10/05/2012

More on Focus.....



I've had a few questions in after my recent posts on focus, so I thought I'd try to answer them here. Here is a comment emailed through the other day which I think pretty much sums them all up....

" I've been researching focus modes and focus points. i'm officially more confused than ever. i'm a Nikon user and have the options of af-a (auto), af-c (continuous), and af-s (single) and the the points options of single, matrix, and auto. i assume that with children i should keep it on af-c and matrix. I'm completely confused about choosing a focus point. I understand holding the shutter button half way, obtaining focus, and recomposing. from what i understand, you can only do that in af-s mode and not af-c mode. but i could be wrong on that. i've been trying to understand the back button technique, and i still don't...arg! do you press the af-on and release then press the shutter? i'm completely confusing myself!"

I'm going to try and explain this a bit further, but if you haven't already done so, please read the posts on Focus Modes and Focus Techniques again, then come back here for a bit more explanation!


Although you have Auto Modes too, you will either be using AF-C / Al Servo, which is for subjects that are moving (or likely to move) or One Shot / AF-S, which is for still subjects.  (I'm not going to cover Auto because there is no need - it will do it all for you).  I'll go through these both individually, and try to use the terminology used by both Nikon and Canon - but please, please bear in mind that there are a lot of different models out there that have different features and capabilities, plus have different names for these (just to confuse matters further) so I'm going to be general, and you'll need to look at your specific camera model. Also, remember I don't use Nikon, I use Canon, so I might not be 100% there with the terms but you should get the general idea.

I'm also going to run through some background so everyone is clear on what Back Button Focus (BBF) is.  Normally, the camera focuses when the shutter release is half-depressed. Once focus is achieved, you then push the shutter all the way down to take the picture. Back button focusing is when you set your camera so that the shutter button doesn't control the focus, but another button on the back of your camera does. (The back button is really just doing the same job as half depressing your shutter) Remember, you will need to manually set your camera to use this back button for focus first. Which button this is will be depends on your camera model.

AF-C / Al Servo 
I like to use AF-C/Al Servo with kids as they tend to move about so much. If you are using this with your shutter locking focus (the normal way) keep your finger half depressed on the shutter until you are ready to take a shot, then push the shutter down fully. (The camera will keep focus on the moving subject until you press the shutter fully) To use AF-C / Al Servo with Back Button Focus (BBF) you press and hold down the back button and use your shutter to take the picture. If you keep holding down the back button you can fire off as many shots as you like with the shutter.  As long as you keep your finger on the back button, the camera will keep focus and is the reason BBF works great with Al Servo / AF-C.

Bear in mind that whether you use the shutter or BBF to focus, you won't hear a beep to tell you that focus is locked in the same way that you would with One Shot / AF-S.

You can't use the focus-recompose method with AF-C (Al Servo for Canon).  If you try to recompose in this mode, the camera will think that the subject has moved and you will lose focus. You can choose your focal point, but Al Servo is most reliable when using your central focal point so shoot a little wider, and simply crop to your chosen composition in editing.  I've gone over how to do this with Canons in the previous posts, but for those with Nikons, Single Point AF is the one single empty box icon with no hashes (see below). Both are the same idea. With Nikons, you also have the additional option for using Dynamic Area AF - this is the icon with one small empty box in the middle, surrounded by four hashes. With this, the camera will track focus and choose a focal point for you, and is a very good option!





BBF & One Shot / AF-S 
If you little darlings are sitting relatively still, you can change to AF-S/One-Shot.  You could still use Al Servo / AF-C, but if you are taking a few shots of a still subject it's better to use One Shot as it will be slightly sharper.  To use BBF in this mode, you press down the back button to lock focus, release, then press the shutter to take your shots. You do not need to press the back button for each shot you take, provided the subject hasn't moved, or the distance between the camera and your subject hasn't changed.  I tend to use the back button to focus every couple of shots even if I haven't moved just to be on the safe side, but perhaps with practice I will get more confident and not feel the need!  You will be using Single Point AF, and you can use the central focal point and use focus-recompose (i.e press the back button for focus, release, recompose, then press shutter) or you can toggle your focus points.

Let's lastly look at changing your focal points and how you would put this to practical use.   Here is a photo I took recently and I've given a rough overlay showing the focus points available. (sorry, I whipped this up in under a minute as you can see - the points aren't even in line!) I can choose to use the central one, or I can toggle to any of the other points that fit my composition better, and save me having to recompose the shot. In this particular instance, I toggled until I got the focal point with the arrow (which was over his eye)  As you can imagine if your toddler is running about this is nigh on impossible, in which case, use focus-recompose.



Lastly, here is a rundown on when I would use what.....


Canon Users 

Sleeping babies or children - One Shot with Single Point AF - Choose your focus point
Toddlers / Kids Playing Sports / On swings etc - Al Servo with Single Point AF - Use central point, or choose focus point but beware not as reliable.  Do not Focus-Recompose.
Older Children / Adults - One Shot with Single Point AF mode - Choose your focus point or use Focus-Recompose.


