What Is It About Autofocus That's so Important?

What Is It About Autofocus That's so Important?

No, it’s not a trick question. I recently asked Fstoppers readers about their absolute must-have in a camera, and the clear winner was autofocus. But there is a lot to autofocus, so I want to examine some of its aspects more deeply.

In a recent article I wrote discussing deal-breakers and new cameras, I asked about the one thing you couldn’t live without and the one thing that you really do need before you're prepared to shell out some serious money for a new body or an upgrade. There was a wide variety of answers and some surprise inclusions on the list such as the tilt screen, ergonomics, dual card slots, buffer speed, and of course, price. But references to autofocus came out on top, which got me thinking about the different facets of autofocus systems that make it so vital to modern photography.

Put simply, autofocus is so important because it provides peace of mind to users. If you know that you’re going to get a high majority of your shots in clear, crisp, perfectly sharp focus, then you can worry about the other things that make a good image, like composition, for example. I mean, there's no more sinking feeling for me than when I look at a shot that has wonderful light or a very pleasingly arranged composition only to zoom in and find that my subject is slightly blurry, or that I haven’t quite nailed focus well enough for a paying client. That moment when you go from “boom baby” to “you gotta be freakin’ kiddin’ me!” is not nice, to say the least. And let me assure you my language is far more colorful than that if I get a well-composed shot that’s not razor sharp.

Sometimes, moments are so fleeting that if you miss the shot, then that light or that composition might not come together again, and the opportunity is gone. So, knowing that you can rely on and trust your autofocus to do its job and provide wonderfully sharp images almost every time is a huge relief and gives you far less to worry about when you’re out in the field.

Frame Coverage

So, what are the different parts of autofocus that make it so important? First, there’s how much coverage you have across the frame. In some modern cameras such as the Sony a9, you get almost total coverage from corner to corner of your frame (93% to be exact), which is extraordinary when I think of the autofocus coverage in my very first Canon Rebel DSLR, which you can see below.

Even now, with my current Canon 5D Mark IV, it has pretty good coverage, but there are times when I really do need a little more, particularly at the top and bottom of the autofocus coverage area, which I've indicated with red arrows in the image below.

Having 93% coverage across the frame like you get with the Sony a9 can make life so much easier and offer up some marvelous opportunities in post-production. For example, if you’re shooting macro and you want to do some focus stacking, it makes it very easy to place the focal point on various parts of the frame quickly and without fuss.

As you can see in this picture of the flower above, there is a lot of the area in the frame that is soft and out of focus. Now, you might like that look and that might be the type of composition that you’re going for, but if you wanted more focus across the frame and different elements of the frame in focus, having an autofocus system that covered the entire frame would make this very, very simple. Say goodbye to focus and recompose forever.

Speed

Another hugely important aspect of autofocus is speed. How fast can your autofocus system lock onto the subject when you press that button? Sometimes, it might only be a fraction of a second delay, but that can often be the difference between nailing the shot and getting a slightly fuzzy image. This is particularly so in scenarios where you might have erratically moving subjects such as surfing or wildlife photography. In cases such as these, you really want to have confidence that your camera’s autofocus system is going to be able to lock on to and keep up with the movements of your subject.

In this shot above (SOOC), I was able to pick up my camera quickly and get my daughter in focus immediately. It’s not often that she runs towards the camera with a smile on her face, as for some reason unbeknownst to mankind, this little diva princess suddenly goes into a shell whenever she sees my camera slung around my neck. So, to have this opportunity and be able to get the shot with one quick press with back-button focus was a testament to the speed of my autofocus system. But it was also a testament to its accuracy, which brings us to our next point.

Accuracy

Of course, there’s a lot of overlap between speed and accuracy, but sometimes, they are separate entities. I’m sure you’ve had an experience where your camera’s autofocus system is telling you that it has locked onto the subject, so you take the shot in good faith, only to then check your shot after the fact and sometimes notice that the focus wasn’t accurate at all. In essence, your camera’s autofocus system lied to you, kind of like telling you it had hit a bullseye on the dartboard when in fact it had barely scraped double twenty.

