{"id":129377,"date":"2019-07-17T12:05:27","date_gmt":"2019-07-17T11:05:27","guid":{"rendered":"http:\/\/amateurphotographer.co.uk\/?p=129377"},"modified":"2019-07-17T12:05:27","modified_gmt":"2019-07-17T11:05:27","slug":"biggest-autofocus-mistakes-avoid","status":"publish","type":"post","link":"https:\/\/other.kelsey.host\/amateurphotographer\/technique\/biggest-autofocus-mistakes-avoid\/","title":{"rendered":"Biggest autofocus mistakes and how to avoid them"},"content":{"rendered":"

There are a few creative exceptions, but one of the fundamental aims of most photographers is to get their subject sharp. Thankfully, that\u2019s the aim of a modern camera\u2019s autofocus (AF) system too but although they are sophisticated, they are not 100% foolproof. There isn\u2019t one go-to setting that guarantees every subject is tack-sharp in every situation. As a result, it\u2019s important to understand how your camera\u2019s AF system works, the conditions that challenge it, and the settings or techniques that will help you to get the results you want.<\/p>\n\t\t

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Using a large AF area makes it easier to track subjects that move erratically. Credit: Angela Nicholson<\/p>\n\t\t<\/div>\n\t\t\n

How does my camera\u2019s AF system work?<\/h2>\n

There are two types of autofocus systems commonly used today: phase detection and contrast detection. Digital SLRs use phase-detection focusing when the viewfinder is in use, but many (recent Canon DSLRs being a notable exception) actually switch to contrast detection in live-view mode.<\/p>\n

Mirrorless cameras operate in full-time live-view mode and use either contrast detection or a hybrid AF system that combines contrast detection with phase detection. Even if they use contrast detection, it\u2019s usually a heck of a lot better than the contrast-detection system used by the average DSLR in live-view mode.<\/p>\n

The mechanics of phase detection are a little different depending upon whether it\u2019s used with a DSLR\u2019s re ex mirror or a mirrorless camera\u2019s sensor, but the principle is still the same. The light exiting the lens falls on to two pairs or groups of matched pixels. If the light intensity of the images from these pixels is the same, the subject is in focus. If there is a difference, the subject isn\u2019t in focus and the camera will then calculate which way to adjust the lens.<\/p>\n

Contrast detection relies on the fact that a subject has the most contrast when it\u2019s in focus. It\u2019s an accurate method but it can\u2019t tell which way to adjust the lens so it may go one way, then the other. This means it\u2019s inherently slower than phase detection. However, mirrorless camera manufacturers have honed this type of focusing so it\u2019s much faster than it used to be. And developments like Panasonic\u2019s Depth From Defocus (DFD) technology helps cameras like the Panasonic Lumix G9 and GH5 get super-sharp shots of fast-moving subjects.<\/p>\n

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Auto AF is useful for subjects like this flower which was intermittently buffeted by the wind. Credit: Angela Nicholson<\/p>\n\t\t<\/div>\n\t\t\n

Which AF mode should I use?<\/h2>\n

There are three main AF modes: single, continuous and auto. Single (S-AF or AF-S) mode is designed for use with motionless subjects. Once selected, the camera attempts to focus when the shutter release or AF-on button is pressed. Once the subject is sharp, it won\u2019t readjust focus until you release the button and press it again.<\/p>\n

In continuous autofocus (C-AF or AF-C) mode the camera continues to focus for as long as the shutter release is held down. This makes it the default choice for shooting moving subjects. It also means that it\u2019s the wrong choice if you\u2019re planning to focus-and-recompose. I\u2019d also steer clear of it with stationary subjects as it can \u2018fidget\u2019, which slows things down and can mean missing the focus.<\/p>\n

Auto autofocus (AF-A or A-AF) mode is a useful choice when you don\u2019t quite know what you\u2019re going to photograph. With street photography, for example, you might shoot a static scene one minute and fast action the next. But if you know what you\u2019re shooting, it\u2019s best to use either S-AF or C-AF because the camera needs to make some decisions in A-AF mode and it sometimes gets things wrong or causes delay.<\/p>\n

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Eye AF can help you nail the focusing with a wide range of people shots. Credit: Angela Nicholson<\/p>\n\t\t<\/div>\n\t\t\n

What has artificial intelligence (AI) got to do with autofocus?<\/h2>\n

Face- and eye-detection AF have been around for a little while, and they are useful for social photography. Eye AF is especially helpful because it gets the most important part of the subject razor sharp, leaving you to focus on aspects such as the composition and timing of your shots.<\/p>\n

One issue with some face-detection systems is that they incorrectly \u2018see\u2019 faces where there aren\u2019t any, which leads to focusing errors. For this reason, it\u2019s a good idea to turn the feature off when you\u2019re not going to use it.<\/p>\n

Thanks to increased use of artificial intelligence and greater processing power it seems likely that we\u2019ll see more subjects being detected automatically in the near future. Sony has now extended its Eye AF system to cover animals\u2019 eyes, or at least cats\u2019 and dogs\u2019 eyes, and Olympus\u2019s OM-D E-M1X can detect cars, trains and aeroplanes, while Panasonic\u2019s Lumix S1 and S1R can detect animals. They all work well, making it easier to get those subjects\u2019 sharp.<\/p>\n

Even though modern AF systems are incredibly smart, there are occasions when it\u2019s better, or perhaps easier, to focus manually. Cameras usually search the distance for their target, which means that it can be quicker to focus manually on a macro subject, for example. And if you\u2019re shooting through something like water spray from a fountain, or foliage, the AF system is likely to get very distracted, thus making manual focusing a better choice here too.<\/p>\n

