Autofocus, and its focusing modes

I know that the sequence of articles is a bit backwards, in my last blog Let’s get focused, I addressed the advanced topic of back button focus, and this time I am focusing on the standard focus modes of Sony Camera.

Autofocus is a very lengthy topic, and it is likely that I will not address all modes or all aspects of it. For me it was absolute key to understand it, how it works and its different modes to harvest the full potential of the system. Because as wonderful as Autofocus can be, as annoying is it when it results in the wrong focus, blurry and unsharp images.

This article is fairly intermediate in its approach, so if you are an advanced user, you might not find much news here J

This article is going to address

Making sense of focus points

The robustness and flexibility of autofocus is primarily a result of the number, position and type of autofocus points. The number and type of focus detection points affects how well the system operates. Sony Alpha A99V features 19 primary focus detection points, including 11 cross-type that are available in all of the autofocus modes. An additional 102 assist points are available in AF-D Mode when using certain lenses.

There is difference between focus detection points and cross-type detection points. Cross-type is a two-dimensional contrast detection which gives higher accuracy than vertical points which only are one-dimensional.

Having 19 points gives you plenty of options when you want to specify the single point that will be active. You cannot select any of the additional 102 points. These points but they are useful, enabling the camera to continue focusing on moving subjects even if it’s not in the part of the frame covered by the 19 primary points.

As the camera collects focus information, its Autofocus processor (AFP) iteratively evaluates the contrast data to determine whether sharp focus has been achieved or additional changes needs to be made to the focusing distance. The focus may rely on information as to whether the subject is moving, and whether the camera needs to “predict” where the subject will be when the shutter button is fully depressed and the picture is taken. The speed which focus is evaluated and the lens elements moved into the proper position to achieve sharp focus, determines how fast the AF system is, but usually this entire process is completed within a fraction of a second.

Modes and how to use them

There are situations where one mode of autofocus isn’t up for the job, that’s why our cameras feature different focus modes. By letting you choose between primary focus modes, your camera is giving you the flexibility to capture exactly what you want.

AF-S, Single-shot autofocus, also called single autofocus. Focus is set once and remains at that setting until the button is fully depressed, taking the picture, or until you release the shutter button without taking a hoot. Activate by pressing the shutter release halfway down. For non-action photography, this setting in usually your best choice, as it minimizes out-of-focus pictures (at the expense of spontaneity). The drawback here is that you might not be able to take a picture at all while the camera is seeking focus; you’re locked out until the autofocus mechanism is happy with the current setting. Single Autofocus is sometimes referred to as focus priority for that reason. Because of the same delay while the camera zeroes in on the correct focus, you might experience slightly more shutter lag. This mode uses less battery power.

AF-A, Automatic autofocus. The Focus Mode is switched between Single-shot AF and Continuous AF according to the movement of the subject. When you press and hold the shutter button halfway down, if the subject is motionless, the focus is locked and if the subject is in motion, the camera continues to focus. Personally I am not a fan of this mode; in my views it is for greenhorns. I much prefer to enable Back Button Focus allowing my full flexibility to activate my auto focus when needed.

AF-C, Continuous Focusing Mode. This mode is also called predictive autofocus. This mode continuously maintains focus on a moving subject while the shutter button is held halfway down. This is great for tracking moving subjects and it is a must for shooting sports, wildlife and other non-stationary subjects. Do mind that this mode uses heaps of battery because it is continuously focusing and refocusing.

This setting is the one to use for sports and other fast-moving subject. In this mode, once the shutter is release is partly depressed, the camera sets the focus but continues to monitor the subject, so that if it moves or you move, the lens will be refocused to suit. Focus and exposure aren’t really locked until you press the shutter release down all the way. The focus confirmation indicator in the viewfinder is flanked by parentheses-like brackets, which indicates that the image is in focus, but the Alpha will change focus as your subject moves. Continuous AF uses the most battery power, because the autofocus system operates as long as the shutter release button is partially depressed.

 

DMF, Direct Manual Focus. This is a wonderful mode that enables you to adjust the focus without switching from autofocus to manual. After AF confirms focus, you are able to rotate the lens’s focus ring to touch up on focus as you sees fit. This is pretty neat in situations where the focus isn’t quite where you want it, the camera might have focused on a person’s nose; and you want to fine-tune the focus manually so the nearest eye gets focuses. Modern lenses often have manual override, which such glass this function is redundant.

AF-D, Depth Map Assist Continuous Autofocus. In addition to the Continuous AF function, the camera continues to focus on a subject using the assist area. As this option only can be used with certain lenses, and that I am a fan on old Minolta lenses, it is not a function that I have much experiences with. What Sony writes about this is function enables the AFP to use an additional 102 points beyond its 19 focus point system. These 102 points assists in determine your subjects motion, and yields increased accuracy.

