The longer you shoot, the larger your repertoire of subjects and assignments you photograph becomes. You start off photographing flowers in the garden, your neighbours dog, your sisters kids, your friends wedding and then before you know it, you're doing product shots for your friends new company. All this happens over time and there is one pretty fundamental skill that must remain paramount throughout this process; properly focused images.

Sure we've all been there, we've all taken that shot once in while which is slightly ‘soft’ (a polite photographers term to describe out-of-focus images), but it's a great shot so we keep it anyway. Even so, we would still of preferred it to be 'tack sharp'.

In focus images has been one of the most fundamental rules of photography right from the dawn of the craft. In the early 1900’s it was a craft in its own right but in the 1960's Leica introduced a rudimentary auto-focus system that changed everything. Since then of course auto-focus has developed dramatically and it's no longer a feature on cameras, its a given.

Modern Auto Focus

So bringing auto-foucs up to date we have a few options to choose from in our modern DSLR's and those are some of the features I just wanted to cover and when to use them. Both Canon and Nikon have very similar settings albeit incorporating different technologies the results are very similar. There are other brands like Sony and Olympus etc. that also follow suit but here I will be discussing the four main focus modes in Canon and Nikon.

This image was shot utilising the 'AF-S/One Shot' auto-focus mode on the camera. Here I focused on the models eyes and then recomposed my image so that she was over to the left of frame allowing for more space in the image where she is looking.

One Shot / AF-S

First off, you have the mode that’s probably been around the longest, Canon's 'One Shot' and Nikon's 'AF-S'. Both of these will do pretty much the same thing and they are predominantly used for stationary objects like model shoots (most of the time, more on when not to use 'AF-S/One Shot on model shoots later) and anything that doesn't require your subject to move around too much in frame. You half press the shutter in this mode and then you can recompose the image, for example you focus on the models eyes then recompose her to be on the left hand side of the image. This auto focus mode is the most common one and will get you through most situations.

Al Servo / AF-C

Next we have the step up from the single focus to Canon's 'AI Servo' and Nikon's 'AF-C'. Essentially what these settings do is continuously track your initial focus point and readjusts the focus accordingly. This setting is ideal for moving subjects like active children and pets that are constantly on the move within your frame.

AI Focus / AF-A

Finally out of the auto focus settings we have Canons 'AI Focus' and Nikons 'AF-A'. Both of these settings actually leave it up to the camera to decide which is best out of the other two focusing modes to use and will choose to continuously track your chosen subject should it decide to move or focus lock if you would like to recompose.

In theory then I needn't have bothered explaining the other two settings as surely this is the best of both worlds? Not quite, I personally have tested this mode a fair amount with stop-start subjects and although the camera does a good job of keeping up with them, it’s always more accurate to use continuous focus mode. The same also goes for its ability to determine when a subject has stopped and when to focus lock for recomposing. Personally I never use this mode as although it has the best of both it also has the worst of both.

So although I have just covered the three basic settings here very briefly, there is of course a whole host of other technological advancements in auto-focus I haven't covered. I know Nikon has extensive, matrix and 3D auto focusing features as well as most modern DSLR also incorporating the 'back button auto focus' which also helps with focus locking, but going over all of that is not the purpose of this article.

Manual Focus

The last focus mode I wanted to cover and one that is rarely used is the Manual focus mode. Now this mode strikes fear into the heart of nearly all modern photographers and that’s simply because they've probably never used it. Do you ever need to use it? That is something that only you can decide and is probably based on the type of photographs you take. If you only ever take portraits of energetic kids or fast paced sports then auto-focus is probably aways your go-to focus mode. If however, you shoot still life, architecture, landscapes and other detailed and relatively motionless subjects then manual focus is probably a good way to go. There's a few reasons for this; landscape photographers will want to find the hyper-focal distance of their scene to maximise the amount of 'in-focus' points in the scene, this is based on an equation so auto-focusing on a specific object is not always a good place to start. Still life photographers will usually have their camera 'locked off' on a tripod so they will not want to focus and recompose once they've set up the shot, its just far easier to focus manually.

There is however another reason to want to use manual mode on some cameras and in certain situations, and this is the catalyst for this article.

This version of the image was shot using the auto-focus mode 'AF-S/One Shot' and meant that after I had focused and recomposed the shot, the models eyes were left 'out' of focus.

The ‘Focus-Recompose’ Issue

I recently purchased a 85mm f1.8 prime lens and I wanted to test the lens out to see what the sharpness was like at f1.8. I predominately only photograph models so I set up my test and went about taking some shots at f1.8 using my usual AF-S/One Shot auto-focusing mode. When I got my shots back to the computer to take a look I was surprised to see that most of them were very 'soft'. It took a few minutes to realise my error and since then, I've adjusted how I shoot with these parameters.

Here you can see that the selected focal node is still situated in the middle of the viewfinder even though I have elected the outer most one when shooting in the portrait format.

I haven't done a lot of very shallow depth of field shots up until this point so I hadn't seen the now exaggerated results of my poor focusing technique previously.

At f1.8 you have a very, very shallow amount 'in-focus', so for example a head shot with the eyes in-focus the tip of the subjects nose is 'out' of focus. For the test I was photographing the model at 3/4 length and shooting up at her so my camera height was probably at her waist height. I was about 6ft/2m away from her and I was focusing on her eyes with my focal point in camera then recomposing my shot to capture the 3/4 length crop. The problem with most (DSLR) cameras is that although they have a lot of focusing points, they're all clustered in the centre of the viewfinder so even though I chose the outer most focal point I still have a dramatic amount of recomposing to do.

This diagram clearly illustrates whats actually going on when you recompose an image after focusing in 'AF-S/One Shot' auto-focusing mode. The actual part of the image that was in focus is now out of focus.

This isn't normally a noticeable problem when recomposing at f8, f11 and f16 but at f1.8 that dramatic shift in the focal plane means the resulting image is very soft around the models eyes. As I recomposed the shot it actually repositioned my focal point further back behind the model, meaning the back of her head and hair were in focus but not her eyes.

There is not too many ways around this pesky little issue, especially as you might not notice it on the back of the cameras little screen. One thing that did resolve it though was by switching to 'manual' focus. I could then compose my shot and manually focus on the models eyes resulting in a fantastically sharp image where I wanted it to be.

Granted there was a few things conspiring together here to really exaggerate the issue. Firstly I was shooting at f1.8; that’s always going to rely on critical sharpness. Secondly I was down low shooting up, this always exaggerates the focal plane shift when recomposing and lastly I was stuck with limited focal nodes. There are many technical reasons why modern DSLR's don't allow focal nodes towards the edges, a lot of smaller frame cameras like the mirrorless, APS-C and micro 4/3 cameras all have selectable focal nodes covering the entire viewfinder, but alas DSLR technology still isn't there yet. Until it does, it’s a good idea to be aware of what’s going on in auto focus modes on your camera and be prepared and ready to switch to manual focus when required. Manual focusing is a skill and as such will take a little practice, but with a little patience you should notice an increase in the hit rate of 'tack sharp' shots. Good Luck!