A friend recently asked me why his camera was so slow to focus.  The answer naturally started with (as do most answers in photography) my standard phrase, “It depends…”  I realized after my conversation with him that this would be an excellent topic for exploring here on the blog.  So, today, we’ll be taking a look at focusing concepts in your camera gear.  To start off, it probably makes the most sense to identify that there are focusing features both on your camera, and on your lens.  On cameras, it’s most often referred to as the “auto-focusing system” or AF.

AF Systems

Canon 7D AF System

The above AF system comes from the Canon 7D, but generally speaking, newer cameras will have better AF systems than older cameras.  What makes them better is the presence of more AF points.  The more points the sensor has, the easier it is to identify a point to focus on.  The AF system identifies these points based on contrast between light and dark, so reflected light does matter to a degree as well. The bottom line though is that with more focusing points, more focus detection, better contrast algorithms, etc. and better sensitivity to light will allow a camera to find a subject quicker than its older counterpart with fewer AF points.  Of course, the AF system is only as good as the lens that it connects to, and the lighting conditions you are in.  If you have a great AF system (camera body) and a slower lens (an aperture in the range of f4 or f5.6), you will still be limited by light.

The upshot:  More AF points = Better Camera (in general)

While there are always exceptions and nuances or unique scenarios for specialty gear, focusing speed is really more a function of the lens than the camera body.  What you will find in bodies though, is that you get better target acqusition and tracking on the higher end bodies than the entry-level models (say an Xsi versus a 1DMark II).   The camera basically does the job of saying “here’s the point to focus on”, and then transfers the job to the lens of actually making that point the sharpest one in the picture.

One question that I often hear though is “So, what defines how fast the focus locks – the AF system or the lens?”  The answer is that the AF system does the focus lock – the “lock” is really just defining the point to focus on…it’s just a fancier way of saying it.  If you have no point locked in, even the fastest lens will just search and search and not bring anything sharp because it doesn’t know what to make sharp.

I would be remiss though if I didn’t acknowledge that newer cameras do work better in lower light too though, because they do.  Here’s why:  you will get better low light performance on a higher end body, simply because the sensor is larger, and more sensitive to light.  But when we start talking about light, we have to actually give more of a nod to the lens.

Lens Focusing

Where camera bodies pull their weight in target acquisition and tracking, lenses pull their weight when it comes to operation in lower lighting conditions.  The larger your aperture (the smaller number actually means a wider open aperture, remember?), the better it will perform in low light because you are simply letting more light get to the sensor.  That lens that opens to f2.8 will let more light in regardless of the body it is connected to!  This is why expensive lenses that can open t f2.8, f2.0 or even wider are referred to as “fast glass” because they let such a large amount of light onto any sensor.

The other part of what really gives a nod to lenses over bodies is that lenses are what actually does the focusing.  Adjusting the focus to something that is “sharp” is done by the lens.  Where the camera said “Hey, cool, I’ve found the point to focus on”, here, the lens uses the AF points from the camera body and basically says “okay, I am going to focus in on this point that you’ve defined.”

The last point to bring up here is to mention the focusing motor of the lens.  Terms that are often bandied about include USM, HSM, and other similar terms.  This is where it can get really confusing because each vendor uses unique terms, and even within a specific vendor, the same letters can mean different things.  Here’s (since it’s the CanonBlogger site), I am going to stick to the Canon nomenclature for now, which is the USM.  In the Canon family, USM is seen in many lenses and has quite a history.  It started in the body before the digital camera revolution.  When Canon introduced the EF lens mount in 1987, the technology was available to put the USM on the lens, which allowed it to operate even faster.  They never looked back!

Nowadays, USM is bandied about within the Canon family, and you have to be careful now, because one is used to identify a lens feature, while the other is more marketing and hype (in my opinion anyway).  The one that really matters are the ones that refer to the Ring USM.  These lenses are more expensive, heavier than their counterparts, are much quieter, and also tend to bring the subject into focus faster.

Canon USM Feature Explained

The alternate motor or Micromotor Ultrasonic Drives are the ones used in cheaper lenses, which by comparison are lighter, noisier, and focus more slowly on the subject.  Most kit lenses are of these variety.


What did we learn here?  First off, hopefully you’ve come away with a better understanding of body AF systems versus lens focusing.  On the most simple of explanations – a camera AF system says where to focus, and the lens is what actually does the focusing.  It should come as no surprise though, that both tie in very tightly to light.  In the body, better sensitivity to light allows for better contrast in the AF points, while in the lens, the wider aperture can bring more light in under conditions where a narrower opening would not suffice.  So, higher sensitivity and wider openings can impact contrast and focusing speed in bodies and lenses respectively.

The bottom line:  neither does this task independently, but when  it comes to speed – most of this comes from your lens, not your body!


Got questions, comments, or additional thoughts to share?  Sound off in the comments!  Have a great weekend, and we’ll see you back here on Monday!

3 thoughts on “Understanding Focus

  1. DSLR’s only use contrast based autofocus in LiveView. Phase detection (a wonderful development useing the wave nature of light) is used when the mirror is in place. in brief, phase detection splits the light into two seperate beams a know distance apart, once split, the light pattern of each beam is measured, and matched up(think peaks and troughs). Since the distance of the split is know as well as the speed of light, they are able to calculate the distance of the subject and adjust focus accordling. Thats why high accuracy “cross type” autofocus points are so important, as they have two phase detecting sensors in a cross pattern for better/faster focus accuracury.

    Small m4/3, and other EVIL cameras only use conrast based autofocus(due to the fact they do not have a mirror to bounce the light to a seperate AF sensor). However,the Nikon 1 Series has phase sensors built into the sensor chip.

    1. What a great insight, thanks for sharing! I am thinking of adding a “featured comment” plugin to the blog, and this would definitely rate there, as it taught me something new too! Again, thanks! 🙂

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