Autofocus tracking and point selection

[DrunkenZombiee]

So I have my 70D now and it has the same AF systems out of the 7D which means its pretty snappy (pardon the pun). I am trying to use my camera differently now as I did the with 550D which only had one crosstype AF point and therefore was more limited. So with the 550D i used to use the central point for AF alone and then use single shot to focus on the subject. Very rarely would i use the Servo mode as quite frankly the AF wasn't quite good enough and I needed to keep the subject central to use the single cross type in the center breaking the rule of thirds rule. Now that I have 19AF points all cross type and the AF is fast and reliable I need to up my game.

I want to stop using only the central point to focus as recomposing changes the DoF plane as this is perpendicular to the camera and its poor show. So I have been experimenting a bit with using the options on my shiny new 70D. I don't really yet understand how the servo mode tracks a subject when using single point AF as it appears that it can track them cross the frame so it no longer is using a single point? Help!

The best method I have  found so far is to use the groupings of AF points with servo mode to track the subject when looking for fast moving objects and gunning them down with 7FPS, but for moving people its all a bit confusing to me still.

So what do you all do in different situations with your AF mode and point selection?

DZ

[smilodon]

A caveat first that I'm only familiar with Nikon cameras and so while I imagine most DSLR's work in a similar way I'm not familiar with Canon, Sony, etc. This might all be a bit obvious for DZ but I thought it might be useful generally to say something about DSLR focus generally as well.

I have three focus settings on my cameras. Manual is self explanatory and I only occasionally use this for studio work where I have a flat featureless subject that autofocus sometimes cannot work with i.e. smooth low contrast products such as bowls, plates, plain fabrics etc. I simply focus by eye turning the focus ring on the lens.

There is Single Servo (focus priority) which is Nikon speak for the camera gaining focus on a fixed point and then only firing the shutter when focus is good. If the subject moves in the frame and focus is lost the shutter will not fire. I use this when it's critical that I get an in focus image, i.e. portraits, architecture, objects. If the resulting image would be useless if it wasn't tack sharp then this is the preferred option. I can then guarantee that all my images will be in focus but not that the shutter will fire every time.

Continuous Servo (shutter priority) is used when the subject is moving or I am trying to grab very quick images i.e. sports, events, news. In this mode the camera will try to gain focus (and usually does) but when I press the shutter release button a picture is taken regardless of the focus. If getting the shot is more important than it being perfectly sharp then I use this feature. I can then guarantee that I will get every picture I try to take but not that every image will be pin sharp.


As for Focus area modes, there are usually three. Auto, dynamic and static. Auto just lets the camera decide what to focus on using it's internal software. It can be good for point and shoot stuff, but you're never going to be sure what is or isn't in focus. If I have a very broad depth of field and am shooting landscapes or street photography, then I occasionally use this mode. I'm not focusing on any one specific thing and pretty much everything from the foreground to infinity will be in focus anyway.

Dynamic allows me to move the highlighted focus point over a subject and tell the camera to track that subject and follow it around the frame. I use this occasionally for moving subjects, sports, kids or pets running about, my goddaughter horse riding and such like. On the Nikon it's uncannily accurate but not perfect and now and then it will jump subjects and start tracking some completely random object.

Single point focus is as it sounds. I have 51 focus points and can scroll the live focus point across any of them. I can then select my subject and lock focus to that. So if I was shooting a portrait I would move the focus point over the subjects eye and lock focus to that. I might then reframe (holding focus) and shoot. I can keep total control over the point of focus all the time. The down side is that I might loose my subject if they are moving rapidly, in which case I might move to dynamic. I'm in this mode 90% of the time especially when I am photographing commercially.

Finally I have less of a problem when photographing moving subjects. Most of my work is going into magazine print or onto the web. Image size isn't critical as I shoot to a far higher resolution (4256 x 2832) than my clients typically require. So it's a viable option for me to simply use the centre focus point in single point focus and then follow a moving subject by moving the camera rather than the focus point. This means that the subject is always in the centre of the image but cropping to get a picture that obeys rule of thirds, golden circle etc. still results in a large enough file size for my needs. if you print out your images poster size then cropping might cause a problem?

I guess the questions I ask is

1. How critical is it that I get a very sharp image versus just getting an image?
2 How critical is it that I get just a specific part of the image in focus (shallow depth of field) versus getting the image generally in focus (broad depth of field)?

[DrunkenZombiee]

Thanks for the response Smilo. Very in depth comments which are great.

It looks like you do what I used to do with the single point focus.

On my camera I have thew following AF modes : One shot, AI Focus and AI Servo.
One shot:
Now I mainly use one shot for portraits and people who are relatively static (obviously use manual if needed if the camera is hunting, Macro work or low light). This will focus once and keep the focus for a long as a hold the shutter half pressed.
AI Servo:
I have been using this for fast moving objects that I want to track it readjusts in between shots even with the shutter half pressed.
AI focus:
Somewhere in between AI servo and One shot as it will attempt to switch between the two with an algorithm. This often doesn't work too well.

Now traditionally I have manually pre-focused on a moving subject like a formula one car or a sprinter as I didn't have the 7FPS machine gun shutter nor a great Servo system and just took the picture when they were in the frame bye eye. So I have never really learned the sports tog skills as my camera was the limitation.

With the 19 Point AF from the 7D I have manual selection of a single point zone selection of groupings of points (5 zones in a sanitary towel shape around the frame) and Auto which is pretty useless unless everything as at the same depth or pretty far away.

Now my concern is around the use of a single point using AI servo mode. Rather than keeping the focus on that single point is will happy track the subject across the frame which freaks me out as sometimes it can get it wrong with fast jerky movements or panning shots with objects in the foreground in front of the subject. Ii this normal as I kinda want the AF point to not move and to stay on that point while tracking the subject. Its all very odd! A check in the manual me thinks.

I really want to be able to use AI servo while locking onto someones eye to keep their face in focus as they move for taking shots of family etc. I recently shot a couple of weddings with my 550d where this would have been useful.

I think so solve this I am going to get used to back button focusing and AI servo mode then anyway as you can change between Single shot and AI servo really with your thumb which is pretty useful as Canon doesn't have the focus and shutter priority like NIKON out of the box.

Cheers for the help.

DZ.

[smilodon]

The nice thing about DSLR's over point and shoots is that there is usually more than one way to get a shot taken. So it's often a case of just trying stuff out until something seems to work naturally for you. It's quite possible that someone else will find it easier to use a different technique to yours or mine. One person might use tracking focus and someone else (me) might use a fixed focus point, snipe the moving target and crop later.

[DrunkenZombiee]

I agree.

Now I have good tech I want to use the AF servo mode to its full potential. I may go back to what I know later with the single point pre-focusing.

I am going to give back button focusing a go on my 70D to see if I get on with it while in servo and single shot. It makes sense to me anyway to have the AF separate from the shutter anyway as I can rattle off photos in bursts stopping when i take my finger off the shutter button, without loosing focus in Servo mode.

DSLR's are pretty much the ultimate in configurability these days. I have more than 10 options for AF buttons on my camera to suit everyone.

The only problem is that anyone else that picks it up is now going to have a really hard time getting it to take a good photo =).

DZ