Nikon Users
Sleeping babies or children - AF-S with Single Point AF & choose your focus point
Toddlers / Kids Playing Sports / On swings etc- AF-C with Dynamic Area AF mode for shooting with high aperture numbers, or Single Point AF if shooting with low aperture numbers. Do not focus then recompose.
Older Children / Adults - AF-S, Single Point AF mode - choose your focus point or use focus recompose.

The most important thing to get your head around is the focus modes of your particular camera. If you don't yet fully understand these - get to grips with them first, before trying back button focus.  Perhaps get confident using one of the modes, with one of the focusing techniques before moving onto the next one. Once you get in nailed, and you know which mode to use when, you will start to find that your "hit" rate of in-focus shots starts to increase!

I'm reading all this again before I post and not sure if I am doing a terribly good job at explaining,  but I'm afraid it may be the best I can do! I hope this has cleared up a couple of points for some of you. If anyone has any questions, please leave them in the comments section below so that I can reply where everyone can see the answer - if you have a query, it's likely others have the same one too!

12 comments:

Jan said...

audrey, this is so clear and concise...beautifully written. thank you! now i feel like i understand enough to start practicing these techniques. you are an amazing teacher and to be able to do that via a blog is a gift. thank you!

Audrey said...

Thank you so much for your kind words and I'm glad I've managed to help a bit!

Anonymous said...

Just wrote long comments to two posts on my Mac but they disappeared! Trying on my iPhone ...
Using canon & 50mm 1.4 lens on f1.4 & 2.8 I did a test on center focal point recompose v left focus point. Center recompose was much sharper (on tube of toothpaste). Did you find this or is it my user error?

Anonymous said...

Also using Ai servo is idea to depress shutter halfway to focus on toddlers eyes (if not using bbf) & try to track toddlers eyes while holding it down & then take shot? Eg when they look into camera (using center focus point on their eyes)

Anonymous said...

Ps I agree with everything jan said!

Audrey said...

Hi there, I've never yet done a test on centre point vs outer focal point, I'll maybe do one and report back! The central point is meant to be more accurate than any of the outer points though, but quickly recomposing can sometimes be enough to move you off focus if using a shallow depth of field.
Regarding Al Servo, if they are moving about pretty fast, I tend to use a bigger f number and just focus the central point high on their torso, as all of them will be in focus if the DOF is large enough. it is easier. If they are slower moving and I want to use a shallower depth of field I try to focus on their face/eye and keep over it. I may sometimes toggle my focus points but it really all depends on how much time I have vs how fast they are!

Anonymous said...

Wonderful easy to understand answer thank you!

Jan said...

just so that i am completely clear, in af-c/al servo mode, i want to keep my camera completely still as long as i have the focus locked on the center point with a larger DOF...so i DO NOT move the camera body with the child. if i need to move the camera body, then i need to start over by refocusing on the torso with the center point and fire away.

i've gone through a lot of my photos in iphoto and thought about whether or not i should have used af-s or af-c for them. will pictures not be as tack sharp if i chose af-s and there's slight movement? for instance, taking pictures of a child in a high chair. the child isn't going anywhere, but there will be slight movement. could i use af-s for that scenario? i'd like using focus/recompose there (i don't have back button capabilities on my current camera body). another example...child playing with blocks on floor or sitting at table with crafting activity. could i use af-s as long as i'm quick with my focus point, or should i choose af-c? i understand that af-c is absolutely necessary for playground, sporting events, and first steps photos.

you are amazing. thanks again!

Audrey said...

Hi Jan, ok answer to first question - no, in Af-c you're not keeping the camera still, you DO move it about, keeping the centre point (or whichever you are using) on your child. Use the camera to track the subject. Bear in mind the camera tries to predict focus with thiis, you won't get every single shot in focus. As for the second, you are right pictures might not be tack sharp if your subject moves after you have focused with Af-s. as for which one to use in situations where there is some movement, I would say personal choice and what you are trying to achieve. You will get sharper images with Af-s but you might get more out of focus ones. If you only get a second or two to take the shot, use AF-c. Personally my camera is on al servo most of the time, but I switch when he is relatively still, as I am more comfortable using focus recompose. Hope this helps x

Jan said...

thank you. the light bulb is shining brightly in my brain, thanks to you! i'm finally having some real "ah ha" moments with this stuff.

Audrey said...

I know that moment!! It's like somehow all this stuff that didn't make much sense just clicks in. Now for the practice part!

Vishal said...

I think you can still re-compose with AI Servo by NOT pressing * (BBF) button after focusing. So just re-compose your shot after focusing and press the shutter button. I've tried and it works like charm.

Thanks for this great article. It does help a lot.

Vishal

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