Ways to ensure you can rely on your camera’s autofocus.

It can be frustrating and dispiriting when you can’t rely on your camera and lens to focus accurately every time you take a shot. And it’s often difficult to track down the source of the problem; so many variables have to be considered and it can take time and effort to find and fix them. In this feature we outline some of the main reasons your camera and lens might be unable to focus precisely.

AF accuracy in mirrorless cameras

In theory, mirrorless cameras should be able to maintain their focus accuracy better than DSLRs because focus is measured on the main image sensor, not separate sensors beneath a reflex mirror. Because the light has to be reflected off one or more mirrors before reaching the AF sensor in a DSLR, errors can happen when the mirror in displaced by less than a millimeter from the correct position.

The use of phase detection pixels on the sensors of both DSLRs and mirrorless cameras have gone some way to improve autofocusing speed and accuracy. However, these ‘hybrid’ systems only work with the Live View modes in DSLRs although mirrorless cameras can use them for shooting with the monitor or the EVF, the latter being handy when recording movie clips.

This illustration shows the difference between Dual Pixel AF and Hybrid AF, the latter embedding phase detection pixels among the imaging photosites. This enables the camera/lens to measure the subject distance from the focal plane very quickly, while the contrast AF can check for exact focus very accurately. (Source: Canon.)

But phase-detection pixels are only part of the solution. While the contrast detection (CD) systems on which early mirrorless cameras relied were slower than the phase detection (PD) systems in DSLRs, they are normally more accurate. In addition, with fewer mechanical parts involved, problems are less likely to occur.

So, the addition of on-sensor phase detection pixels has mainly solved the speed problem, rather than issues associated with accuracy. Mirrorless cameras can start to hunt when they switch between PDAF and CDAF as shooting conditions change, for example when light levels change abruptly.

Improvements in technology have expanded the low-light limit for phase detection recently from an average of about 1.0EV (which is full moonlight) to -3.0EV (which is light from a crescent moon low in the sky, assuming no other ambient lighting). As a result, hunting, when it occurs, is usually very brief, especially in the latest cameras where it may be undetectable.

Manual focusing has also been made easier in mirrorless cameras with aids like focus peaking and image magnification, which are provided in most cameras.

Regardless of the conditions in which focusing fails, it’s always best to do the simplest, cheapest things first. Switch off the camera then detach and re-fit the lens. Turn the camera on and see whether the problem persists. Power-down the camera again and remove and replace the battery; check the battery has sufficient charge.

Check the type of focusing motor in the lens. Did you shut the camera down before switching between auto and manual focus? This is essential when using lenses with micro-motors. Lenses with ultrasonic and stepping motors can tolerate manual override of AF but those with micro-motors might be damaged.

If none of these checks help, consider the following factors that can influence focusing accuracy:

1. The subject

Regardless of whether they are using contrast or phase detection, all AF systems rely on a difference in contrast to achieve focus. Where there is no contrast difference – for example in a plain blue sky or on a smooth one-colour wall – most cameras and lenses will struggle to focus accurately and ‘hunt’ through the focus range of the lens.

Most AF systems will find it difficult to focus on high-key subjects that lack hard edges to focus upon. Using wide-area autofocus when framing this scene will probably cause the lens to hunt for a second or two due to the low overall contrast, although there might be enough edges to focus upon in the centre of the horizon.

Autofocusing can also be difficult with subjects consisting of highly repetitive patterns and those with specular reflections. Overlapping objects in a scene can also confuse some AF systems and you may need to switch to spot focusing to ensure the area that interests you is sharp.

Autofocusing on a fast-moving subject can be difficult when there are repetitive patterns in the background. In this case, spot focusing was used to capture the subject at the start of his slide. The camera was panned with the subject during a 1/20 second exposure at f/7.1 with an 80mm lens at ISO 1600.

Solving these problems can be relatively simple. Either switch to manual focusing or find an object with adequate contrast at the same distance as the main subject, focus on it by half-pressing the shutter button or  holding down the AF button. Then reframe the shot and capture the image.

2. Incorrect camera and/or lens setting

Some cameras make it too easy to overlook focus mode selections by burying the settings deep within the menu. If you haven’t used the camera for a while, you may be shooting with manual focus settings while thinking autofocus has been selected.

When either the camera or the lens is set to Manual Focus (M/F) mode, the camera won’t even attempt to focus. Make it a regular practice to check the camera and lens settings at the start of each shoot.

The AF/MF switch on the Z-Nikkor 24-70mm lens. (Source: Nikon.)

Check the AF point or focus area settings, making sure you can see clearly where the main focus is being placed. An LED point in the viewfinder or on the monitor screen will be illuminated in one-point AF mode, while an area will be outlined in any of the area-AF modes.

If more than one focal point lights up in one-point mode, or a different point lights up, switch off the camera then turn it on again and re-set to single-point AF and test again. Use the same procedure with the area AF modes.

In cameras with an AF-ON custom function, half-pressing the shutter button will not engage autofocusing when this setting is switched on, even though the autofocusing might seem to be working. Check that AF-ON has not been set by selecting Reset in the custom setting menu, disable it if necessary and then test the autofocus again by half-pressing  the shutter release button.

