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DSLR Focus Modes Q&A - Your Questions Answered

This is a Q&A on DSLR focus modes that will seek to answer some of your most burning questions on the subject. I'd like to put to bed any confusion and uncertainty surrounding this area.

I particpate on several photography forums and have learnt a lot from them. But often there is a tendency amongst contributors to delight in jargon. Some people seem to adore making simple things seem complicated, as though to affirm their own sense of knowledge and expertise. This is very true of discussions about focus modes.

But explaining things is really about making complicated things seem simple. So (not that DSLR focusing is complicated, frankly!) this Q&A will be a waffle free, jargon busting resource for you.


1. How does the auto-focus system on my camera actually work?

Basically, the whole thing is run by auto-focus sensors. These sensors are very clever at detecting areas of contrast within a scene, which help them to understand the likely point on which you intend to focus.

Some sensors are cleverer than others though! Those known as 'cross type' sensors are superior and feature in better DSLR models. They are able to detect contrast in a wide range of scenes. Meanwhile the poor relation are 'vertical line' sensors, which can struggle in some situations, although are excellent at picking up on horizontal lines.

DSLR's have varying numbers of auto-focus sensors, the amount tending to increase with the quality of the camera. You can select which sensors (or single sensor) are used to pick out a focus point manually.


2. Why does my auto-focus system sometimes 'search' for a focal point for ages, before just giving up?

Well, there are limits to what autofocus can do. It's seriously irritating when the focus ring swivels violently one way then the next, without success. But it's understandable why it happens. Auto-focus needs contrast in order to detect something to lock on to. Where there is a lack of contrast, like in a clear blue sky, it doesn't really have a fighting chance.

Equally AF systems require good light and lack of movement (whether from the subject or camera shake). Think of it as something with really terrible eyesight! It can only just make things out and lock on to them, provided the edges are clear, the light is good and everything is nice and still.

When one or all of those things are not satisfied, you'll get that annoying search for focus going on. So, for example, the worst subject to auto-focus for might be a street performer wearing brown clothes, in front of a brown building, in dim evening light. Conversely, the best subject for autofocus could be a smiling couple standing on the beach in broad daylight, posing for a photo.


3. But why do I sometimes get out-of-focus shots even when conditions are perfect for the autofocus system?

There are several possible reasons for this. The 3 most common tend to be:

1) The focus points (explained below) you have selected to use have been changed without you relaizing/remembering (as in the example image below). It's really easy to choose which focus points to use, and we all sometimes switch to one option for a certain shoot and forget about it the next time. On my camera, the controls are also physical switches on the back of the camera which could easily be knocked inadvertently.

2) You may need to go into the AF menus and specify that a shot cannot be taken until focus is locked in. Sometimes the default option may be to prioritize taking the shot(s), regardless of whether focus has been found (i.e. in order to always get maximum possible fps). If that's the case it can be easy to take pictures before focusing properly.

3) You're using a really narrow depth of field, like f.1.8 or f.2.4, and weren't quite as accurate when focusing as you'd thought. Using a narrow depth of field makes focusing more demanding, because there is less margin for error. You only have to slip very slightly away from the intended focal point for it to become blurry with these settings. Practice makes perfect (and sometimes manual focusing is necessary).


4. So what are my options for selecting AF areas/points?

Whilst the options and names vary from camera to camera, the basic options are similar across modern DLSLR's, so I'll refer to those I'm familiar with on my Nikon D200. Firstly, I select which focus point/area to use using the arrow keys on my camera. The centre point is the most accurate, so I usually select that. Next, I choose the Area Focus Mode, of which there are 4 options:

1. Single Area. This uses a single one of the points you see in the viewfinder and is best for static subjects. The central focus point is the most accurate.

2. Dynamic Area. Whilst the manually selected focus point is used, the surrounding focus points are brought into play if the subject should move. This way you can always be sure of good focus, even when the subject slips out of the primary focus point being used.

3. Group Dynamic. This is the same as Dynamic Area, but instead of one focus point being used, a group/cluster of them is used (i.e. left, centre, right). So if you're photographing something that suddenly moves from the right to the centre of the frame, Dynamic Are focus mode will pick it up and re-focus accordingly.

Group Dynamic mode uses a group of focus points, which can switch from those on the left, right or centre when you cannot move quickly enough to keep up with the subject.

4. Dynamic Area - Closest Subject. This, again, is the same as Dynamic Area Focus, only the focus does not follow a subject, but switches to whatever is closest to you in the frame.


5. When is AF not a good idea?

It's best to use Manual focus mode when AF struggles to lock on. So, as we know from earlier, that can be in low light, where there is minimal contrast or when there is lots of movement.

It's also worth switching to Manual when it's necessary to really fine tune focus carefully. The most obvious example of this is in macro photography, where the depth of field is so small that anything but perfect focus is going to miss the subject.

Another example is some portrait shots. The eyes are the focal point of portraits more often than not, and this requires very careful focusing to get right. To be honest, I still trust be autofocus system to get it right here, but sometime Manual mode is better.


6. Any other tips for shooting in Manual focus mode?

Some lenses feature a switch with the options M and M/A. The latter is suitable for switching between both Manual and Autofocus modes, but leaving it set on M can give the best results when manual focusing.

When you're not 100% sure that a manually focused shot is in-focus, lots of cameras have a little indicator light to give you a clue. For my D200, I just half press the shutter release, as though auto-focusing, and check to see whether the in-focus light appears in the viewfinder. It begins to flash when nearly there and becomes more solid when focus is spot on.

Hyperfocal distance is a concept that takes a bit of explaining. I recommend this tutorial from the always brilliant Cambridge In Color to get to grips with it. But one of the simplest ways to find hyperfocal distance is to set the focus ring to infinity, and then draw back the focus as far as possible before the background areas start to become become blurry.


7. What's the difference between 'Single' AF mode and 'Continuous' ('AI' for Canon) AF mode.

Single AF mode locks on to a single point and is best used for stationary subjects. If the subject moves, the focus remains at the original poistion, or relocates on to whatever has appeared where the subject was. This is the mode to use for subjects like portraits, landscapes or still life, where there's plenty of time to set things up.

Continuous mode responds to a moving subject by constantly tracking it. In fact, it doesn't so much track it as predict where it is about to move to based on where it has come from, thereby accounting for shutter lag. Used in conjunction with Dynamic Area focus mode (which alters the auto-focus sensors being used when the subject suddenly moves), Continuous mode is a great solution for sports and wildlife subjects.


8. Is it possible to use Single Area focus mode, with the central focus point selected, and recompose the scene once focus is locked?

Yes, absolutely. There are 2 common ways of doing this, both of which amount to autofocusing and then disabling the focusing capacity of the shutter release button when half pressed, so you can move the camera and take the shot without re-focusing somewhere new.

Option 1 is to autofocus, switch to Manual mode, then recompose and take the shot. Option 2 is to autofocus then hold down the autofocus lock button (AE-L/AF-L on the back of Nikons) and press the shutter release button. Keep the AF-L button held down the whole time, as this is what disables the focusing of the shutter release button.

First focus on the subject (clearly this image doesn't show what you'd see in the viewfinder, which would be more of the scene to the top right)...

...Then compose the scene as you wish with the focus point locked in.

Ok, so I hope this Q&A on DSLR focus modes has been helpful for you. Please do email me with any questions about DSLR focusing. If I get lots of questions on certain subjects I'll add them to this list.

As ever, feel free to leave feedback, ask questions etc in the comments below. Cheers.

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