That’s particularly maddening, because you should be able to trust your camera when it says it has locked on to the subject you have asked it to. Such scenarios might be forgivable if you’re shooting landscapes on a tripod and you can just do it again or even manually focus yourself to get the shot, but in a situation like high-action sports or a wedding where moments are absolutely crucial, then this is unforgivable and perhaps even a deal-breaker.

Different cameras use different autofocus systems. However, regardless of whether they use a phase detection system or a contrast detection system, the accuracy rate should be high if you’re going to use that camera long-term. When I was first learning about autofocus and trying to ensure that my camera could find the subject quickly and accurately, I was always told to look for contrasts in colors or lines in my frame. That way, you give your camera’s autofocus system the best opportunity of locking on to the desired subject. On the other hand, sometimes you might notice when you try to focus on a single solid color that the camera has a lot of trouble, and you hear the lens struggling or you get the flashing light in your viewfinder.

For example, in the picture above, if I were to set my camera on the little dark spot, it would find focus quickly and accurately within the blink of an eye. Conversely, if I tried to focus on the plain white wall, it might struggle a little bit. I can’t speak for any other camera, but I do know that using the 5D Mark IV, it doesn’t handle solid colors like that particularly well. So, in this day and age, when cameras are becoming such extraordinary pieces of technology, you should be able to get an autofocus system that’s fast, accurate, and covers almost all of the screen. That way, you can feel safe in the knowledge that your camera's autofocus system will do its job properly and you can focus on colors, or light, or composition — the things that really separate good images from great images. Autofocus should not be a concern, and that’s why I presume it ranked most highly in Fstoppers’ readers’ responses to my deal-breaker article.

What are your thoughts? I’d love to hear from you in the comments below.

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22 Comments

AF, for me, is critical because there's no saving a blurry picture. I can deal with low dynamic range, poor contrast and even bad ergonomics but give me a camera with a low keeper rate due to poor autofocus and I'll chuck that camera off a cliff!!!

Kidding aside I can't think of anything more frustrating with a camera.

Scott Choucino's picture

Interesting article.

I have a similar one coming up where I got in touch with over 100 pros from different fields and tried to break down to the specific genres of what is wanted. It was really interesting (to me) and I think I have similar findings.

Iain Stanley's picture

I look forward to reading it!

To me AF is a crutch. Seems to me I have 15/15 on a Mamiya 645 Pro, 10/10 on Asahi Pentax 6X7, 36/36 on Konica Autoreflex T3, and on my AF speed demon that is the F4S with 1 AF point 36/36 in focus . I find the more cluttered the VF The moe you can bd confused. I will never let a piece of equipment decide what I am focusing at. Can remember covering hockey and football on an F2AS. But That's just my opinion .

I guess it depends on what you shoot. If you have ever tried shooting a bald eagle or osprey fishing you may not consider AF a crutch. Sure it can be done without, but I bet the guys who shot film and manual focus in the past through out a lot of out of focus shots to get one keeper.

Iain Stanley's picture

As Kurt says below, I think the genre/field of photography can have a big impact on your feelings towards AF. In some situations you simply don’t have time to manually focus continuously on things going on around and expect 95%+ keeper rates. Plus, if you’re outlaying large sums of money as with modern cameras, certain expectations must be met.

The need for AF was actually there back in the manual focus film days. The camera makers worked very hard to make VFs that assisted in focusing as much as possible.
Today the resolution in imges is so high that slight errors in focus are very obvious. Most people do not print and many images are seen only at web res in small-ish sizes but the propensity to zoom in on images in PS reveals the focus errors.
For me, the AF issue is two-fold.
First, the hype about AF is overstated. AF is faster than ever however the speed of AF for Canon and Nikon DSLRS plateaued a number of years back. Yes, each iteration got a smidge faster but half micro second improvements might be a large percentage but not a total time improvement that is meaningful.
Also, Canon in particular seems to have segmented their performance to ensure the 1D series has the snappiest response.
Second, the ability for the camera to figure out the subject is still very weak. I have often had to use modes that rely on the camera to select a subject from the BG in fast moving situations only to have it fail reliably. Irrespective of the claims of "intelligence" the camera still really has no clue as to what is of interest to the operator. Eye AF (at least for the Canon EOS R in its latest upgraded form) is still unimpressive. In many cases it cannot even find a human in the scene.
So yes, AF is important but the awareness that is is driven by a witless algorithm must be front and center in the photographer's mind and not get lulled into disaster in a "must have" situation by the happy talk from manufacturers and their paid "influencers".