If you\u2019re using a DSLR, don\u2019t struggle with the tiny viewfinder image while you focus. Instead, activate live view so that you can magnify the focus point and use focus peaking to get the subject completely sharp.<\/p>\n

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A small AF point gives you precise control over the focus. Credit: Angela Nicholson<\/p>\n\t\t<\/div>\n\t\t\n

Never use auto-AF point mode: true or false?<\/h2>\n

Most cameras offer a collection of AF-point selection modes that help you to choose the point(s) you want to use. These include options such as single- point, multiple-point, all-point or auto-point, zone and tracking. The smaller the selection area, the more precise you can be with the focusing. However, if your subject is moving, it\u2019s worth selecting multiple-point mode or zone mode as it\u2019s easier to keep at least part of a larger area over it.<\/p>\n

In the past I would have avoided using an all-point or auto-AF point mode that puts the camera in full control over which AF point is used (it always just focused on the nearest or most central object), but Sony\u2019s recent AF systems have changed that. Its Wide and Zone Focus Area modes work brilliantly and in continuous autofocus (AF-C) mode in models like the A6400 and Alpha 7 III, they\u2019re great for tracking a moving subject. But it\u2019s worth noting that they usually target the nearest part of the subject and if it stops moving, it\u2019s prone to focusing just in front of it.<\/p>\n

Sony\u2019s Lock-on AF and Real-Time Tracking modes work in a similar way to other manufacturers\u2019 tracking modes, but are better than most. You start by placing the point over the subject, and then press the shutter release to activate the focusing. Once you start shooting, the camera is in control over where the AF point goes. It usually follows the subject, but if it strays you have to stop shooting to reposition it.<\/p>\n

Most tracking AF modes aren\u2019t as fast or accurate as Sony\u2019s, but can be useful with slower or motionless subjects. For example, you can place the starting AF point over a critical part of a subject and it should stay with it as you recompose.<\/p>\n

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When shooting a portrait wide open with an 85mm lens it\u2019s best to select an AF point rather than focus and recompose. Credit: Michael Topham<\/p>\n\t\t<\/div>\n\t\t\n

I like to focus and recompose \u2013 is this OK?<\/h2>\n

When cameras had one AF point we often had to focus and recompose the shot before taking the image. However, simple trigonometry tells us that this can lead to focusing errors. If you\u2019re shooting a portrait with an 85mm f\/1.4 lens wide open, for example, the depth of field is so shallow that using the central point to focus on the eye and then recomposing can result in the eyebrow or nose being sharper than the pupil. Consequently, it\u2019s much better to compose the shot and use the AF point that\u2019s closest to the object you want to be sharpest.<\/p>\n

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Why use the back button?<\/h2>\n

Even with high frame rates, it\u2019s important to time your shots carefully. Back-button focusing can help with this by taking away the shutter release\u2019s AF activation duty. Instead, you use the AF-on button on the back of the camera (or via a customised button) to control the AF system, and only press the shutter release when you want to take a shot.<\/p>\n

This is especially useful when you\u2019re capturing action. With motorsports, for example, you might want a car to appear at the apex of a bend. To help with this, you can set the focus with the AF-on button and it won\u2019t shift when you press the shutter release to take the shot.<\/p>\n

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Most cameras struggle to focus on a silhouette unless you put the active AF point over the edge where there\u2019s some contrast. Credit: Angela Nicholson<\/p>\n\t\t<\/div>\n\t\t\n

My AF system is struggling to lock on – what should I do?<\/h2>\n

Focusing systems need light and contrast to work. It\u2019s the lack of contrast that can leave a camera struggling to focus on a well-lit sheet of paper or a smooth wall. If you can find some point of contrast, even if it\u2019s just a join in the wallpaper, it will often snap into focus quickly.<\/p>\n

Because the phase-detection sensors in a DSLR are often arranged in lines, the detail needs to cross a line rather than run along it for it to be \u2018visible\u2019 to the AF system. Consequently, it\u2019s a good idea to understand your camera\u2019s AF sensor arrangement. It\u2019s also why cross- type AF points are useful \u2013 they can detect details in more than one direction.<\/p>\n

If your camera is struggling to focus, look for an area of contrast or consider switching to the central AF point. As well as being cross-type, this is usually the most sensitive in a DSLR.<\/p>\n

Cameras sometimes have multiple cross- type AF points and it\u2019s worth knowing where these are, especially when shooting action in low light.<\/p>\n

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Knowing how your subject moves can help you decide on the ideal AF customisation settings. Credit: Angela Nicholson<\/p>\n\t\t<\/div>\n\t\t\n

Is it worth customising my AF system?<\/h2>\n

Some cameras have a series of customisation settings for the AF system. These affect aspects such as how the AF responds to a change in the subject distance or if the subject moves from the selected AF point.<\/p>\n

Many photographers assume that it\u2019s best to set the camera to react quickly to any change, but a little pause can be a good thing. For example, if you\u2019re photographing a swimmer doing the breaststroke towards you, with a fast setting the camera will refocus on the far end of the pool every time they bob underwater. Then when they reappear it will refocus again, making a major adjustment. A slower reaction will help the camera stay on the subject.<\/p>\n

To get the best from these customisation systems you need to consider what you\u2019re photographing as well as the shooting conditions \u2013 for example, whether any objects are likely to come between you and the subject.<\/p>\n

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AF mysteries explained<\/h2>\n