MF, Manual focus. This function is often redundant on newer lenses, as they mostly are equipped with an AF/MF switch on the side of the lens. With manual focus your Sony Alpha let you set the focus yourself. Battery life will be slightly longer in manual focus mode. This mode is great for low light situations, backlit scenes, or scenes where the AF system just isn’t up for the job.

The dreaded focus hunt

When autofocus fails, it might result in the dreaded “focus hunting” scenario. The camera tries to focus back and forth repeatedly without achieving focus lock. Obviously this is very unfortunately, while it may cause you to miss the shoot you are looking for. This is particularly true when shooting birds in flight.

Sony Alpha A99 features an AF Range acts as a virtual, adjustable focus limiter. This is function is handy when knowing the range, in which your subject movement is within. With this function you can tell the camera not to focus outside that distance range, and by this preventing the camera getting distracted by movement in the foreground or background.  

 

Sony – “Focus Magnifier” vs. “MF Assist”

The A99V enables the displayed image to be enlarged up to 11.7x by pressing the Focus Magnifier button. This is particularly handy when doing focus manually. Yet it can be very cumbersome, “MF assist” seems to be a further development of this feature which takes some of the cumbersomeness away, an evolvement which the A99 is missing.

The newer software setups on cameras like RX1 have “MF Assist” option, which, when turned “On” instantly zooms in to 5.9x as soon as you turn the focus ring. No buttons need to be pressed; it just magnifies the focus and gives you the focus peaking if you have that turned on as well. On A99 we have the “Focus Magnifier” option that can be custom assigned to a button to perform a similar function: zoom in to 5.9x+ when you press a “Focus Magnifier” button.

This is clearly a low hanging fruit of which Sony has chosen not to give us A99V users. Frankly Sony… you can do better for us high end users!

Focus Peaking

To determine the focus correctly you have to use the magnification feature and that is quite a pain. Focus peaking is a fast way to determine what is in focus. Thus not showing the actual focus, it still highlights contrast edges. If used at the right level of peaking and color it can give you a strong cue to where the focus lies. It takes a little practice to learn how to ‘read’ the peaks as they build up and drop away while you focus, but with a little learning effort you can get very fast rough focusing that you can quickly refine by zooming in.

But be careful, focus peaking is based on the low resolution jpg also shown in your viewfinder. The actual sensor data is of much more resolution. So the best way of using focus peaking is to use it to initially find the contrast edges, and then using imagine magnification go in an make you’re your focus is correctly

Focus peaking works really nice in normal light scenes, and especially well if your subject isn’t moving too fast. You can tweak it a little by shooting in Raw+JPG and setting the camera to a black and white preset so that the focus color shows up more distinctly on the screen. So for the accurate focus peaking setting is B+W mode with +3contrast and +3sharpness, red color peaking.

Do watch the video here to find more info.

What the High, Medium, Low peaking level really mean. In the manual, there is no explanation, just stated how you set the different levels. As previously written Peaking only “sees” contrast, and not focus. The “level” is just a sensitivity setting, with “low” highlighting only areas of high contrast and “high” highlighting many more, lower contrast areas. Which is “best” comes down to a combination of contrast in the scene and not least lens sharpness. There is no golden rule, of which setting that fits the best, you will have to experiment for yourself. Use the focus magnification and see if the focus is as intended. Aim for the minimum amount of visible peaking consistent with you actually being able to see it when deciding which level to use for best accuracy.

 

Setting the Autofocus Area

Wide, the camera chooses the appropriate focus area from its 19 autofocus points

Zone, let you choose between the 19 points grouped by left, center or right. The camera will decide which of the points within the group that will be used. There are six points in the left and the right groups, and seven in the center zone.

Spot, with this function set will the camera always focus on the center autofocus point.

Local, gives you absolute freedom to choose from which of the 19 points you want the camera to focus on.

How to camera communicate Autofocus to you

Your camera communicates the autofocus states to you through an indicator.

Focus indicator

Status

Lit

Focus locked. Ready to shoot.

Lit

Focus is confirmed. Focal point moves following a moving subject. Your camera is ready to shoot.

Lit

The camera is still focusing. You cannot release the shutter.

Flashing

Cannot focus. The shutter is locked.


 

One thought on “Autofocus, and its focusing modes”

  1. Nice job, I’m a big fan of DMF when I do use AF. And I would like MF Assist on the A99 but focus magnification becomes second nature after a while.

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