To determine whether the camera or the lens is at fault, try fitting the lens to a different camera body and/or using a different lens on the body you suspect is the source of the problem. If autofocusing fails with multiple lenses, the fault lies with the camera; where only one lens won’t focus, that lens is the source of the problem.

3. Poor lens connections

No lens can focus correctly if it can’t communicate properly with the camera body. If there is a partial connection, focusing will be erratic. Start by checking that the lens has been fully rotated (it should click firmly into place). Then inspect the camera body and lens to see whether either has suffered any impact damage, such as dents or scratches.

Check the contact points, both in the lens and in the camera body. If the gold plating on their surfaces is scratched or appears dull or tarnished, they can make a poor connection, resulting in the possibility of intermittent focusing errors.

It can be easy to clean contacts with low levels of contamination, but care is needed to prevent dust and grime from entering the camera body when the lens is removed. When cleaning the contacts in the camera, remove the battery and hold the camera body with the lens mount facing downwards.

Use a fine nylon brush to wipe off dust, brushing gently downwards. Grease on contacts may be removed with a trace of alcohol (e.g. diluted methylated spirits) on a lint-free cloth or lens tissue. Blot up excess liquid before touching the contacts and be very gentle to minimise the risk of damaging or dislodging them. Put the body cap back on the camera and leave it face down for about five minutes before fitting the lens again.

The same processes may be used to clean the contacts on lenses. But here you can use a blower brush to dislodge dust. This might be all that’s needed. Make sure you don’t touch the lens elements if you’re not wearing rubber gloves.

Human sweat is greasy and acidic, which means your fingerprints can become etched into the delicate multi-coating on the front or rear lens elements. This will create problems with flare as well as making the lens worth less on the secondhand market.

Test the autofocusing system to make sure it’s working properly and, if your efforts have not delivered the improvement you need, professional servicing should be considered.

4. Contamination in the camera body.

Poor focusing accuracy in older DSLR cameras can usually be attributed to dust, hairs or other contaminants within the camera body; specifically inside the AF sensor system that sits at the bottom of the mirror box. Anything that interferes with the light reaching the AF sensors is a potential source of problems. Because only a small percentage of the light that enters the camera through the lens actually reaches the sensors, they will struggle to detect it.

Just as dust and grease on a lens will reduce the contrast in the images the camera records, so too will dirt on the AF sensor surfaces reduce the contrast the focus detectors will see. Reduced contrast will make it very difficult for the sensors to actually detect areas to focus upon, as is evidenced when photographing very low contrast subjects.

In DSLR cameras, slight displacement of mirror alignments (a problem that won’t affect mirrorless cameras) will impact on focus performance.  If any of these factors is a potential problem, your camera will probably need to be professionally serviced since the mirror box components might also require re-alignment. In general, cameras should be cleaned, checked and, if necessary, re-adjusted every year or two, particularly if you’ve been shooting is dusty places and/or places with high humidity.

AF microadjustment

The main reasons for AF microadjustment are to correct for tiny deviations in the manufacturing process that can influence focusing accuracy. At lens apertures smaller than the f/3.5-f/5.6 range found on standard consumer-level zooms, the depth of field is often wide enough to mask minor focusing errors, although they will be evident at wide apertures with faster lenses. Microadjustment, which lets users fine-tune AF settings for different lenses, can provide the necessary corrections.
Theoretically each camera and lens combination should focus at the same point; but if you’re unlucky, one might be off-focus in one direction while the other is off in a totally different one. Moving the exact point of focus slightly forwards or backwards can keep the camera and lens in perfect alignment. While this can improve the accuracy of the focus, it does nothing to correct its precision (whether the main AF point sits precisely where it should).

Two screen grabs showing the AF microadjustment settings in a Canon DSLR camera.

In a DSLR camera you can check the accuracy of the focus using contrast detection in Live View mode (where the image sensor is used to determine the best focus), then compare that to the phase detection AF. Here’s how to do it:

1. Mount the camera on a tripod, switch off image stabilisation, set the AF area to single point and select Live View shooting.

2. Set up a target to focus on at a distance no less than 50 times the focal length of the lens. If you’re shooting with a 50mm lens, that’s approximately 2.5 metres. Open the lens to its widest aperture and focus as sharply as you can using the monitor screen.

3. Without moving anything, switch the camera from Live View to one-shot AF and half-press the shutter button (or press the AF button) while watching the focusing ring on the lens. If your AF system is accurate, this ring should not move. If it does move remember which way and by how much. If it moves closer, that indicates front focusing; if it moves further away, the camera is back focusing.

When checking the results of AF microadjustment, Canon recommends taking three consecutive images at -10, 0, and +10 adjustment steps and then reviewing them at 100% magnification on a computer monitor. Adjustments can be saved in the camera for future use when the tested lens is fitted.

Datacolor’s SpyderLENSCAL enables photographers to make accurate measurements of the focusing accuracy of their lenses.

Datacolor’s SpyderLENSCAL is a useful tool that can be used with cameras that support AF microadjustment as well as those that don’t. Photo Review’s in-depth review is available at www.bit.ly/pr81-spyder.

It’s important to remember that no camera will produce a perfectly focused image every time at every distance, and no adjustment procedure will change that. However, a proper AF microadjustment can increase the percentage of shots that are correctly focused.

Article by Margaret Brown (see Margaret’s photography pocket guides)

Excerpt from Photo Review Issue 81

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