Phase Detection Autofocus

Lens Calibration Explained

Oct

2012

October 10, 2012

If you are wondering about how to calibrate lenses, this article has detailed explanations and different methods of AF fine tuning. Due to the nature of the phase detect autofocus system that is present on all SLR cameras, both cameras and lenses must be properly calibrated by manufacturers in order to yield sharp images. Various factors such as manufacturer defects, sample variation, insufficient quality assurance testing/tuning and improper shipping and handling can all negatively impact autofocus precision. A lot of photographers get frustrated after spending thousands of dollars on camera equipment and not being able to get anything in focus. After receiving a number of emails from our readers requesting help on how to calibrate lenses, I decided to write this tutorial on ways to properly fine tune focus on cameras and lenses. Lens calibration is a complex topic for many, so my goal is to make this guide as simple as possible, so that you could manage the process by yourself, while fully understanding the entire process. In addition, I strongly recommend to follow these tips every time you purchase a camera or a lens in order to identify and address any potential focusing issues. But I have to warn you – this article is NOT for beginners. If you just got your first DSLR, you might get very quickly frustrated with the calibration process.

1) Why Calibrate?

Why is there a need to calibrate lenses? With the release of new, high-resolution cameras like Nikon D800, it seems like calibration is becoming an important and hot topic. Why is that? As I have explained in a number of my photography articles and reviews, while the increase of megapixels in our cameras has a number of benefits (see benefits of high resolution cameras), it can also expose potential focus problems. A slight focus issue might not be as noticeable on a 10-12 MP sensor, but will be much more noticeable on a 25+ MP sensor (assuming both sensors are of the same size). Especially when viewed at 100%, which is what we, photographers unfortunately like to do too much. Hence, the need for a properly calibrated camera setup today is bigger than ever.

How Phase Detection Autofocus Works

Jul

2012

July 11, 2012

By Nasim Mansurov 31 Comments

When it comes to DSLR technology, there seems to be quite a bit of confusion on how exactly phase detection autofocus works. While for most people this might not be a topic of great interest, if you are wondering how and why a camera could have an autofocus problem, this article will shed some light into what happens inside the camera in terms of autofocus when a picture is taken. There is an overwhelming amount of negative feedback on autofocus issues on such fine tools as the Canon 5D Mark III, Nikon D800, Pentax K-5 and other digital SLR cameras and it seems like most photographers do not seem to understand that the underlying problem is not necessarily with a specific model or type of a camera, but rather with the specific way these cameras acquire focus. If you search on the Internet, you will find thousands of autofocus reports on all kinds of DSLRs dating back 10+ years. Hence, the front focus and back focus issues we see in modern cameras are not anything new – they have been there ever since the first DSLR with a phase detect sensor was created.

How DSLR Cameras Work

To understand this issue in more detail, it is important to get to know how a DSLR camera works first. The typical DSLR illustrations only show a single reflex mirror positioned at a 45 degree angle. What they don’t show, is that there is a secondary mirror behind the reflex mirror that reflects a portion of light into a phase detect sensor. Take a look at the below simplified illustration that I made from a sample Nikon D800 image:

How to Quickly Test Your DSLR for Autofocus Issues

Jul

2012

July 1, 2012

By Nasim Mansurov 128 Comments

Below is the easiest and quickest way to test if your DSLR has an autofocus issue, along with a recommendation on what to do if there is a problem. This test can be used to detect front focus or back focus issues with a particular lens or a camera body. I will be using the Nikon D800E as a reference camera for this article, but any modern DSLR with Live View capability can be used for the same test (even entry-level DSLRs such as the Nikon D3200 have a Live View mode). Why would you want to test your camera for autofocus issues? Because if your camera or your lenses are defective or have a calibration problem, then you will not be able to obtain critically sharp images.

1) What You Will Need

For this test, you will need the following:

Any DSLR with Live View mode capability such as the Nikon D7000.
At least one lens, but preferably 2-3 lenses if you want to isolate the problem to the camera or your lenses.
A good stable tripod.
A flat vertical surface in a very brightly lit area. For example, your garage door or a wall inside your home that is adjacent to a very large window will do fine.
Print out either this Siemens Star Focus Chart or this Focus Test Chart on regular letter size paper. You can print it on a laser printer or inkjet (doesn’t really matter). Make sure to print on regular paper, not anything glossy like photo paper.
Scotch tape or some other adhesive material to keep the focus chart on the wall.