockquote>

<p>CF IV 1-2 is what I use.</p>

<p>thanks for all the responses. i have decided to use custom function 1 or 2. here is my question. do i have to keep my finger on the rear af button in function 1 for it to lock focus. if so, i think i prefer function 2. any comments?</p>

<p>Howard,</p>

<p>In the 1 position the AF button will STOP AF working, it will never lock focus (it will stop the AF working) in selection 1. In the 2 position AF ONLY works when you press the AF button.</p>

<p>Scott, the 5D mk I behaves completely differently from what you're describing. Did they change the meanings of the settings? I've never used a 5D II.</p>

<p>Alan,</p>

<p>Yes they did, it has caused a lot of miss use and negative reports of 5D MkII AF performance. The 5D MkII, that Howard says he is using, is totally different to the 5D. The list I quoted above is how the CF-IV : 1: 0-4 selections work on the 5D MkII.</p>

<p>Hope this helps, Scott.<br>

P.S. It seems the confusion is in the awful wording they use, for "lock focus" read 'stop focus' it locks the focus from moving, it does not lock it on to a subject or make it work, it turns it, AF, off.</p>

<p>Position 2 works better IMO as it is a positive control, push it=AF, don't push it=no AF --just an on/off switch.</p>

<p>I use a 7D, so I see that I have a button that does this already. I just need to customize the shutter release to begin metering. Does full auto override this, if I were to give my camera to someone not DSLR-savvy to take a picture?</p>

<p>thanks for all the responses. i have decided to use c.fn iv-1, option 2. this will turn the auto focus on and lock it with a push of the af button, but will still allow exposure to be constantly monitored. i think that the suggestion to think of "stop" to be substituted with "lock" clarifies the confusing wording.</p>

<p>if set the custom function to '1', do i have to hold the af button constantly to lock focus?</p>

<p>Howard,</p>

<p>Yes, if you are in CF-IV : 1:1 then when you press the shutter button half way the focus and metering initiates and they both work all the time you are half pressing the shutter button (obviously in Servo AF the focus constantly, in One Shot it just acquires and stops). When you push the AF button, AND you are in Servo AF, the AF stops all the time you press that AF button, it will restart when you let it off.</p>

<p>A good use for this mode would be following a bear through some trees, or a soccer player where other players come in front of your subject. Press the shutter button to make the camera work like normal, but when something (a tree or other player) gets between you and the subject then push the AF button, this will suspend AF operation and prevent focus acquiring on spurious closer objects.</p>

<p>Hope this helps, Scott.</p>

<p>I just started looking into this myself. I havent yet had much use with it but I can see it's appeal. One thing to remember, it will only work in the "creative" modes. AV, tV, or manual.<br>

Cheers,<br>

Mark</p>