Iain Stanley's picture

I must admit, I’m hugely fussy when it comes to shots in focus. My wife is sick and tired of hearing me look at photos in exhibitions and plaintively cry “but it’s not even in focus” haha

Black Z Eddie .'s picture

Assuming the AF is fast and accurate, higher keepers with less shots. And, one less thing for me to worry about. Just frame the shot, check settings and histogram, and fire. I move around a lot while shooting so having a great AF brings high confidence.

Iain Stanley's picture

Exactly. As I said early in the article, AF brings peace of mind (it should anyway!!)

I for one definitely rely on my auto focus. As I`ve gotten older I now have a need for reading glasses this becomes difficult, you cant really wear reading glasses all day while run and gun style shooting say while out on a full day hike, however if I set my diopter to compensate all photos must be shot using the viewfinder rather than the larger back screen, then later you grab the camera for a quick shot with glasses on and everything looks blurry again.
Even with an fast and accurate auto focus system I often don`t discover till I get home and on a full computer screen, that some of my images I thought were good, turn out to be soft.

One of the places the camera always seems to be struggle is shooting through a window, Understandably the camera doesn`t know if I`m trying to focus in the room or out the window. My camera does have a touch screen to focus but when using the eyepiece its difficult to simultaneously work a touch screen, and the joystick can be a bit clunky for a quick response time.

Also, I`m not always sure which of auto focus setting I should be using for best results.

David T's picture

Not just the raw speed/accuracy of the AF system matters, but also the handling/ergonomics.

I find it very hard to navigate on Sony, e.g. if there are multiple faces. Press AF mode, switch to flexible spot, move point to face, press Face AF Button, hope it locks in.

With Panasonic I just point in the general direction and tap on the face that I'd like to lock in.

Iain Stanley's picture

It’s interesting reading and learning about the nuances of the different systems. I wonder if Custom Controls can fix your issues on Sony

David T's picture

I think I already have it set up the most efficient way possible on Sony by re-appropriating the AEL button on the lens to Face-AF. It's just a caveat I have to live with as a hobbyist/side-business who is looking for best bang for buck. Sony A7iii is overall a great deal.

I generally use the center point and reframe on my D750 as the full focus points might pick the background. Are my fears unwarranted or are you guys picking your focus point with the joystick(or whatever) on every shot? If so, that can't be very fast. The D750 has a "3D" option as well that i don't often use, is that the better choice?

William Salopek's picture

Jason I've wondered that as well...whats the quickest way to pick a focus point? With face/eye AF, and shooting people of course, the camera can be quite good at focusing on the correct part of the frame...but when there is a bird in the sky, or in a bush, the camera focuses on whatever, which most of the time is the wrong item. And activating tracking is cumbersome...but maybe I'm missing something. So I also often use centerpoint, then focus (single or continuous) and recompose.

I loved my manual focus Hasselblad 500CM's. When men were men. All day weddings. 220 backs. Kodak VPS 160.

Ahh, the hoary days of yore! Good stuff though.

Tim Foster's picture

I only own one autofocus lens. It's certainly not a priority for me.

A good article, thank you Iain. For me I do use both, manual when I need the point of sharp focus to be in a specific spot, i.e. Macro etc.
But then we read about calibrating our lenses in Auto with a specific black and white printed card and that is a whole new ball game.
I have done it myself but found it a tad tedious which is why I also resort to manual with lots of practiced gained as I also shoot with old film cameras as well as attach beautiful vintage lenses onto my digital

The flower image is a manual focus as Auto got very confused as you can imagine.
The moon eclipse shot was a 70-200mm lens in Auto, manual just didnt work at all. Not perfect but good enough.

Having said all of the above, I do love and hugely rely on my good ol' faithful Auto Focus

Iain Stanley's picture

I went through the process of calibrating my Sigma Art 50mm. Tedious yes, but well worth it in the end. Great shots!